Introduction to UMTS Optimization
Short Description
Radio optimization...
Description
Introduction to UMTS Optimization Course Code: SC2804
Duration: 2 days
Technical Level: 3
Radio Principles and Planning courses include:
Radio Principles
Principles of Radio Site Engineering
Digital Radio and Microwave Link Planning
Cell Planning for GSM Networks
2G/3G Indoor Coverage Planning
3G Indoor Coverage Planning
Introduction to GSM Optimization
Drive-Test Data Capture and Analysis
Cell Planning for UMTS Networks
Introduction to UMTS Optimization
Specially prepared for Safaricom Limited
www.wraycastle.com
Introduction to UMTS Optimization
INTRODUCTION TO UMTS OPTIMIZATION
First published 2004 Last updated October 2008 WRAY CASTLE LIMITED BRIDGE MILLS STRAMONGATE KENDAL LA9 4UB UK
Yours to have and to hold but not to copy The manual you are reading is protected by copyright law. This means that Wray Castle Limited could take you and your employer to court and claim heavy legal damages. Apart from fair dealing for the purposes of research or private study, as permitted under the Copyright, Designs and Patents Act 1988, this manual may only be reproduced or transmitted in any form or by any means with the prior permission in writing of Wray Castle Limited. © Wray Castle Limited
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INTRODUCTION TO UMTS OPTIMIZATION
CONTENTS Section 1 Section 2 Section 3 Section 4 Section 5 Section 6 Section 7
Introduction and Overview Optimization Software Tools Optimizing Coverage and Capacity RAN Configurations and Dimensioning Idle Mode and System Access Connected Mode and Radio Link Control UMTS Features and Techniques
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SECTION 1
INTRODUCTION AND OVERVIEW
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CONTENTS 1
Optimization or Planning? 1.1 What is Optimization? 1.2 Typical Planning/Optimization Distinction 1.3 Differences for UMTS
1.1 1.1 1.3 1.5
2
The Optimization Process 2.1 Identifying Optimization Opportunities 2.2 Key Statistics and Analysis 2.3 Drive Tests and Signalling Analysis 2.4 Change Implementation 2.5 Monitoring 2.6 Database Update
1.7 1.7 1.7 1.7 1.9 1.11 1.11
3
Exercise 1 – Discussion about Optimization Options and Priorities
1.13
4
Drivers for Optimization 4.1 Overall Quality of Service (QoS) 4.2 Set-up Failure 4.3 Dropped Calls
1.15 1.15 1.17 1.19
5
The Coverage–Capacity–Quality Relationship 5.1 Interference Sources 5.2 The Coverage Loop
1.21 1.21 1.23
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Summary of Optimization Strategies
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OBJECTIVES At the end of this section you will be able to: • • • •
explain the close relationship between planning and optimization in a Wideband CDMA (WCDMA) radio network describe the overall optimization process as distinct from purely planning functions list typical key metrics relating to optimization outline, in general terms, how the air interface may be optimized through the use of cell parameters, activation of features and other techniques
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OPTIMIZATION OR PLANNING? 1.1
What is Optimization?
The term optimization is used in connection with almost any engineering design task. It is usually taken to mean fine tuning for optimum performance. This general understanding of the term can be applied comfortably in the context of a UMTS network, but its precise interpretation can vary a great deal in practice. Ideally, the optimization of a Universal Mobile Telecommunications System (UMTS) network would take place in the assumption that the network is not under performing because of some fault condition or configuration error. In practice, however, the output of the optimization process will often be the identification of a fault or incorrectly-set parameter value. The optimization process may also stray from its purest interpretation into the area of future planning. The nature of UMTS network design is such that it benefits from giving consideration to future direction even when planning for current needs. The optimization team is in a good position to estimate the likely future behaviour of the network and may provide a valuable input into future planning needs.
1.1
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Optimization – Theoretical fine tuning for optimum performance
Optimization – Practical fine tuning for optimum performance fault/configuration error detection identification of network development requirements setting planning goals
Figure 1 Optimization Definition SC2804/S1/v1.1
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Typical Planning/Optimization Distinction
Most people distinguish between the planning and optimization processes. This is true whatever the technology because it would be impossible to perform any kind of optimization on a network that had not yet been planned. Therefore, planning can be considered as a process that is carried out and completed before optimization commences. Furthermore, the optimization process will need a goal, for example a certain minimum level of dropped calls. Therefore it also makes sense to consider that until a network’s performance can be observed and judged, it cannot be optimized. This idea emphasizes a division in time between planning and optimization. Much of this is true of the Global System for Mobile communications (GSM). The GSM planning process is generally one of ensuring sufficient radio coverage based on assumptions made in formulating link budgets. The process of coverage planning can be independent of capacity planning. This means that the initial planning process can be performed without optimization involvement.
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Set targets for radio coverage and capacity
Perform link budget calculations and planning for radio coverage
Dimension for capacity requirements Build the network
Gather performance statistics
Optimize radio network design and configuration
Plan for continued network development
Figure 2 Planning and Optimization Relationship in GSM SC2804/S1/v1.1
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Differences for UMTS
For UMTS, coverage and capacity planning must be linked. This is because the mutual interference between calls has a direct impact on radio performance, hence on coverage. This means that even at the earliest stage a proposed radio network design should be tested, evaluated and optimized in traffic-loaded conditions. The only way to do this at the design stage is by simulation. A realistic and detailed simulation will be beneficial. Similarly, the earlier the optimization process can be carried out the better. This can be thought of as ‘optimization in advance’. However, no simulation is perfect and traffic characteristics can only be guessed. This means that constant modification is required as the real network is rolled out and real traffic characteristics become apparent. In UMTS, planning and optimization are ongoing processes that will always remain closely linked.
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Set targets for radio coverage and capacity Perform link budgets and traffic analysis to determine cell characteristics and configuration
Optimize through simulation
Plan radio network including expected expansion after rollout
Optimize through simulation
Build the network Optimize radio network design and configuration
Gather performance statistics Plan for continued network development
Figure 3 Planning and Optimization Relationship in UMTS SC2804/S1/v1.1
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THE OPTIMIZATION PROCESS 2.1
Identifying Optimization Opportunities
The first step is to distinguish between optimization problems and faults and configuration problems. Information is therefore required from a number of sources, for example: • key performance statistics • problem reports from customers • radio planning information • recent configuration changes • completed and ongoing work • existing data on problem areas Analyzing this data and correlating the information will enable true optimization opportunities to be identified. 2.2
Key Statistics and Analysis
The next step is statistical analysis of all the sites with an optimization problem. Radio planning will give information about anticipated problems such as interference and coverage. Historical data on previous problems may indicate a new issue has arisen, perhaps due to expansion or an increase in load factor on one or more cells. There may now be enough information to suggest a solution. If not, further information may be obtained by drive testing. 2.3
Drive Tests and Signalling Analysis
Performing a drive test in the area where the problem exists may result in further data. Failing that, detailed analysis of the signalling information passed between the Node Bs and Radio Network Controllers (RNC) may uncover the problem. To make the drive test, call trace and signalling measurements valid they should be performed under the same conditions as those prevailing when the original problem occurred. For example, at the same time of day, in the same traffic conditions, on the same route and in the same place.
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Inputs: Identifying an QoS targets, problem reports, optimization opportunity planning information, ongoing work Statistical analysis of all sites of interest
Sufficient information
Inputs: radio planning, historical data No
Perform drive test
Yes Identify an appropriate change Implement change
Monitor results No
Success
Reverse change
Yes Update database
Figure 4 The Optimization Process SC2804/S1/v1.1
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2.4
Change Implementation
When the problem has been identified, an appropriate change should be recommended. However, before considering a change, the impact on the rest of the network needs to be assessed. This is critical in UMTS because of the interaction between User Equipment (UE), and also between Node Bs, in terms of interference effects. Parameters that may be considered for optimization on a cell basis include: • physical channel elements • Power Amplifier (PA) maximum transmit power • power control parameters • selection and reselection parameters • handover parameters • neighbour lists • common channel configuration • dedicated channel configuration • antenna orientation • antenna tilt • antenna type (beam width, beamforming, adaptive) • antenna height It may be desirable to implement network features such as: • transmit diversity • site selection diversity transmit • hierarchical cell structures • multi-user detection • secondary scrambling codes
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Inputs: Identifying an QoS targets, problem reports, optimization opportunity planning information, ongoing work Statistical analysis of all sites of interest
Sufficient information
Inputs: radio planning, historical data No
Perform drive test
Yes Identify an appropriate change Implement change
Monitor results
Success
No
Reverse change
Yes Update database
Figure 4 The Optimization Process (repeated) SC2804/S1/v1.1
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2.5
Monitoring
Having made the change it is important to perform post-implementation monitoring to ensure it has the desired effect. This can be done by monitoring the statistics or, better still, by using the same method as was used to identify the problem initially. Statistical analysis should also be carried out to assess the impact, if any, on the rest of the network. In UMTS this monitoring must include observation of surrounding cells. 2.6
Database Update
If the changes have been successful (or not), the databases in the network management systems need to be updated. This way the history of the problem, and hopefully its solution, can be logged and used by others.
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Inputs: Identifying an QoS targets, problem reports, optimization opportunity planning information, ongoing work Statistical analysis of all sites of interest
Sufficient information
Inputs: radio planning, historical data No
Perform drive test
Yes Identify an appropriate change Implement change
Monitor results
Success
No
Reverse change
Yes Update database
Figure 4 The Optimization Process (repeated) SC2804/S1/v1.1
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EXERCISE 1 – DISCUSSION ABOUT OPTIMIZATION OPTIONS AND PRIORITIES Working in groups of two or three, complete the following exercise and summarize your group’s answers in the work space on the opposite page. Allow about 10 minutes, after which all groups will compare answers. 1
List techniques, features or solutions that reduce interference either directly or indirectly (e.g. antenna downtilt).
2
List techniques, features or solutions that increase capacity either directly or indirectly (e.g. secondary scrambling codes).
3
List techniques, features or solutions that improve radio coverage or produce better utilization of existing coverage (e.g. cell repeater).
4
List techniques, features or solutions that combat slow fading and fast fading and their effects, either directly or indirectly (e.g. transmit diversity).
5
List techniques, features or solutions that improve link quality either directly or indirectly (e.g. multi-user detection).
1.13
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Exercise 1 Work Space and Summary of Results SC2804/S1/v1.1
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DRIVERS FOR OPTIMIZATION 4.1
Overall Quality of Service (QoS)
For a network to be successful in the highly competitive mobile phone market, it must be customer driven. This should be reflected in the setting of appropriate Quality of Service (QoS) targets against which network performance can be measured on a regular basis. The QoS targets must be reviewed regularly as part of a policy of constant improvement. The UMTS standards associate a specific technical meaning to the term QoS in describing the expected performance characteristics of a channel. These are valid in this context, but the term is also being used in a wider sense. Here it includes a customer’s personal feeling about the success and usability of a service. Thus it includes what may be termed ‘human factors’. Measurement of the QoS may be carried out either by the network operator or by an independent agency or a combination of the two. In terms of air interface performance for real-time services such as voice, customers are usually concerned primarily with call success rate and secondarily with call quality. For non-real-time services such as messaging or data exchange, this prioritization may be reversed. Call success rate could be defined in a number of ways, but a simple definition classifies calls as successful when they set up without a problem, do not suffer handover failure and clear normally, i.e. the call is not cleared abnormally or dropped. Given the slight differences in processes, it is wise to measure call success rate independently for mobile-terminated calls and for mobileoriginated calls. Call quality may be measured in a number of ways depending on the type of call. Voice or video may be judged subjectively, but for optimization purposes an objective target in terms of bit error rate or frame erasure rate is preferable. Data and messaging services can also be considered in terms of bit error rate and frame error rate, but a retransmission factor should also be considered. Data services will also have delay requirements in terms of latency and delay variation. Finally, the quality of the radio channel may be a good indicator of overall quality and this may be monitored in terms of radio signal strength and signal-to-noise ratio.
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Quality of Service (QoS) Call Success Rate
Set-up failure
Dropped calls
Mobile originated
Mobile terminated
Link Quality Signal-tonoise ratio
Bit error rate Frame error rate
Radio signal strength
Retransmission rate
Figure 5 Quality of Service (QoS) SC2804/S1/v1.1
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4.2
Set-up Failure
Call set-up failure is attributable to a variety of causes. There may be a hardware and/or software failure in the network or in the mobile equipment; alternatively, the UMTS Subscriber Identity Module (USIM) may be invalid or faulty. In relation to the air interface, congestion may be the cause, possibly within the Random Access Channel (RACH) or Paging Control Channel (PCCH). Generally this type of congestion only affects mobile-terminated calls, but PCCH congestion may also affect some types of ongoing data calls. The congestion of traffic-carrying channels will be a significant concern for optimizers. When the cell’s noise rise limit is reached, Radio Resource Control (RRC) will not allow new calls to be established. This situation in UMTS is complicated by the simultaneous provision of different service types with different QoS requirements. For example, a real-time voice call or higher-bit-rate video call may be blocked because of the noise rise limit. Yet, at the same time, a low-bit-rate non-real-time call may be allowed to go ahead. Additionally, the noise rise in a cell to be partly a factor of traffic load in neighbour cells, so it is possible for congestion in one cell is caused by overloading in a neighbour cell. Care must be taken to ensure that the cause is the focus of optimization, not the symptom. Calls may also fail at setup because of poor radio coverage, fading, or interference causing failure in access channels. Coverage can never be perfect. Interference is always present and can become too strong. Fading effects are also inevitable in a cluttered, multipath environment. The most obvious sources of interference are other users and other intra-frequency cells. However, interference contributions will also be present from inter-frequency cells, some of which could belong to other operators. This may be an important consideration in some optimization scenarios. The multimedia nature of Third-Generation (3G) services means that not all networks will support all services in all locations. Therefore it is possible that calls may fail simply because the network does not support the requested service or channel configuration. Incorrect cell parameter settings could also cause set-up failure, for example by causing mobiles to select an inappropriate server in idle mode or use inappropriate transmit power for access. UMTS presents particular challenges for the optimizer in this respect because there are so many parameters and because of the interdependency between cells.
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Set-up Failure
Hardware limits Soft capacity Service type and QoS variation Air interface channel types
congestion poor radio coverage
Intra-frequency Inter-frequency Inter-operator Pilot pollution External Noise
interference fading service not supported
incorrect or suboptimal cell parameter settings
Many parameters Interdependency
hardware/software problem in the network, mobile equipment or USIM
Figure 6 Set-up Failure SC2804/S1/v1.1
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4.3
Dropped Calls
Many of the reasons why calls drop are closely related to those that cause set-up failure. For example, calls may drop because of a hardware or a software problem in the network or mobile equipment, or because of problems in the radio channel. The potential causes of problems with the radio channel in terms of signal strength or interference are the same as those for set-up failure. One additional problem when considering dedicated channels could be the inappropriate setting of parameters that relate to closed loop power control. Calls requiring dedicated channels will also need handover functions. These may be a mixture of soft and hard handovers. In most UMTS networks there is also a requirement for inter Radio Access Technology (RAT) handovers. There are many parameters that relate to measurements and subsequent handover decisions. Incorrect or inappropriate setting of these parameters could result in handover failure. Problems with coverage or interference could also result in handover failure. In extreme cases call drops may be forced on a priority basis at times of congestion. If pre-emptive channel allocation is adopted for emergency (112) calls, then a routine non-emergency call may be dropped to provide emergency capacity. Key metrics relating to dropped calls include poor signal level, high interference level and handover success/failure rate.
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Dropped Calls
Intra-frequency Inter-frequency Inter-operator Pilot pollution External noise
interference fading poor radio coverage
Different bit rates Different QoS
handover/reselection failure
Soft (intra-frequency) Hard (inter-frequency) Hard (inter-RAT)
fast power control
Capacity Quality
incorrect or suboptimal cell parameter settings
Measurements Power control Handover
pre-emption for emergency call channel allocation
Figure 7 Dropped Calls SC2804/S1/v1.1
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THE COVERAGE–CAPACITY–QUALITY RELATIONSHIP 5.1
Interference Sources
The capacity available in a UMTS system is ultimately limited by the amount of interference present. Downlink capacity may be thought of as limited by the total amount of transmit power available from the Node B. Nonetheless, downlink transmit power is only a factor because the inability to raise power beyond a limited point restricts the ability to overcome interference. The amount of interference tolerated by a given system is variable. It can be considered a factor of three key considerations: • services offered • features supported • local environment Different services have different QoS requirements and can therefore tolerate different amounts of interference. Optional features such as Multi-User Detection (MUD) can be used to increase tolerance to interference. The local environment determines a channel’s exposure to potential interference sources. An uplink channel is separated from other channels by uplink scrambling codes. An individual channel will experience interference predominantly from other in-cell and neighbour-cell intra-frequency channels. However, there will also be some adjacent channel interference, which may be most problematic if the interference source belongs to another operator. A downlink channel is separated from other channels on the same cell by the Orthogonal Variable Spreading Factor (OVSF) codes. These are highly orthogonal, but where different-length codes are used simultaneously in a multipath environment there will be a significant interference contribution. Downlink channels in neighbour cells are separated by scrambling codes, but this will also present an interference source. Additionally, as for uplink channels, adjacent radio channels will contribute some interference.
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Other UEs in neighbour cells
UL Int. DL Int.
UL Int.
DL
UL Int.
Intra-frequency neighbour
DL Int.
UL DL Int. Inter-frequency neighbour Other UEs in the serving cell
Serving Node B UE
Other UEs in neighbour cells
Figure 8 Interference Sources SC2804/S1/v1.1
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5.2
The Coverage Loop
Conventional planning practices deal with capacity and coverage as fundamental but independent processes. This approach is not applicable for a Code Division Multiple Access (CDMA)-based system. UMTS is both CDMA based and it provides multimedia support, hence capacity and coverage calculations cannot be separated. Any tool used to simulate network performance for planning or optimization of a UMTS system must link these calculations. The link budget is a normal starting point for any coverage estimate. However, in a CDMA-based system the link budget must account for interference levels. The interference level for a cell can be calculated if the load on a cell and its neighbours is known. If traffic distribution and traffic types are known, then cell load can be calculated for a given coverage area. In order to calculate cell coverage it is necessary to calculate a link budget. To establish an initial entry point to this loop, an assumed load is used, allowing an iterative process to begin. This will ultimately converge on a solution. The result of this convergence will be used for planning a network in the rollout stage. Once the network is operating and carrying live traffic these calculations may need to be revisited. Discrepancies between the assumptions made at the planning stage and real traffic characteristics could lead to coverage or interference problems. It is an optimization function to verify load, capacity and coverage assumptions as part of the analysis of optimization tasks.
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Coverage
Link Budget
Capacity
Figure 9 The Coverage Loop SC2804/S1/v1.1
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SUMMARY OF OPTIMIZATION STRATEGIES Most optimization solutions involve the use of network features, adjustment of one or more cell parameters, adjustment of antenna orientation, tilt, height or type, and redimensioning of traffic or control channels. More serious issues may require the addition of macro or micro sites, provision of in-building coverage, or cell splitting. In all cases, optimization activity must be carefully prioritized, keeping QoS and the customer in mind. There is little point in trying to optimize a cell working at 90% of potential capacity if one of its neighbours is suffering a 50 percent handover failure rate, for example. The optimizing engineer must always look for a practical solution that acknowledges the real constraints. For example, in a site suffering very high blocking, it may not be possible to install a second radio carrier (existing cabinets full, lack of space for more, perhaps) and another solution must be found (maybe a new micro cell and use of a Hierarchical Cell Structure (HCS) perhaps). It is also important to look for the simplest solutions first. For example, downtilting an antenna to modify coverage before considering a complete change of antenna type or complicated and risky parameter changes. Another complicative factor can be the use of Radio Network Subsystem (RNS) equipment from a number of different vendors within a single network. This can cause compatibility problems as not all vendors offer the same features and facilities. Adjustment of cell parameters is not a precise science. Some trial and error is often required. It is important to adjust only the minimum number of parameters simultaneously (one at a time if possible) in order to determine the parameter producing the changes (desirable or otherwise). Parameter changes can be implemented locally or from the Operations and Maintenance Centre (OMC). In all optimization activity, it is important to consider possible knock-on effects before taking action. Reorienting an antenna could solve coverage problems but cause serious interference problems elsewhere. It is important to consult others, discuss the issues, and perhaps consider alternatives before selecting the final solution. Equally, the appropriate company procedures must be followed when implementing changes. Finally, timing is important. Busy hour is not the best time for potentially serviceaffecting changes of parameters, features, etc. It is necessary to choose the time carefully and ensure all procedures are followed.
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Key Optimization Options antenna adjustment omni to sector transmit parameter tuning new cells additional radio carriers channel types/configurations deployment of features
Prioritize Activity customer quality of service
Select the Solution practical solutions within constraints simplest solution first knock-on effects consider alternatives multi-vendor issues company procedures timing
Implement the Solution monitor results customer quality of service reassess if required
Figure 10 Selecting and Implementing Optimization Solutions SC2804/S1/v1.1
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SECTION 2
OPTIMIZATION SOFTWARE TOOLS
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CONTENTS 1
Software Tools for Optimization 1.1 Introduction
2.1 2.1
2
Planning and Simulation Tools 2.1 Planning Tool Capabilities 2.2 The Graphical Display 2.3 Monte Carlo Simulation 2.4 Dynamic Simulations
2.3 2.3 2.5 2.11 2.15
3
Drive Test Tools 3.1 CW Testing 3.2 Live Network Drive Testing
2.17 2.17 2.19
4
Network Performance Data 4.1 Collection, Storage and Processing of Statistics 4.2 Key Statistics
2.21 2.21 2.23
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OBJECTIVES At the end of this section you will be able to: • • • • • • •
identify a range of different software tools that are applicable to the optimization process describe the desired capabilities of different tool types when used to optimize a WCDMA radio network describe how drive tests, ongoing radio coverage tests and traffic measurements relate to capacity and network optimization describe how simulations can be used to analyze optimization problems and identify potential solutions state the role of the NMC/OMC in providing statistical data of various types recognize the need for hardware and software tools in relation to testing and optimization recognize the limitations of tool and simulation capabilities
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SOFTWARE TOOLS FOR OPTIMIZATION 1.1
Introduction
There is a wide range of tools available to the optimizer to assist with the optimization process. Some of these are the same as those used for the planning process, for example planning software or drive test tools. Others are specific to the optimization role. These include system databases, network statistics analysis tools, dynamic simulation software, protocol analyzers, network simulators and parameter tuning tools. Figure 1 provides a summary of some of the key software tool types that are utilized for optimization. These tools can be very complex when applied to UMTS and it is important that the optimizer is familiar with their operation and capabilities. The optimizer must be able to interpret fully and correctly output information from the tool. While these tools can be very powerful they also have limitations that must be appreciated and allowed for if the correct significance is to be applied to results.
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Radio planning tools Static simulation tools Dynamic simulation tools Parameter tuning tools
Protocol analyzers
Build and configuration databases
OMC/NMC KPIs
RNC Drive test tools Node B
Figure 1 Optimization Tools SC2804/S2/v1.1
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PLANNING AND SIMULATION TOOLS 2.1
Planning Tool Capabilities
Planning tools may be modified versions of Second-Generation (2G) planning tools or they may be dedicated 3G tools. Many operators have both 2G and 3G networks and it is beneficial if the same tool can show and process information about both systems simultaneously. Planning for GSM is usually a simple process of creating coverage predictions based on balanced uplink and downlink link budgets. However, for UMTS, radio signal strength predictions are not sufficient. Even if uplink and downlink link budgets have been performed that include allowance for system load, specific simulations are required to model the effects of traffic. Realistic mixed offered traffic must be simulated as accurately as possible. Therefore the tool needs to have a facility for modelling a variety of traffic and channel characteristics. These are most commonly brought together to form a service reliability prediction using a Monte Carlo simulation The optimizer may also be interested in a number of other radio characteristics. For example, prediction of soft handover areas, pilot pollution, Ec/Io values and active set sizes are very important when considering optimization solutions.
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Traffic Modelling
Radio signal strength prediction
mixed traffic channel characteristics demographics mobility user characteristics
CDMA Factors soft handover areas pilot pollution Ec/lo UE transmit power active set size
Monte Carlo Simulations used to produce service reliability maps
Figure 2 Planning Tool Capabilities SC2804/S2/v1.1
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2.2
The Graphical Display
The graphical display in any planning tool will contain both foreground and background data. Background data includes things like terrain contours, clutter data and vector data showing roads and railways. It may also be possible to overlay aerial photos or maps. The display shown in Figure 3a is typical and is taken from the Atoll planning tool produced by Forsk. The display is currently showing terrain data with clutter and vector data on top. Foreground data includes an indication of site positions, typically with graphical and text annotations giving an indication of site configuration. On top of this the tool will display the results of predictions and simulations. Figure 3b shows sites displayed with radio signal strength. Figure 3c shows a mixed traffic Monte Carlo simulation. Figure 3d shows predicted soft handover areas.
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Figure 3a Example Graphical Display
Figure 3b Sites and Radio Signal Strength SC2804/S2/v1.1
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Figure 3c Mixed Traffic Monte Carlo Simulation
Figure 3d Soft Handover Prediction SC2804/S2/v1.1
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2.3
Monte Carlo Simulation
The Monte Carlo simulation is a critical process in the planning and optimization of UMTS networks. It is not an ideal simulation type in that it is static, but it is a good compromise that gives the optimizer a fairly quick and relatively realistic view of likely network operation. It is particularly useful for the optimizer to test the probable impact of a proposed optimization change. To simulate network operation it is necessary to account for the effects of interference between users in both the uplink and downlink directions. It is also necessary to model the effects of power control and mixed traffic. To do this, the Monte Carlo simulation creates a series of snapshots (or drops). For each of these snapshots users are randomly scattered over the ground area with weightings for expected traffic density. The tool then uses defined radio parameters to estimate transmitted power, soft handover requirements and, ultimately, call success rate. A number of snapshots can then be combined to produce a statistical analysis of the probability of coverage for various service types. 2.3.1
Monte Carlo Simulation Inputs
Figure 4 shows some of the most significant input parameters that are required before a Monte Carlo simulation can be performed. Tools vary in the way traffic profiles are entered, but typically traffic layers are built up by mapping services to user types and then user types to geographical areas. The result is a map showing the combined requirement for different services across the map area. Numerous radio parameters may be required. Many are related to site configuration and radio transceiver performance capabilities. However, some parameters may be adjusted through the optimization process.
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Bit rate Required Eb/No Activity factor PS/CS Channel type
Terminal type Service profile Service usage Mobility
User types User density
Service A
User type A
Area type A
Service B
User type B
Area type B
Service C
User type C
Area type C
Service n
User type n
Area type n
Monte Carlo Simulation
General Radio Parameters Site details (antenna height, gain, position, etc) Path loss Total transmit power Pilot power weighting Common channel power weightings Noise rise limit Ec/Io limit Soft handover thresholds Maximum active set size Power control step size Orthogonality factor
Figure 4 Monte Carlo Simulation Inputs SC2804/S2/v1.1
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2.3.2
Monte Carlo Simulation Output
The output of a snapshot produced through the Monte Carlo simulation will be an indication on the map of user distribution, requested services and connection success or failure. The example in Figure 5 shows a snapshot based on a simulated system supporting three different user types, each with access to the services listed in the displayed legend. The tool can provide specific data indicating the uplink and downlink channel performance for each user instance, as shown. Similar collective statistics can be produced for site performance. It is then possible to combine the outputs of a number of snapshots to produce a statistical map for each service type and user type combination.
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Figure 5 Monte Carlo Simulation Snapshot SC2804/S2/v1.1
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2.4
Dynamic Simulations
An advantage of static simulations is that they are quick to perform and the results are quite easy to interpret. Nevertheless, their accuracy is limited. When statically simulated, a call is either active or not, it is either in soft handover or not and power control is stabilized. In a real system there is a lag between measurements and control activity for power control and handover control. Similarly, open loop power control for Physical Random Access Channel (PRACH) establishment and signalling will precede all call attempts; even those that are unsuccessful. These can be allowed for to some extent in static simulations by including error variables, for example by adding a random error to required transmit power levels, but the most accurate results are produced with dynamic simulations. Dynamic simulations use specialized software that model user activity and movement over a continuous time frame. This enables much more detailed analysis of network behaviour with a specific set of parameter and configuration settings. This method is more time consuming but is of great value to the optimizer, especially in areas that are sensitive to small changes in settings. This method may also be used to generate correction factors that will improve the accuracy of results produced in static simulations. Care should be taken when setting up dynamic simulations to ensure that they have a clear objective goal. The results can be difficult to interpret if too many changes in settings are made.
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Trajectory of UEs is modelled following a map vector such as a road. DCH activity including closed loop power control
DCH activity including closed loop power control and soft handover
RACH activity including open loop power control UE inactive
DCH activity including closed loop power control
Figure 6 Dynamic Simulations SC2804/S2/v1.1
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3
DRIVE TEST TOOLS Drive testing often provides a primary source of information for optimizers investigating recognized performance problems. Drive testing can be used for a wide variety of network optimization functions including network performance assessment, fault analysis and model tuning. Two basic forms of drive testing are commonly performed, Carrier Wave (CW) testing and live network testing. 3.1
CW Testing
This involves the use of a calibrated receiver connected to a data storage device, typically a laptop or a Personal Digital Assitant (PDA). The receiver may be capable of measuring more than one frequency simultaneously. For UMTS it is useful if the receiver is capable of providing measurements of Receive Signal Code Power (RSCP) and Ec/Io for individual cells. However, basic CW testing measuring radio signal strength may be used on individual frequencies from a test transmitter for basic path loss estimation. CW testing is most commonly used for propagation model tuning and verification. The example in Figure 7 shows an overlay of CW test data on an empirically generated signal strength prediction. These differences can be analyzed to calculate a standard deviation for the cell. This can then be used to modify the ‘k’ values in the empirical model.
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Figure 7 CW Testing SC2804/S2/v1.1
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3.2
Live Network Drive Testing
This type of drive testing involves the connection of a test mobile (usually incorporating test software) to a logging device such as a laptop or PDA. A series of calls are made, either manually or automatically, and all events and signalling during the calls are recorded. It is particularly useful to record measurement data from the test mobile, both during calls and while in idle mode. UMTS offers the possibility to provide modified measurement commands to individual mobiles. This would mean that test mobiles could be asked to measure a larger neighbour set and provide more detailed measurements. The range of measurements that can be specified for UMTS is extensive. The recorded data captured during a drive test is then replayed using a drive test analysis tool. This may be a specialized tool, but many planning tools will also overlay some drive test data. Drive test analysis tools will typically use recorded positional information to provide a rolling map display for real-time or slow-time replay of drive test logs. Many analysis tools provide a protocol analysis function so that signalling can be decoded. This is particularly useful when analyzing the reasons for call failure. Figure 8 shows part of a drive test log overlaid on a graphical display in a planning tool.
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Figure 8 Live Network Drive Testing SC2804/S2/v1.1
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4
NETWORK PERFORMANCE DATA In a UMTS network, performance data is available in the form of raw statistics from all major network elements in the core network such as Mobile-services Switching Centres (MSC), messaging platforms, databases and other service platforms. Similarly, performance data can be gathered form all network elements in the UMTS Terrestrial Radio Access Network (UTRAN) such as RNCs, Node Bs, transmission nodes and Location Management Units (LMU). These statistics are essential for the day-to-day operation of the network, providing data for ongoing performance evaluation against targets. This information is also critical for the optimizer because it may be used for problem analysis and provides a means of assessing the success or otherwise of optimization solutions. 4.1
Collection, Storage and Processing of Statistics
All network elements, for example an RNC, collect and store statistical data locally. These raw statistics, of which there are many different types, are uploaded to the OMC/NMC at regular intervals. Usually they can also be read locally using a laptop. The uploads are carried out using Operations and Maintenance (O&M) data links, normally utilizing part of the transmission infrastructure. The upload interval could be as short as every five minutes, but is more likely to be every 15 or even every 30 minutes. It is possible for the most important statistics to be uploaded more frequently than other data in some systems. Raw statistics are sometimes called counters. The raw statistics can be viewed as tabular or graphical data, or further processed to provide key statistics, which are also known as metrics or Key Performance Indicators (KPI).
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Statistical reports (KPIs)
Storage in relational database
Reporting tool
Tabular
Graphical
Data collection process
OMC/NMC
Local access to data
Figure 9 Gathering Network Performance Data SC2804/S2/v1.1
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4.2
Key Statistics
Key statistics are the KPIs that are used to judge whether the network is working to its design criteria. They are created through the processing of raw statistics. For example, raw statistics may be uploaded from each cell regarding the number of call requests, the number of successful attempts and the number of unsuccessful attempts. These would all be provided for a defined measurement period, perhaps every 15 minutes. If all these results are summed for all the cells on an RNC over a 24-hour period, then a KPI could be produced representing average call success rate for each day. Typically this would be divided into success rates for each definable call type, for example voice, video telephony, low-rate packet data and high-rate packet data. KPIs will be required for many different aspects of the operational network’s performance. Figure 10 provides some examples of things that may be included, but it is up to individual operators to determine the most appropriate KPIs. KPIs falling below an expected threshold may trigger optimization activity. These statistics in themselves may be useful for the optimizer, but more detailed analysis is often required to isolate a problem. For example, the call success rate mentioned above may be studied on an hourly basis in order to identify a time period when the problem occurs. More detailed analysis may also be set up when a new feature is introduced on a trial basis. Because of the potentially very large amount of data generated, it is beneficial if particular information about performance is targeted for detailed analysis in respect of the new feature. However, standard statistics should also be monitored in case the feature has an unexpected effect.
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Idle Mode Related success rate location update routing area update UTRAN registration area update total attempts location update routing area update UTRAN registration area update
Set-up Related paging success rate RACH success rate successful channel allocations successful PDP context activations average duration for call establishment average range from which call attempts are made
Connected Mode Related number of dropped calls number of soft handovers number of hard handovers handover success rate average percentage of calls in soft handover average transmit power (uplink and downlink) by call type cell throughput RNC throughput QoS statistics for packet data average call hold time average mobility of users per cell average range of users in a cell
Figure 10 Typical Key Statistics SC2804/S2/v1.1
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SECTION 3
OPTIMIZING COVERAGE AND CAPACITY
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CONTENTS 1
Link Budgets 1.1 Load Factor 1.2 Load Factor and Noise Rise 1.3 Optimization Considerations for Load Factor 1.4 Mixed Traffic and Load Factor
3.1 3.3 3.5 3.7 3.9
2
Coverage and Capacity Optimization Issues 2.1 Coverage Solutions 2.2 Capacity Solutions 2.3 Adaptive Voice Channels 2.4 Secondary Scrambling Codes
3.11 3.11 3.15 3.27 3.31
3
Traffic Scenarios 3.1 Introduction 3.2 Uplink Limited Systems 3.3 Downlink Limited Systems
3.33 3.33 3.33 3.33
4
Evolving Radio Access Architecture 4.1 Rollout Architecture 4.2 Antenna Azimuths and Beamwidth 4.3 More Sectors or More Cells? 4.4 Use of Repeaters 4.5 Basic Considerations for Indoor Coverage
3.35 3.35 3.37 3.39 3.41 3.51
5
Exercise 1 – Urban Capacity and Coverage
3.53
6
Location Services (LCS) 6.1 Introduction 6.2 Quality of Service 6.3 Factors Affecting Accuracy of Location Information 6.4 Response Time 6.5 Cell ID Based Positioning Mechanism 6.6 Observed Time Difference Of Arrival (OTDOA) 6.7 Network-Assisted Global Positioning System (GPS)
3.57 3.57 3.59 3.61 3.63 3.65 3.69 3.79
7
Propagation Modelling 7.1 Empirical Models 7.2 Deterministic Models 7.3 Comparing Models and Their Effects
3.81 3.81 3.85 3.87
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OBJECTIVES At the end of this section you will be able to: • • • • • • • • • •
perform link budget calculations to verify cell size and traffic load capabilities in mixed traffic scenarios describe the impact of coverage and capacity expected for a range of mixed traffic scenarios describe the conditions in which a cell may become uplink or downlink limited describe conditions in which a system may be coverage or interference limited describe how the rollout architecture for a UMTS network can be evolved to expand capacity and coverage discuss the merits of cell splitting and multicell sites discuss the merits of using repeaters to improve coverage describe how masthead amplifiers can be used to improve coverage and capacity in a UMTS system identify suitable propagation models and explain the need for accurate model tuning state the requirements for optimization of location capabilities in the radio access network
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1
LINK BUDGETS A link budget must be performed in both the uplink and downlink directions. For GSM this only involves radio factors such as transmit power, receiver sensitivity, feeder losses and antenna gains. The aim is to find a maximum path loss that is acceptable in both the uplink and the downlink directions. For GSM, the result of this calculation is static since it is not altered by cell load. In UMTS the link budget is not static because it is affected by cell load. There are two related aspects to this: the fact that the technology is CDMA-based and also the need to support mixed traffic. In a link budget for a CDMA-based system, account must be taken of the interference present due to other users. This is a factor of serving cell load and also, to a lesser extent, of neighbour cell load. The resulting interference level is known as noise rise. It is necessary to allow a margin for noise rise when calculating the link budget. This margin is referred to as the interference margin. The noise rise is calculated from the load factor of a cell. The value of load factor is largely dependent on two factors: the channel processing gain and the required value of Eb/No at the receiver output. Both these factors will be different for different services with different QoS requirements. Thus a realistic value of load factor can only be achieved if realistic mixed traffic cases are considered. An important consideration for the optimizer will be the degree of correlation between the estimated traffic load used at the planning stage and the real traffic load when an optimization problem arises.
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Downlink link budget Uplink link budget
Maximum acceptable path loss Node B
UE
Radio Parameters
Interference Margin
Noise Rise
Load Factor
Mixed Traffic
Figure 1 Link Budget Inputs SC2804/S3/v1.1
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1.1
Load Factor
In practice the received signal power at a cell and at the UE contains both wanted channel data and unwanted interference. The theoretical maximum load on a cell would be when all the received power was wanted channel data. The load factor is the ratio of wanted power to unwanted power and is a measure of how close a cell is operating in relation to its theoretical maximum load. The calculation of uplink and downlink load factors differs slightly because of the relative positions of the transmitters and receivers. In the uplink direction the channels are transmitted from different locations, but are all received in the same location. This means that the effect of neighbour cells can be considered constant for all channels. In the downlink direction all channels are transmitted from the same location but received in different locations. The effect of neighbour cell interference varies as a result of the UE’s location and, ideally, this should be included in the load factor calculation. Additionally, a factor must be also allowed in the downlink to account for lack of orthogonality between variable-length codes in a multipath channel. Figure 2 provides expressions for uplink and downlink load factor calculation. Note that these expressions do not allow for a mixed traffic case as shown. However, this could be accounted for simply by summing the load factor estimate for each individual traffic type.
3.3
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η
=
+ =
η
+
ν
η η
UL
= UL load factor
DL
= DL load factor
−α +
= =
+
ν
N = number of UEs in the cell N j = an individual UE W = chip rate Eb = energy per bit No = noise spectral density Rj = bit rate for UE j j
= activity factor for UE j
j
= orthogonality factor
j
= neighbour cell interference factor
Figure 2 Uplink and Downlink Load Factors SC2804/S3/v1.1
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1.2
Load Factor and Noise Rise
Noise rise is derived from both the uplink and the downlink load factors (η) in the following way: Noise rise =
1 1–η
Noise rise is more usefully expressed in decibels for inclusion in the link budget as an interference margin; in which case the expression becomes: Noise rise (dB) =
–10log10(1 – η)
Figure 3a shows the relationship between load factor and noise rise expressed in decibels. It can be seen that noise rise tends to infinity as load factor approaches 100%. It is not advisable to plan a system with very high load factors. The shape of the curve indicates that at high load factors small changes in load give rise to dramatic changes in noise rise. A system planned to carry such loads would require an impossibly high interference margin or it would suffer extreme cell breathing effects. This is perhaps most graphically illustrated when looking at a linear representation of the curve as shown in Figure 3b. If the maximum load factor is planned to be in the region of 60% to 80% then the curve is relatively flat. A system planned in this way requires a more manageable interference margin leading to achievable link budgets. In addition, it should show minimal cell breathing up to the intended cell capacity limits.
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18 16 14 12 10 Noise Rise (dB) 8 6 4 2 0 0%
20%
40%
60%
80%
100%
Load Factor ( )
Figure 3a Load Factor and Noise Rise (Logarithmic)
50
40
30
Noise Rise 20
10
0
0%
20%
40%
60%
80%
100%
Load Factor ( )
Figure 3b Load Factor and Noise Rise (Linear) SC2804/S3/v1.1
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1.3
Optimization Considerations for Load Factor
At rollout a UMTS network will have a relatively small number of subscribers, who are not likely to make full use of high-rate data services. The operator’s aim at this stage will be to maximize coverage. Capacity in the network is unlikely to be a problem. Therefore it makes sense to select a fairly low load factor as a basis for coverage planning with macro cells only. Consider Figure 4. A load factor of 50% gives rise to a 3 dB noise rise. Including this as the interference margin in the link budget places a small, but still significant, limitation of maximum acceptable path loss. For example, if the operator wished to provide contiguous coverage in an urban area offering at least 144 kbit/s to class 3 UEs, a typical link budget might suggest a maximum acceptable path loss of about 145 dB. This can be interpreted in terms of cell radius using, for example, the COST231-Hata model. When not considering the interference margin this gives a cell radius of about 1.2 km. When allowing for a 50% load factor it is necessary to add another 3 dB interference margin. This reduces cell radius to 1 km. The 50% load factor would be enforced by the Call Admission Control (CAC) policy in the RNC. As traffic levels rise in the network, the cell load factor limit will begin blocking calls with a resulting fall in grade of service. Simply increasing the permitted load factor to alleviate this is not a sensible solution. For example, if the CAC policy was modified to allow a load factor of 75%, then noise rise would be increased to 6 dB. When factored into the link budget as interference margin cell radius is reduced to approximately 800 m at busy times. This could leave coverage gaps in the network. This could be dealt with by the introduction of either in-fill cells or a hierarchical cellular architecture incorporating micro cells. Micro cells used simply to absorb traffic load rather than provide extended coverage could be planned on the assumption of high load factors. Typical load factor figures for macro cells would be in the range 50% to 60%. This gives a good compromise between maximizing coverage potential and maintaining a reasonable traffic load. Micro cells are added with less emphasis on cell radius and more emphasis on capacity. Typical load factors for micro cells could be in the region of 75% to 80%.
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Example: 144 kbit/s, urban area, class 3 UE Max. path loss 145 dB
Load factor = 0% Noise rise = 0 dB Cell radius 1.2 km
Using COST231-Hata
Load factor = 50% Noise rise = 3 dB Cell radius 1 km
Noise Rise (dB)
6 3 50%
75%
100%
Load Factor ( )
Load factor = 75% Noise rise = 6 dB Cell radius 800 m
Figure 4 Load Factor Illustration SC2804/S3/v1.1
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1.4
Mixed Traffic and Load Factor
It can be seen from the load factor equations that Eb/No requirements for a particular service contribute to determining the load factor. The output Eb/No requirement itself is dependent on the service type and the error protection being applied in the channel. For example, a typically allowed figure for a standard voice service would be 5.5 dB, whereas high-rate data is often taken to be much lower, perhaps as low as 1.5 dB or even 1 dB. The reason for this low Eb/No figure is the assumed use of more powerful error protection schemes such as Turbo coding and the relaxed delay constraints permitting retransmission. A lower Eb/No figure means that total cell throughput can be higher for a given load factor. The mobility and the geographical location of the UE may also influence Eb/No requirements because the prevailing channel conditions will have an impact on error characteristics. Another important factor is the level of neighbour-cell interference contribution. This is usually assumed to be higher in macro cells than in micro cells. This is because micro cells tend to be sheltered by street canyons and therefore suffer less from neighbour-cell interference. Again, a lower interference factor means more cell throughput for a given load factor. Finally, it can also be assumed that a higher load factor can be tolerated on micro cells than on macro cells because coverage and ultimate cell range is less of a concern. The UE is likely to be much closer to a micro cell and therefore a larger interference margin can be included in the link budget. Figures 5a and 5b show calculations of cell throughput in kbit/s for different cell types and service types. Calculations have been performed for the macro cell with 50% and 60% load factors, and for the micro cell with 75% and 80% load factors. This illustrates the extremes of variation that are to be expected in usable cell capacity for UMTS cells. These calculations assume that all users in each scenario will be using the same service type. In reality, a cell could be expected to deal with a dynamic mix of service types, in which case the throughput will be some amalgam of the values shown here.
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Service
Bit Rate (kbit/s)
Activity Factor
Eb/No (dB)
Load Factor
N-cell Number Total Cell Interference of Throughput Factor Channels (kbit/s)
Voice
12.2
0.6
5.5
50%
1.3
57
695.4
Low packet data Medium packet data High packet data
64
0.9
2.5
50%
1.3
14
896
144
0.9
1.5
50%
1.3
8
1152
384
0.9
1
50%
1.3
3.7
1420.8
Voice
12.2
0.6
5.5
60%
1.3
68
829.6
64
0.9
2.5
60%
1.3
18
1088
144
0.9
1.5
60%
1.3
10
1440
384
0.9
1
60%
1.3
4.5
1728
Low packet data Medium packet data High packet data
Figure 5a Macro Cell – Mixed Traffic
Service
Bit Rate (kbit/s)
Activity Factor
Eb/No (dB)
Load Factor
N-cell Number Total Cell Interference of Throughput Factor Channels (kbit/s)
Voice
12.2
0.6
5.5
75%
1.1
101
1232.2
Low packet data Medium packet data High packet data
64
0.9
2.5
75%
1.1
27
1664
144
0.9
1.5
75%
1.1
15
2160
384
0.9
1
75%
1.1
6.7
2572.8
Voice
12.2
0.6
5.5
80%
1.1
108
1317.6
64
0.9
2.5
80%
1.1
28
1792
144
0.9
1.5
80%
1.1
16
2304
384
0.9
1
80%
1.1
7.1
2726.4
Low packet data Medium packet data High packet data
Figure 5b Micro Cell – Mixed Traffic SC2804/S3/v1.1
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2
COVERAGE AND CAPACITY OPTIMIZATION ISSUES Coverage and capacity are closely linked in UMTS; nevertheless, it is possible to consider independent optimization strategies for each characteristic. In some cases benefits arising from successful optimization activity may result in improvements to both coverage and capacity, but even here it is possible to weight the effect to influence one or the other more noticeably. 2.1
Coverage Solutions
Coverage is likely to be of prime concern when a network is in the rollout phase. The main limiting factor will be the low transmit powers from a UE, most UEs being class 4 with a maximum output power of 21 dBm (0.125 W). This, coupled with an operating frequency in the region of 2 GHz, means a restricted uplink power budget. Well-established radio techniques and some CDMA-specific techniques can be used to improve coverage. These include: • antenna height • antenna gain/types • antenna alignments • low noise amplifiers • repeaters • soft handover gain 2.1.1
Antenna Solutions
Rollout Node Bs will be predominantly macro cells with antennas mounted relatively high compared to average building height. When such cells are used to maximize coverage they will probably be unbalanced such that the potential downlink radius is significantly greater than the uplink radius. This means that different antenna gains need to be used to balance the link. A common approach is to use omni transmit and sector receive over three sectors. Optimization attention will be focused on uplink antenna types, gains and alignments to maximize coverage and minimize interference. Close attention should be paid to simulation of performance effects caused by antenna installation errors that are within the tolerances set for site build and acceptance. It may also be worth considering higher gain antennas, perhaps with more than three sectors.
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Antenna Coverage Improvements antenna configuration (omni transmit) antenna type antenna gain antenna alignment build tolerances
Weak uplink link budget
Node B
Unbalanced downlink link budget
UE Typically class 4 21 dBm (0.125 W)
Figure 6 Antenna Coverage Solutions SC2804/S3/v1.1
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2.1.2
Low Noise Amplifiers (LNA)
The use of Low Noise Amplifiers (LNA) is a well established technique for boosting uplink power budget performance. These are sometimes referred to as Mast Head Amplifiers (MHA) or Tower Mounted Amplifiers (TMA). They reduce the noise figure at the input to the receiver, which helps to compensate for the low UE transmit power. The reduction in noise floor created by an LNA could also be used to increase capacity because it allows for more noise rise. 2.1.3
Repeaters
Repeaters may be used for coverage improvement in areas that are not likely to present high traffic loads. They should not be used where it is predicted that traffic load will increase significantly over time unless the site can be upgraded to a Node B with ease. If planned with care a repeater may also provide some increase in capacity. 2.1.4
Soft Handover Gain
While in soft handover the UE is benefiting from uplink and downlink spatial diversity in the link. This produces a gain usually referred to as soft handover gain. Soft handovers reduce overall capacity in a network because a call requires multiple channel resources. However, in areas where coverage is of prime concern it may be possible to reduce handover margins to increase the soft handover area.
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Low Noise Amplifiers (LNA) reduce receiver noise floor boost uplink link budget increase capacity
Repeaters low traffic or rural areas in-building coverage cheaper than new Node B could provide some capacity benefits
Weak uplink link budget
Node B
UE Typically class 4 21 dBm (0.125 W)
Figure 7 Low Noise Amplifiers and Repeaters SC2804/S3/v1.1
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2.2
Capacity Solutions
As a network matures the customer base will increase, as will the range of services offered to subscribers. An early and very important function for optimization teams will be to evolve the radio access network from a coverage-oriented design towards a capacity-oriented design. This will involve a mixture of architectural changes and the introduction of new features as they become cost effective. This will include: • use of more frequencies • use of UMTS Time Division Duplex (TDD) mode • in-fill cells • Hierarchical Cell Structures (HCS) • indoor coverage solutions • more sophisticated 2G interworking • antenna configuration changes • antenna orientation/downtilt • adaptive voice channels • secondary scrambling codes • Multi-User Detection (MUD) • transmit diversity 2.2.1
More Spectrum
Most UMTS operators have licences for enough spectrum to operate more than one FDD carrier pair. Typically an operator may be able to implement two or three carrier pairs. These could be used in a variety of ways, but essentially an operator may choose to use then as independent cell layers or to provide more capacity within a cell layer. Overall the highest capacity will probably be achieved through the use of hierarchical cell structures partitioned by frequency. It is important for optimizers to bear in mind that different solutions may suit different locations and an operator can use different strategies in different geographical regions if appropriate. Even where UMTS operators have only one Frequency Division Duplex (FDD) carrier pair there may still be scope for spectrum sharing. This option would increase capacity and reduce infrastructure costs for the operators and is therefore worthy of consideration.
3.15
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Example UMTS Licence
TDD (x1)
5 MHz
FDD (x3 pairs)
5 MHz
5 MHz
Used at rollout on macro cells
Progressively introduced as a micro cell or pico cell layer
5 MHz
Progressively introduced as second carrier on macro cell sites or for a micro cell layer Progressively introduced as a micro cell or pico cell layer
Figure 8 Additional Radio Carriers SC2804/S3/v1.1
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2.2.2
UMTS TDD Mode
Many UMTS operators have licences that include spectrum for TDD mode radio carriers. Typically this will be a single carrier, but TDD mode is a very flexible technology solution. Although it is a UMTS technology the optimizer will need to treat it as a different radio access technology and integrate it as such. Potentially the cell sizes for a TDD mode cell and an FDD mode cell are the same; however, the TDD technology is more suited to non-symmetric data applications. This makes TDD mode a candidate technology for the implementation of pico cells and indoor coverage solutions. It may also be the preferable technology solution for special project cell where, for example, it may be desirable to stream high-quality video.
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Example UMTS Licence
TDD (x1)
5 MHz
FDD (x3 pairs)
5 MHz
5 MHz
Used at rollout on macro cells
Progressively introduced as a micro cell or pico cell layer
5 MHz
Progressively introduced as second carrier on macro cell sites or for a micro cell layer Progressively introduced as a micro cell or pico cell layer
Figure 8 (repeated) Additional Radio Carriers SC2804/S3/v1.1
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2.2.3
In-Fill Cells and HCS
Ultimately, the need for more capacity will always lead to a need for more cells. The first step in this process may be to in-fill new cells between the macro cells of a rollout architecture. This will need considerable attention from the optimization team to target new capacity appropriately and minimize the potential negative impact on existing cells. As a network develops the new cells may be implemented as overlays on existing coverage. In this case parameters and procedures required for the effective operation of HCS will need to be introduced. These should be monitored and tuned by the optimization team.
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Coverage areas of rollout cells reduced with downtilt and pilot power reductions. Optimization needed to ensure new in-fill is beneficial not detrimental to network performance.
Rollout Node B
New in-fill Node B Rollout Node B
Rollout Node B
Figure 9 In-Fill Cells and HCS SC2804/S3/v1.1
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2.2.4
Interworking with 2G
Most operators are overlaying a new UMTS network onto a mature and (usually) well-optimized GSM/General Packet Radio Service (GPRS) 2G network. UMTS offers capabilities above a 2G network, but many of the offered services can be carried adequately on a 2G network; for example, voice or messaging services. Therefore, balancing the traffic load in the most appropriate way between the 2G infrastructure and the 3G infrastructure is an important optimization task. The main mechanism for this will be effective and appropriate settings of triggers for inter-RAT handovers, but it may also impact on admission control.
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Split traffic according to QoS requirements
3G
2G
Optimize handovers and reselections to account for QoS requirements
Figure 10 Interworking with 2G SC2804/S3/v1.1
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2.2.5
Antenna Solutions
Several aspects to antenna optimization can influence capacity in a UMTS network. Firstly, antenna type, orientation and downtilt will need changing as more cells are added in the system. As in-fill cells are added considerable reorientation may be required; typically, a 30° azimuth change is applied in an area where cells are placed in an even hexagonal pattern. In theory this maintains an even geographical area split between original and new cells. However, in more realistic and variable environments the exact orientation changes need to be chosen for best performance. It is generally to be expected that increasing amounts of downtilt will be applied as traffic load and the number of cells increase. In some cases reduction in antenna heights may be considered appropriate. Cells may also be changed from omni to sector transmit, which may require the addition of new antennas or simply the rerouting of feeder runs and the addition of new power amplifiers. One option for capacity increase would be the introduction of cells with more than three sectors. Again, this will require new antenna and feeder runs at the site. The extra equipment required at a site may also mean that more space and mast reinforcement are required. The cost of this will be a factor determining whether this approach is used or not. Finally, there are more advanced antenna types that could be used to increase capacity. These may initially take the form of beam-forming antenna arrays, followed by dynamically adaptive beamforming arrays (sometimes called smart antennas).
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Omni transmit to sector transmit
Sites with more than three sectors
Beam forming antennas
Figure 11 Antenna Solutions for Capacity SC2804/S3/v1.1
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2.2.6
Multi-User Detection (MUD) and Transmit Diversity
Multi-user detection and transmit diversity are two optimal features that could be introduced to gain extra capacity from an existing cell plan. Multi-user detection is a form of noise cancellation. It utilizes knowledge about the nature of the interference from one channel onto another to correct for the distortion caused. It is applicable only to the uplink direction. New software and possibly some new hardware would be required to upgrade a Node B for this capability. Transmit diversity utilizes two transmit antennas mounted so as to provide space diversity. Transmissions are marked such that the UE can identify which antenna a particular copy of the received signal was transmitted from. The result is that the UE can optimally combine multiple copies of the received signal with a significant diversity gain figure. There are several different modes of operation available for transmit diversity, but all would require software upgrades and a significant amount of new hardware adding on the site.
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Multi-User Detection (MUD) uplink channel performance improvement hardware and software changes in Node B significant capacity improvements
Transmit Diversity downlink channel performance improvement significant hardware and software changes in Node B significant capacity improvements
Figure 12 Multi-User Detection and Transmit Diversity SC2804/S3/v1.1
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2.3
Adaptive Voice Channels
The Adaptive Multi-Rate (AMR) voice codec is designed to utilize variable bit rates for achieving an optimal balance between quality and capacity in carried voice traffic. The AMR codec is applicable to both GSM and UMTS. It provides eight voice coding rates set between 4.75 kbit/s and 12.2 kbit/s. In addition, it supports discontinuous transmission with Silence Descriptor (SID) frames at an effective rate of 1.8 kbit/s. The voice coding rates are designed to be adjusted dynamically according to radio conditions. In theory the rate can be changed every 20 ms, but air interface delays and processing time mean that in practice the adjustment rate will be lower than this. The voice-coding rate will be lowered, and error protection overhead increased, as channel conditions worsen. This results in a more robust channel exhibiting more consistent voice quality. Using AMR in this way can provide an overall improvement in perceived voice quality for users and an increase in capacity for the operator. There are several ways in which the benefits of the AMR coder can be applied, and its operation in GSM and UMTS is slightly different. For UMTS the eight bit rates are used to provide a smooth trade off between voice quality and capacity in the network as a whole. Additionally, the 7.4 kbit/s codec mode provides compatibility with legacy networks in North America, and the 6.7 kbit/s codec mode provides compatibility with those in Japan. For GSM, coarse adjustment is provided through the selection of a full-rate or a halfrate channel mode. These two channel modes relate to a gross channel bit rate of either 22.8 kbit/s for the full-rate channel mode, or 11.4 kbit/s for the half-rate channel mode. The half-rate mode for AMR (7.95 kbit/s) provides significantly improved voice quality when compared to the standard GSM half-rate codec. In both the full-rate and half-rate channel modes the codec mode may then be changed dynamically between the defined voice coding rates according to channel conditions. This provides a very flexible tool enabling operators to achieve an effective balance between capacity and quality.
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GSM HR Channel Mode (kbit/s)
Codec Modes
UMTS Operation and GSM FR Channel Mode (kbit/s)
AMR_12.20
12.2 (GSM EFR)
AMR_10.20
10.2
AMR_7.95
7.95
7.95
AMR_7.4
7.4 (IS-136 EFR)
7.4 (IS-136 EFR)
AMR_6.7
6.7 (PDC EFR)
6.7 (PDC EFR)
AMR_5.9
5.9
5.9
AMR_5.15
5.15
5.15
AMR_4.75
4.75
4.75
AMR_SID
1.8
1.8
Figure 13 AMR Codec Modes and Application SC2804/S3/v1.1
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2.3.1
Adaptive Voice Channel Benefits
The AMR voice coder enables the operator to change the characteristics of the voice channel for more capacity (or more coverage) by accepting lower voice coding quality. Figure 14 shows how the net rate of the voice channel, which represents coded voice plus error protection overhead, remains constant. The channel adapts to changing quality by varying the ratio of coded voice and error protection overhead. With more error protection overhead the required Eb/No is reduced and the processing gain is increased. Figure 14 also shows the effect this may have on cell capacity for a macro cell supporting only voice calls.
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Channel quality
Net channel rate
Error protection Coded voice according to selected codec mode
Service
Bit Rate (kbit/s)
Activity Factor
Eb/No (dB)
N-Cell Number Total Cell Load Interference of Throughput Factor Factor Channels (kbit/s)
AMR_12.20
12.2
0.6
5.5
60%
1.3
68
829.6
AMR_7.95
7.95
0.6
5.0
60%
1.3
118
938.1
AMR_5.9
5.9
0.6
4.5
60%
1.3
178
1050.2
Figure 14 Adaptive Voice Channel Benefits SC2804/S3/v1.1
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2.4
Secondary Scrambling Codes
Separation of channels in the downlink direction is facilitated by the OVSF codes. These are designed to be fully orthogonal within their set providing that the rules for allocation of codes from the code tree are followed. These rules are illustrated in Figure 15a. Once a code has been allocated, no code derived from it or from which it is derived to the root of the tree may be used. An efficient code allocation algorithm is required to make best use of the available codes. Potentially most problematic is the allocation of high spreading factor codes. In this case the allocation of one code to a low-bit-rate user removes a substantial portion of the tree. The most efficient allocation strategy is to allocate high spreading factor codes from the same or from related branches if possible. However, even with an efficient code allocation algorithm there may be no available codes even when the cell has not reached its load factor limit. The result is that a cell may become code limited rather than interference limited. Secondary scrambling codes can be used to overcome this problem. There are 15 secondary scrambling codes associated with every primary scrambling code. The allocation of secondary scrambling codes provides additional partitioning between a cell’s downlink channels. They may be used on any of the downlink channels other than the Common Pilot Channel (CPICH) and the Primary Common Control Physical Channel (PCCPCH). This means that capacity will not be limited by the possibility of running out of codes. Additionally the use of multiple code lengths simultaneously in a multipath channel leads to a reduction in orthogonality. This is usually allowed for by an extra interference factor in the load factor calculation. Secondary scrambling codes could be used to partition different channels using different spreading factors and thus reduce intra cell interference, ultimately increasing capacity.
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To the top of the tree
Root
Allocated code Non-allocatable codes
Figure 15a Secondary Scrambling Codes
Primary Scrambling Code
OVSF codes can be reused when covered by a secondary scrambling code Secondary Scrambling Code
Figure 15b Secondary Scrambling Codes SC2804/S3/v1.1
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3
TRAFFIC SCENARIOS 3.1
Introduction
Second-generation systems can be broadly categorized as being either uplink or downlink limited. Once categorized as one or the other they can be treated as such irrespective of traffic load. The determining factors are radio related, and typically most second-generation technologies, particularly those operating at the higher end of UHF, are uplink limited. System balance is usually achieved by adjusting gains and losses in the antenna path. UMTS is also affected by radio factors and an unloaded cell is generally assumed to be uplink limited. This results mainly from the high transmit frequency and the low UE transmit power capability. 3.2
Uplink Limited Systems
The relationship between load and coverage in the uplink direction is fairly straightforward. An increase in load is accompanied by an increase in noise rise. This is accounted for by including an interference margin in the link budget. The result is a radius that reduces as load increases; the familiar cell breathing effect. Figure 16 is a generalized graph showing a typical relationship between the maximum acceptable path loss in decibels and the cell load in kbit/s. It can be seen that in this example the cell is uplink limited when load is below about 650 kbit/s. This is likely to be the case for most cells in the rollout phase of a UMTS network. 3.3
Downlink Limited Systems
In the downlink direction noise rise also increases with traffic load. However, the relationship between noise rise and the maximum acceptable path loss is further complicated by the limitations of the cell’s power amplifier. There will always be a finite amount of power available, which must be divided between all the downlink channels. As the number of channels increases with cell load, so the amount of transmit power available per channel in the downlink decreases. Thus the interference margin and cell transmit power become load-dependent variables in the link budget. The result is that at high cell loads the cell becomes downlink limited. This effect is further accentuated when higher utilization of packet data services becomes more common. Many of these services are typically unbalanced such that they place more load in the downlink than the uplink.
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165
Uplink Downlink
160 155 Maximum Path Loss 150 (dB)
Downlink limited Uplink limited
145 140 135 100
200
300
400
500
600
700
800
900
1000 1100
Cell Load (kbit/s)
Figure 16 Coverage and Capacity Limitations SC2804/S3/v1.1
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4
EVOLVING RADIO ACCESS ARCHITECTURE 4.1
Rollout Architecture
Rollout topology can be expected to be wholly, or at least largely, implemented with single-carrier macro cells. Ideally these should be placed in an even hexagonal pattern with identical radio configurations. In reality, topological and morphological considerations will result in localized variations. Additionally, most operators will need to reuse existing sites and integrate an element of site sharing into the plan to reduce build costs. The result will be an irregular pattern of cells with variations in antenna height, type, and alignment as well as transmit power. Simulations suggest that making small deviations from the regular hexagonal pattern to suit the nature of the planned area has very little effect on system performance. Perhaps not unexpectedly, the regular hexagonal pattern turns out not to be the optimal plan in realistic non-homogeneous areas. However, if these variations do not suit the terrain and are random in nature then they can lead to a deterioration in system performance. There are probably two main reasons for this. Firstly, a lack of accurate terrain and demographic data in the simulation tool. Secondly, the need to reuse existing cell sites that will not be ideally suited to UMTS operation.
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Figure 17 Rollout Architecture SC2804/S3/v1.1
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4.2
Antenna Azimuths and Beamwidth
As the network is expanded and evolved it may be necessary to realign antennas in order to optimize traffic distribution between cells. For example if in-fill cells are added or if hierarchical cells are added without using a second frequency. The diagram shows one of the considerations for azimuth changes. The angle between two adjacent antennas has been reduced. This is likely to have been done in an effort to improve the effectiveness of coverage with planned benefits for link budgets and capacity. The result will be that the overlap area between the two cells is increased. There are two significant consequences of this. Firstly, it will change the soft handover relationship between the two cells. The location of the soft handover area will move and, depending on handover triggers, antenna type and local topology it may either increase of decrease in size. Inevitable these changes will have an impact of total load carried by these two cells. Secondly, the isolation between the two antennas will be reduced. The extent by which it is reduced would depend on antenna type and their relative mounting positions. The reduction in isolation will increase mutual interference in both the uplink and downlink directions; again reducing capacity. Each case must be considered independently and, if possible, simulations carried out to identify potential problems. Any simulation should allow for build tolerances. Using antennas with a narrower horizontal beamwidth might be considered if problems are suggested by the simulations.
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e.g. 30° Isolation reduced
Soft handover area will change in location and size
Figure 18 Changing Antenna Azimuths SC2804/S3/v1.1
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4.3
More Sectors or More Cells?
When attempting to find more capacity from an existing infrastructure it is always best to look for the most economic solution. Building sites with more than three cells in GSM is rare because of high costs for a relatively low return. However, the added interference tolerance gained by using more cells on a site in UMTS translates directly to increased capacity. This makes such sites potentially more cost effective. In some locations adding more cells to a site may be preferable and cheaper than building new in-fill or hierarchical cells to deal with increased traffic load. However, this will not always be the case. It is important for the optimizer to consider space and facilities at a site as well as aesthetics in some sensitive locations. Micro cells and pico cells have much less visual impact than the large tower required to support a six-cell site. Possible evolution should also be considered. The more common application of advanced optimal features such as multi-user detection or beamforming antennas may be a better longer-term evolutionary path for a site. Micro cells and pico cells may need to be built at some future time so there may be some benefits in early site acquisition even if this is not cost effective in the short term. Finally, six-cell sites require refitting of much narrower aperture antennas, but even with these in place, parameter setting for cell reselection and handover may be more difficult.
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Multi-Cell Sites
More Cells
Cost Visual Impact Targeted capacity Advanced features Evolution to HCS Long-term solution Parameter setting Service differentiation
Figure 19 More Sectors or More Cells? SC2804/S3/v1.1
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4.4
Use of Repeaters
4.4.1
Introduction
Repeaters are bidirectional amplifiers designed to be used in locations where coverage from a cell is poor and requires enhancement. They are commonly used to extend more reliable coverage indoors from a donor cell or to fill in coverage holes that exist because of terrain. In the early rollout phase of a UMTS network they may be used to increase the general coverage rapidly and at low cost in rural areas, residential areas or along roads and railways. 4.4.2
Donor Antenna Alignment
Repeaters can be made channel selective, but Node Bs in the same HCS layer are separated only by code. This means that great care must be taken when considering the position, type and alignment of the donor antenna for a repeater. Any other Node B signals arriving at the repeater will also be amplified and reradiated in the repeater area. The result of this could be that UEs in the repeater area are in continuous soft handover with consequential loss of capacity in the system. It is recommended that the donor antenna should be positioned such that there is at least an 8 to 10 dB margin between the donor cell signal and other cells in the repeater area. This must then be reflected in handover thresholds.
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Node B
Risk of continuous soft handover
Node B
Plan for 8 to 10 dB margin Intended donor Node B
Repeater
UE in repeater area
Figure 20 Repeaters and Antenna Alignment SC2804/S3/v1.1
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4.4.3
Repeaters to Add Capacity
Where a repeater is used to add new coverage to the system in the sense that communication from the repeater area would be impossible without the repeater, it will not add capacity to the system. However, if a repeater is used to improve the reliability in an area where communication is possible to some extent without the repeater then there may be some capacity gain. The repeater will have the effect of improving the radio link in both uplink and downlink directions. The result is that the closed loop power control process will reduce UE and Node B transmit power, thus reducing interference contribution. This in turn translates to an increase in net system capacity. A good example of where this effect could be utilized to some advantage is illustrated in Figure 21. The building shown would be served directly by an external macro cell at network rollout. The building penetration loss means that a UE inside the building would require a disproportionately large power weighting in the downlink direction. This reduces the capacity of the cell, and also neighbour cells because ultimately more transmit power is radiated. Once the repeater is installed as shown, the reduction in overall downlink path loss means a lower proportion of downlink power is allocated to UEs inside the building. This means that more calls can be established on the cell for a given transmitter power amplifier capability. A similar argument can be made for the uplink direction.
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Disproportionately large power weighting to compensate for building loss reduces capacity Serving Node B
Reduced power weighting due to repeater gain increases capacity Donor Node B
Figure 21 Repeaters to Add Capacity SC2804/S3/v1.1
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4.4.4
Antenna Isolation and Gain Setting
The coupling between the two antennas connected to the repeater is referred to as ‘isolation in decibels’. Normally the donor antenna will be highly directional. The antennas in the repeater area may be either omnidirectional or directional. Any signal coupled from the output of the repeater back to the input via the antennas will be reamplified. There is a danger that this could lead to positive feedback. The resulting transmitted noise would have severe implications for capacity in the system as a whole. It is critical therefore to ensure that the repeater gain is kept below the level of isolation to prevent self-oscillation. It is recommended that in the downlink direction repeater gain is kept at least 15 dB below the level of isolation. This margin may be reduced by up to 5 dB in the uplink direction. Typically a repeater gain can be set independently in the uplink and downlink directions up to a maximum of about 90 dB. Isolation between the antennas should be determined once they are fixed in their final locations. Driving the repeater antenna from a suitably calibrated test transmitter and measuring the power level received at the donor antenna can achieve this. For in-building solutions, the physical structure of the building is interposed between the antennas. This should lead to a better degree of isolation between the antennas than for outdoor applications.
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Isolation (dB)
Repeater Gain up to c. 90 dB Donor antenna
Recommended maximum gain 15 dB less than isolation
Repeater antenna
Figure 22 Antenna Isolation and Gain Setting SC2804/S3/v1.1
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4.4.5
Node B Desensitization
Consider the repeater shown in Figure 23. Its uplink gain is set to 80 dB and it has a noise figure of 6 dB. Assuming thermal noise of –174 dBm/Hz the noise at the input to a channel selective repeater when allowing a channel bandwidth of 4.8 MHz will be –107.2 dBm. This background input level is amplified by 80 dBm and the noise figure must also be added. –107.2 + 80 + 6 = –21.2 dBm Assume an input sensitivity level at the input to the donor cell’s receiver of –102 dBm. If the total coupling loss between the repeater output and the donor cell’s input is less than 80.8 dBm the amplified noise being transmitted back to the donor cell will be above the threshold of –102 dBm. In these circumstances the repeater is increasing the noise rise and therefore reducing the cell’s capacity. The coupling loss includes all antenna gains, the path loss and other forms of gain or loss between the donor cell input and the repeater output. A coupling loss in the order of 80.8 dB could occur with a spacing between repeater and donor cell of about 400 m, although exact figures will depend on the antennas used and the propagation path. If a donor cell and repeater are closely located, then it is worth calculating the coupling loss and checking in relation to the gain of the repeater whether desensitization seems likely. A reduction of repeater gain may be necessary to correct the problem.
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Gain 80 dB Repeater
NF = 6 dB Coupling loss
6 dB NF
Donor Node B
–21.2 dBm
80 dB gain
Coupling loss = 80.8 dB
–102 dBm
–107.2 dBm
Figure 23 Node B Desensitization SC2804/S3/v1.1
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4.4.6
Time Delay in Repeaters
Another consequence of using a repeater is the propagation delay through the repeater. The specific value of delay caused depends on the particular device in use; the value will be available from the vendor as part of the device specification sheet. Typical delays for a repeater will be in the range 5 to 8 µs. A typically delay of 6 µs translates to a distance travelled of about 1.8 km for a normally propagating radio signal. This is not a problem for normal UMTS operation, but it could cause difficulties when trying to estimate range or position for the UE. For example this would mean that a round trip time measurement used to estimate range from a Node B would show an error of approximately +1.8 km. Also, if using the observed time difference of arrival method for position determination, the error would be at least 900 m and could be considerably higher depending on the relative positions of the Node Bs and the UE. It may also cause a problem if the receiver can see both direct and repeated versions of the transmitted signal. The delay in the repeater could mean a delay spread greater than the search window for the rake receiver. Thus some channel paths would be treated as interference.
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e.g. 6 µs delay each way
Node B
Repeater
UE in repeater area
Round trip time measurement indicates UE is 1.8 km further away than it really is. May cause problems because of limited search window size in Node B and UE.
Figure 24 Time Delay in Repeaters SC2804/S3/v1.1
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4.5
Basic Considerations for Indoor Coverage
The provision of reliable coverage and sufficient capacity for the indoor environment presents specific challenges for the UMTS optimizer. Building loss has a significant impact on link budgets such that more power must be used by indoor UEs and more power must be allocated from the Node B. this reduces system capacity. Furthermore, in buildings where there is coverage from more than one outdoor Node B, UEs are more likely to be engaged in soft handover, thus further reducing system capacity. In addition to this, the indoor environment is much more likely to generate higher data rate traffic. This puts further power and capacity demands on UEs and outdoor Node Bs. Use of outdoor to indoor repeaters is a cost effective solution that for UMTS can improve both coverage and capacity. However, capacity gains with this type of solution are limited. The potentially very large traffic density for some in-building scenarios and likely different traffic profiles mean that dedicated indoor Node Bs will in many cases be a longer term solution. There are several options for providing coverage from a repeater or an indoor cell. The primary aim will be to provide sufficient and even coverage across the whole building area. This may be achieved with a distributed antenna system or with a radiating cable system.
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Drivers for Indoor Coverage indoor UEs need more power and reduce capacity buildings represent very high traffic densities poor coverage in buildings may result in more soft handovers in-building areas may generate a different traffic profile expected in mature networks could use differentiated tariffs could be a way to compete in different markets
Indoor Coverage Options repeater dedicated indoor Node B copper distributed antenna system fibre distributed antenna system radiating cable system
Figure 25 Basic Considerations for Indoor Coverage SC2804/S3/v1.1
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EXERCISE 1 – URBAN CAPACITY AND COVERAGE The urban area shown in the diagram measures approximately 2 km in each direction. The rollout plan assumed a load factor of 65% on each of the cells shown. Each cell has three sets of directional antennas with a maximum gain of 16 dBi. 1
Complete the link budgets using the values shown in this and the next diagram and verify (approximately) that the planned coverage was valid for class 4 UEs requiring up to 384 kbit/s in the downlink and 128 kbit/s in the uplink. Note that the maximum allowed channel code power on the cells is set to -20 dB relative to the maximum power.
You can convert path loss (Lp) to range in kilometres (d) using COST231-Hata with the following relationship: d = antilog Lp – 144.95 37.2 2
Are the cells likely to be uplink or downlink limited?
Customers are starting to complain that they are not able to establish calls in this area. The call types and locations vary but the times of day coincide with busiest times for these cells. 3
Consider what you think might be happening.
4
Suggest some information you may seek from network statistics to verify your suspicion.
5
Suggest two things you might consider as a solution.
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approximately 2 km
Antenna gain 16 dBi Diversity gain 3 dB
Antenna gain 0 dBi Duplex filter (1dB)
45 W TX (46.5 dBm)
UE Class 4 0.125 W (21 dBm)
Feeder loss 3 dB
RX
Figure 26 Exercise 1 – Urban Capacity SC2804/S3/v1.1
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Uplink
Downlink
UE (TX)
Node B (TX)
Max (TX) power
Max (TX) power
Antenna gain
Feeder/connector losses
EIRP Node B (RX) Receiver noise power in channel
Duplexer loss –102.2 dB
Antenna gain EIRP
Interference margin
UE (RX)
Processing gain Required Eb/No
2.5 dB
Receiver noise power in channel
Antenna gain
Interference margin
Feeder/connector losses
Processing gain
Duplexer loss
Required Eb/No
Fade margins
10 dB
–102.2 dB
1.5 dB
Antenna gain
Diversity gain
Fade margins
Minimum required signal level at antenna
Minimum required signal level at antenna
Maximum acceptable path loss
Maximum acceptable path loss
10 dB
Comments and Conclusions
Figure 27 Workspace and Proposed Solutions SC2804/S3/v1.1
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6
LOCATION SERVICES (LCS) 6.1
Introduction
The ability to provide location information for a UE is an important aspect of the UMTS system. Location-based services are a potentially significant revenue generator for an operator, but additionally the ability to record information regarding a UE’s position may be very helpful for the optimizer. Location Services (LCS) and information used by optimizers will only be effective if the type and accuracy of positioning information is appropriate to the function intended. It may be part of an optimizer’s role to consider the different positioning techniques used and the accuracy of the positioning information. 6.1.1
LCS Clients
There are four categories of LCS Client. These are: • Value Added Services LCS Clients • PLMN Operator LCS Clients • Emergency Services LCS Clients • Lawful Intercept LCS Clients Value Added Services LCS Clients use LCS to support VAS, while a PLMN operator may use it to improve operations and maintenance functions or supplementary services. By employing LCS the emergency services can assist subscribers who have made emergency calls. Lawful Intercept LCS Clients may perform services that are required or sanctioned by law.
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Value Added Services Clients
PLMN Clients
list of restaurants places of interest navigation application
enhance network operations location assisted handover traffic engineering
Police FBI
E.911 999, 112, 911 weather warnings
Lawful Intercept Clients
Emergency Services Clients
Figure 28 LCS Clients SC2804/S3/v1.1
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6.2
Quality of Service
One aspect of QoS for the geographic location of a UE is accuracy: horizontal accuracy and vertical accuracy. If appropriate, speed and direction of travel can also be taken into account. It may be possible to provide both velocity and geographic location, or they can be supplied individually. Important QoS issues are response time and accuracy. Accuracy of horizontal and vertical data may be considered independently, even where both are requested by a particular location service. 6.2.1
Horizontal Accuracy
Not all services require the same level of accuracy. For example, the provision of weather reports or traffic information does not need to be pinpointed within a tight geographical area; it could be an area covering several kilometres. However, tracking information, such as tracking of delivery vehicles or personnel, may need tighter coordinates. Subscribers requiring very localized information, perhaps in a town or city, may need location information that has been calculated down to a few metres or tens of metres. The emergency services locating an incident require the most precise information that can be provided. A range of values is presented to estimate a UE’s position, even for a stationary UE. This is to ensure that the information provided is the best possible within a required response time. Figure 29 illustrates a range of location services and their estimated accuracy requirements. 6.2.2
Vertical Accuracy
It may be possible to provide vertical location information in terms of the actual height/depth of the target UE, or the estimated height/depth relative to its position at ground level. Vertical accuracy may range from approximately ten metres to hundreds of metres.
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Service Type Stock prices; sports reports Services for a particular country or PLMN Weather reports Local news; traffic information People/animal tracking; emergency services; manpower planning Emergency calls; network-based positioning SOS; local adverts; ‘where is my nearest?’ Emergency calls; asset location Emergency calls; route guidance
Location PLMN/ Independent Country
Regional District 500 m (200 km) (up to 1 km) to 1 km
100 m 75–125 m acc. 67%
50 m
10–50 m
Figure 29 Horizontal Accuracy SC2804/S3/v1.1
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6.3
Factors Affecting Accuracy of Location Information
While information needs to be as accurate as possible within QoS requirements, several factors may impact upon the provision of a location service. Some of these factors are shown in Figure 30 and are described below. The optimization process can influence most of these factors. • positioning technique • geography • signal attenuation • multipath propagation and repeaters • network coverage patterns There are three defined positioning techniques for UMTS, cell ID based, Observed Time Difference of Arrival (OTDOA) and network-assisted GPS. The type used may depend on the QoS requirements and on UE capability. Geography may affect LCS in a number of different ways. For example, the number and relative position of the base stations; the number of visible satellites, and height variation of mobiles and base stations. When a signal is weakened due to attenuation, it becomes more difficult to make reliable measurements. This is applicable to all the positioning techniques listed above. Multipath propagation alters the path length of the signal relative to the geometric length, giving the impression that the mobile is further away from the base station than in fact it is. Perhaps even more significant will be the distortion in measured propagation time caused by repeaters. These things are most applicable to the cell ID and OTDOA positioning techniques. The size of a cell may affect LCS, depending on the type of positioning mechanism in use. If a mobile is out of coverage, no positioning information will be available for it.
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Factors Affecting Accuracy
Network coverage patterns
Multipath propagation and repeaters
Signal attenuation
Geography
UMTS Positioning Techniques: cell ID based Observed Time Difference of Arrival (OTDOA) network-assisted GPS
Figure 30 Positioning Techniques and Accuracy SC2804/S3/v1.1
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6.4
Response Time
Response time is an important QoS issue. Different clients and location services will require different response times; the issue is particularly important, for example, where a rapid response time is required, as in the case of an urgent positioning request. The response time may be a negotiable QoS parameter. There are three QoS parameters in respect of response times: • no delay • low delay • delay tolerant When a ‘no delay’ response time is specified, the response will be the Last Known Location or Initial Location (if the system holds one for the Target UE). If not, the system will return a failure indication and, optionally, initiate procedures to acquire an estimate should it be required. A ‘low delay’ response time will place speed of response above accuracy of information, although it is still with the aim of providing the greatest possible degree of accuracy relative to the accuracy requirement. However, any attempt at accuracy should not incur additional delay. A ‘delay tolerant’ response time places accuracy above speed. If necessary, a response will be delayed while the accuracy requirement is fulfilled. A timestamp will always be provided in respect of location estimates, detailing the time at which the estimate was obtained. The network may allocate priority levels to different location services. Requests that carry a higher priority level will be processed more quickly than lower-priority ones, and with a greater degree of accuracy. Requests from the emergency services will take highest priority.
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a) No Delay LCS Client
b) Low Delay
LCS Server
LCS Client
c) Delay Tolerant LCS Client
Figure 31 Response Times for LCS SC2804/S3/v1.1
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6.5
Cell ID Based Positioning Mechanism
The positioning mechanism used for a particular request is dependent on UE/UTRAN capabilities and on the QoS requirement for the request. For many types of service the accuracy provided by simply identifying the cell in which the UE is currently located will be sufficient. In addition to the basic identification of the cell, this position mechanism also allows for other information to be included in the position calculation. For FDD mode the Round Trip Time (RTT) may be requested; in TDD mode the receive timing deviation may be requested. In both cases these can be measured in terms of chip periods. This would enable a range estimation to be made with step increments of about 40 m. 6.5.1
Calculation of Geographical Coordinates
If the only information provided for position estimation is the cell ID, then the accuracy, which in turn will depend on cell size, will depend on the type and location of the cell. In rural areas cells are likely to have radii greater than several kilometres, but in urban areas cell radii could be measured in hundreds or even tens of metres. For large and small cells alike the UE’s position is only identified within the coverage area of the cell. Where geographical coordinates are required as the response to the location request, a default position within the cell must be defined. It would make sense for this to be the geographical centre of the cell. However, knowledge of traffic distribution within the cell (for example if it was covering a major road or included a busy shopping street) could be taken into account when defining a default location. If distance information is included, then defined default position may take the form of a line across the cell. 6.5.2
UE State
At the time of the location request the UE’s associated cell ID may or may not already be known, depending on the current RRC state of the UE. If the UE has a current RRC connection a cell ID may already be known for the UE. However, if the UE is in the URA_PCH state or if it has no RRC connection and is in idle mode, a cell ID will not be known. For UEs in the URA_PCH state a transition to the CELL_FACH state can be forced by paging initiated by the SRNC. If a UE is in idle mode, paging will need to be initiated from the core network.
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RTT measurements
Node B
Default location for coordinates
X
UE
Error margin
Figure 32 Cell ID Based Positioning SC2804/S3/v1.1
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6.5.3
Cell ID During Soft Handover
When the UE is in soft handover it will be associated with more than one cell, so a strategy is required to determine which cell ID should be used for indicating the UE’s position. Several methods are suggested, including: • quality measurements of cells in the soft handover • the cell on which the call was set up • the cell provided by the closest Node B to the UE • the cell most recently added to the soft handover In the rollout phase it is likely that one policy will be adopted for all location requests. However, as the network matures the policy could be refined through optimization. In the example shown in Figure 33 the UE is in a three-way soft handover. It is closest to Node B 1, but the call was first set up on Node B 3 and Node B 2 was most recently added to the soft handover. Which of these may be most appropriate to use as a cell ID for location could depend on why the location is being requested. Node B 1 may best represent the UE’s physical location, but Node B 2 may give a better indication of where the UE is going. However, the call was set-up on Node B 3, and the location request may be associated with a service related to call establishment.
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Node B 2
Node B 1 UE
Closest – Node B 1 Call Set-up – Node B 3 Newest Added – Node B 2 Node B 3
Figure 33 Cell ID During Soft Handover SC2804/S3/v1.1
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6.6
Observed Time Difference Of Arrival (OTDOA)
6.6.1
Hyperbolic Position Calculation
This mechanism involves the UE taking measurements of the OTDOA between the downlink transmissions from pairs of Node Bs. As shown in Figure 34, a constant measured time difference between the downlink signals from Node B 1 and Node B 2 describes a line. The line takes the form of a hyperbolic curve and is known as a Line Of Position (LOP). In general, to estimate the position of a UE in a two-dimensional plane, measurements are required from at least two pairs of Node Bs, i.e. a minimum of three Node Bs are involved. This provides two intersection curves; this is known as trilateration. However, it is possible for two curves to have two points of intersection. In such cases it is necessary to add measurements from a third pair of Node Bs in order to give an unambiguous position. A software function called the Position Calculation Function (PCF) translates measurements into position coordinates. In UE-assisted mode the PCF is resident in the Serving RNC (SRNC), with the UE only returning measurement results. In UEbased mode the PCF is resident in the UE. The system sends assistance data to the UE, which then performs both the measurements and the calculation in order to return the position coordinates to the SRNC. It is possible to use the OTDOA mechanism to derive a three-dimensional position for the UE. To do this it is necessary to consider a plane of constant difference rather than a line of constant difference. This plane will be hyperboloid in shape. Two hyperboloid planes will not provide an unambiguous position since their intersection will be elliptical. If three hyperboloids are identified then their two elliptical intersections may provide a unique point. Ideally, however, four hyperboloid planes would be used.
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Example UE position described by two Lines Of Position (LOP) Node B 3
Node B 2
Node B 1
LOPs representing equal distance: Node B 1 to Node B 2 Node B 1 to Node B 3
Figure 34 OTDOA Mechanism SC2804/S3/v1.1
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6.6.2
Position Accuracy for OTDOA
There are many factors that will influence the accuracy of this positioning mechanism, including: • measurement resolution • measurement accuracy in the UE • radio channel and propagation conditions • accuracy of the Node B’s known position • relative position of the Node Bs The measurement resolution for time difference is the chip period of 260 ns. This equates to approximately 40 m difference between the LOP curves for a given pair of Node Bs. As shown in Figure 35, noise, propagation conditions, measurement errors and measurement quantization will result in a Probability Distribution Function (PDF) surrounding the assumed exact position of the LOP. If the bounds of the PDF are taken to be σ then it represents 68% confidence. In Figure 35 this has been done for LOP1 and LOP2. At their intersection the overlap of the two PDFs and the probability of the UE’s position being contained within it is the product of the two PDFs, i.e. 46.6%. It is possible to construct an ellipse inside this area with axes ‘x’ and ‘y’, which gives a good indication of the effects of measurement error. To maintain 68% confidence it is necessary to construct a circle that has a radius equal to the square root of x2 + y2. It is then apparent that the size of this circle is a function of the angle of intersection between the two LOPs. Thus the closer their intersection is to a right angle, the more confidence there can be in the UE’s position. This is an important factor when deciding which Node Bs are used for measurements. Ideally, it should also be a consideration when selecting Node B sites, but in practice there are other, more inflexible requirements that drive site selection. Therefore, to increase the level of accuracy, it is possible to install a network node called a Location Measurement Unit (LMU). These produce signals the UEs can use for measurements and can be placed with regard only to improving measurement accuracy.
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68% confidence
LOP 2
x
LOP 1
y
r PDF for LOP 1 PDF for LOP 2
r = x 2 + y2
Figure 35 Position Accuracy for OTDOA SC2804/S3/v1.1
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It can also be seen that the space between the adjacent LOP curves increases with increasing distance from the centre of each curve. This is known as the Lane Expansion Factor and is another reason why the relative positions of the Node Bs and LMUs is important. The degradation in accuracy due to the relative position of measured transmitters is sometimes called ‘geometric dilution of position’. Radio propagation conditions may have a significant effect on positioning accuracy. Reflections and diffractions will increase the path length relative to the geometric distance between the UE and the Node B. The degree to which this affects accuracy is a factor of the cell’s location. In order to translate time differences into a position, it is necessary to have accurate positional information for the Node Bs. This means that the location of the transmitters must be accurately surveyed. It is worth noting that the position must be that of the electrical centre of the transmitting antenna and not the position of the Node B. If an antenna array is being used for beam steering then this point may change and, depending on the accuracy required, may need to be accounted for in the position calculation.
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68% confidence
LOP 2
x
LOP 1
y
r PDF for LOP 1 PDF for LOP 2
r = x 2 + y2
Figure 35 (repeated) Position Accuracy for OTDOA SC2804/S3/v1.1
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6.6.3
Real Time Difference (RTD)
In order to perform the position calculation, account must be taken of the Real Time Difference (RTD) between downlink transmissions. UMTS TDD systems are usually fully synchronized, so RTD will be a constant value, which can be entered into a database. However, UMTS FDD systems are non-synchronized. The non-synchronization between FDD Node Bs means that the RTD between Node Bs will slowly drift. For example, it would be possible for the RTD between two Node Bs operating within specified tolerance to drift by one chip period in about 2.5 hours. In a non-synchronous system it is the function of the LMUs to measure and update values of RTD between Node Bs. The updated values are then passed to the SRNC.
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Node B 1
Node B 2 t
t RTD
Ref
Ref
UE
OTD t
GTD Geometric Real Time Difference
RTD
– Local time at Node B 1 – local time at Node B 2.
Observed Time Difference – The timing difference between Node B 1 and 2 as measured at the UE. Geometric Time Difference – The difference between the reception of signals from two stations due to geometry.
Figure 36 Real Time Difference (RTD) SC2804/S3/v1.1
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6.6.4
Use of Idle Periods
In order to take measurements of observed time difference a UE must be able to hear neighbour cells. In a CDMA-based system this is very difficult for a UE near a Node B because of traffic and signalling transmissions to other UEs in the cell. This is known as the ‘hearability problem’. This is solved by a mode of operation known as Idle Period Downlink (IPDL). In this mode of operation the Node B periodically ceases downlink transmission on all channels. This provides UEs with a silence period during which they can take reliable measurements of neighbour cell timing. The Node B informs UEs of idle periods in higher-layer signalling. The UMTS specifications1 contain a number of parameters that are used to define how idle periods will be operated in a cell. It is possible that these parameters could be subject to optimization activity.
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Weak signal
Node B 2
Strong signal
Node B 1
UE
Idle
Idle
Serving Cell Neighbour Cell t
Figure 37 Use of Idle Periods SC2804/S3/v1.1
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6.7
Network-Assisted Global Positioning System (GPS)
The GPS is a constellation of low earth orbiting satellites operated by the US Department of Defence. It is very widely used for providing timing and positioning information in varied applications. GPS satellites transmit synchronized CDMA signals, timing and constellation position information. This enables a terminal to calculate a three-dimensional position by measuring the observed code phase shift for several (ideally at least four) satellites. The reduction of cost and size of GPS reception equipment makes it feasible for it to be incorporated into a UE. Additionally the selective availability feature, which limited civilian access to an accuracy of 100 m, was turned off in May 2000. This means that in the right conditions this method can offer a high degree of vertical and horizontal accuracy. This may be operated in either UE-assisted or UE-based modes. In the UE-based mode the UE contains a full implementation of the GPS receiver so that it can perform both the measurements and position calculation internally. In the UEassisted mode the UE can contain a simpler, limited-function GPS receiver so that it is able to carry out timing measurements only. The measurement results are then returned to the SRNC, where the position calculation is carried out. 6.7.1
Network Assistance
There are some disadvantages with using GPS. These include unreliability in weaksignal cases (in-building), long acquisition time and very high power consumption, particularly while a fix is being taken. For its application in UMTS, GPS assistance data is provided for the UE in order to alleviate some of these problems. This assistance data includes information about satellite visibility, timing and position. The aim is to improve performance in terms of position calculation accuracy, reduced acquisition time, lower power consumption and improved performance in low signal strength conditions.
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GPS GPS GPS
Assistance Data SRNC UE
Node B
Figure 38 Network Assisted GPS SC2804/S3/v1.1
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7
PROPAGATION MODELLING There are many widely accepted propagation models that have been used in the planning and optimization of GSM and other 2G systems. However, UMTS introduces a need for new levels of accuracy in propagation prediction. This is because the application of CDMA within UMTS makes it very sensitive to small changes. In particular there is a high coupling between channels and cells such that the behaviour of one Node B or even one UE can have wide-reaching effects on large geographical areas. Additionally, there are the large number of parameters required to control system access, power control and handover. The mutual interaction between channels and cells means that it is important for the optimizer to be confident that predictions used for simulating optimization changes are an accurate representation of real system conditions. There are two main categories of propagation model, empirical models and deterministic models. 7.1
Empirical Models
Empirical models are based on a power law modified to align with best-fit curves derived from real-word measurements. Perhaps the best known of these is the Okumura–Hata model. The COST231-Hata model is based on measurements taken in several modern European cities and is a development of the Okumura–Hata model. It is widely used and generally considered to be suitable for planning UMTS macro cells. The urban variant of COST231-Hata is shown in Figure 39, modifications for suburban, quasi-open and open areas are also available. It is very important to tune an empirical model to suit the specific location in which it is to be used. Ideally this should be done for every cell. In practice this would be very costly and may be impossible during the initial planning and rollout phase for a new UMTS network. Nevertheless, great accuracy is required for effective UMTS optimization.
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Signal level
Variables frequency antenna height region type
x x x
x x x x x x x
x x
x x x
x x
x x
xx
x
x
x
x
Distance
Best-fit curve
COST231-Hata Lp (urban) = 46.3 + 33.9Logf – 13.82Loghb – a(hm) + (44.9 – 6.55Loghb)Logd + Cm Where hb and hm are in meters, d is in kilometres and f is in MHz. and a(hm) = 3.2(Log(11.75hm))2 – 4.97 for a large city or a(hm) = (1.1Logf – 0.7)hm – (1.56Logf – 0.8) for a small to medium city and Cm is 3 dB for metropolitan centres and 0 dB for medium sized cities or suburban areas.
Figure 39 Empirical Models: COST231-Hata SC2804/S3/v1.1
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7.1.1
Accuracy for Simulations
The potential for making system performance worse with a change that has not been accurately simulated is significant in UMTS, and would be far more likely to occur without accurate propagation prediction. Therefore, accurate radio measurements and tuning of the radio model should ideally be performed on established cells before optimization activity begins. This will mean that more accurate simulations can be performed to check the likely impact of proposed optimization solutions. A number of comparisons have been made between the performance of empirical models and that of deterministic models. In general, they work well in open areas but with degraded performance in urban areas. Even when well tuned, the effects of street canyons and building loss means they can exhibit considerable localized errors. These errors are tolerable in a TDMA-based system such as GSM, but probably not in a CDMA-based system such as UMTS. This suggests that more accurate modelling methods should be used for UMTS, at least for optimization purposes.
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Signal level
Variables frequency antenna height region type
x x x
x x x x x x x
x x
x x x
x x
x x
xx
x
x
x
x
Distance
Best-fit curve
COST231-Hata Lp (urban) = 46.3 + 33.9Logf – 13.82Loghb – a(hm) + (44.9 – 6.55Loghb)Logd + Cm Where hb and hm are in meters, d is in kilometres and f is in MHz. and a(hm) = 3.2(Log(11.75hm))2 – 4.97 for a large city or a(hm) = (1.1Logf – 0.7)hm – (1.56Logf – 0.8) for a small to medium city and Cm is 3 dB for metropolitan centres and 0 dB for medium sized cities or suburban areas.
Figure 39 (repeated) Empirical Models: COST231-Hata SC2804/S3/v1.1
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7.2
Deterministic Models
These are physical models based on knowledge of wave theory and on detailed knowledge of the morphological and electrical characteristics of the local environment. The most widely used deterministic technique is ray tracing. Ray tracing models calculate specific reflections and diffractions for rays launched into the modelled environment. The aim is reproduce as closely as possible real-world propagation. The most accurate prediction comes with three-dimensional environmental data, but two-dimensional predictions can also be effective in some environments. For ray tracing to be effective it is necessary to have accurate data about the environment to be modelled. This kind of data is now more widely available, which makes ray tracing more viable. Another limiting factor for ray tracing in the past has been the lack of sufficient processing power for it to be performed on a large scale. In recent years this too has become a much less significant problem. A number of trials have shown that ray tracing is significantly more effective for predicting signal level in urban and in indoor areas than empirical techniques. This makes it a much more effective tool for the optimizer.
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Node B
Figure 40 Deterministic Models: Ray Tracing SC2804/S3/v1.1
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7.3
Comparing Models and Their Effects
It is possible to compare real measurements with predictions based on empirical and deterministic models in the most effective way to assess their accuracy. Studies in which this has been done suggest that the impact of buildings (canyon effects and penetration loss) are not well modelled by empirical methods. In general, this means that empirical models tend to underestimate signal level in streets at the edge of a cell’s predicted coverage area. The consequence of this is that they underestimate the overlap for adjacent cells. Dependent on street layout this error can be extreme. This concept is illustrated in Figure 41. The likely result of this error is that the real network will show a higher level of interference and greater occurrence of soft handover than the simulations suggest. Thus the real network will have less capacity than the simulated network. It follows that the pessimistic coverage estimates of empirical models may result in a cell plan containing more cells than necessary. This raises the probability that the conclusion of an optimization study may be to suggest the removal of cells in order to reduce interference and thus increase capacity. The need to do this would only become evident with very accurate coverage predictions. Indeed, simulations performed using inaccurate coverage predictions could lead to optimization changes that degrade system performance rather than improve it.
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Typical empirical model results Small predicted overlap area
Typical deterministic model results Large predicted overlap area
Figure 41 Comparing Models SC2804/S3/v1.1
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SECTION 4
RAN CONFIGURATIONS AND DIMENSIONING
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SECTION CONTENTS 1
UMTS Channels 1.1 Access Stratum (AS) 1.2 Non-Access Stratum (NAS) 1.3 The AS on the Air Interface 1.4 Logical Channels 1.5 Transport Channels 1.6 Downlink Physical Channels 1.7 Uplink Physical Channels 1.8 Channel Mapping Options
4.1 4.1 4.1 4.3 4.5 4.7 4.11 4.15 4.17
2
Cell Configuration 2.1 Example Downlink Channels 2.2 Example Uplink Channels 2.3 Configuration Options 2.4 Using More than One Frequency
4.19 4.19 4.19 4.21 4.23
3
Cell Transmit Power 3.1 Downlink Power 3.2 Downlink Power Weightings 3.3 Varying the CPICH Weighting 3.4 Utilizing Soft Capacity and Dynamic CPICH Power 3.5 Pilot Pollution
4.25 4.25 4.27 4.29 4.31 4.33
4
Antenna Configurations 4.1 Use of Downtilt 4.2 Calculations for Beamtilt 4.3 Practical Antenna Types and Tilt Effects 4.4 Choice of Antenna
4.35 4.35 4.37 4.39 4.53
5
Radio Performance 5.1 Minimum Coupling Loss 5.2 Adjacent Channel Leakage Ratio (ACLR) 5.3 Radio Carrier Spacing 5.4 Adjacent Channel Interference (ACI) 5.5 Reducing ACI
4.55 4.55 4.57 4.59 4.61 4.63
6
Interaction and Interference with GSM 6.1 Transmitter Noise and Spurious Emissions 6.2 Receiver Blocking
4.65 4.65 4.69
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SECTION OBJECTIVES At the end of this section you will be able to: • • • • • • • • • •
list the logical, transport and physical channels applicable to UMTS FDD mode describe FDD mode channel mapping options, channel characteristics and traffic applicability describe typical UMTS cell configurations for a range of traffic scenarios describe and justify options for downlink power weightings identify how different antenna configurations can be used to optimize coverage and capacity identify appropriate antenna types and configurations for a range of cell types describe how an operator may use multiple carrier allocations to optimize coverage and capacity describe downlink channel power allocations and limitations in terms of coverage and capacity describe the impact of Adjacent Channel Leakage Ratio (ACLR) and describe optimization options to combat it identify how capacity and coverage may be limited by spurious emission and receiver blocking characteristics
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1
UMTS CHANNELS Isolation of radio-related functions from the data networking functions is achieved by splitting the air interface into two distinct areas: the Access Stratum (AS) and the Non-Access Stratum (NAS). 1.1
Access Stratum (AS)
The AS provides communication between the UE and the UTRAN, managing the UMTS radio interface and providing services, called Radio Access Bearers (RAB), to the NAS. The AS can be considered as being layers 1–2 of the OSI Seven-Layer Model, with some layer 3 functionality. The main AS functions are: • provision of physical channels • control of physical channels • link establishment and clearing • channel coding • some security functions 1.2
Non-Access Stratum (NAS)
The NAS provides communication between the UE and the Core Network (CN). The NAS acts transparently through the UTRAN and can be considered as being carried by, rather than being, the air interface. The NAS can be considered as providing layers 3–7 of the OSI Seven-Layer Model. The NAS is used to invoke and provide overall control of a number of air interface procedures.
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OSI Layers L7
Core Network
UE Non-Access Stratum
Relay
L3 L3
Access Stratum
L1 UTRAN
Uu
Iu
Figure 1 UTRAN Architecture SC2804/S4/v1.1
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1.3
The AS on the Air Interface
The AS covers functionality from layers 1–3. At layer 1, signalling and traffic data is carried across the air interface in physical channels that are defined in terms of either code set and frequency for FDD mode, or code, timeslot and frequency for TDD mode. Layer 2 is divided into two sublayers. The lower sublayer is the Medium Access Control (MAC) layer. It is responsible for a wide range of functions including random access procedures, physical link control, multiplexing and channel mapping to the physical layer. The upper sublayer is the Radio Link Control (RLC) layer, which is responsible for Logical Link Control (LLC) and acknowledged and unacknowledged data transfer. Ciphering may be provided by either RLC or MAC. Layer 3 in the AS provides only the lower part of layer 3 in the control plane. This is known as the Radio Resource Control (RRC) layer. It is responsible for the coordination and control of a range of functions including bearer control, monitoring processes, power control processes, measurement reporting, paging and broadcast control functions. 1.3.1
Logical and Transport Channels
There is a complex array of user and signalling requirements. In order to define a process for each type of information, sets of logical channels mapping into transport channels and ultimately to physical channels are defined. Logical channels are defined between RLC and MAC. Transport channels are defined between MAC and the physical layer.
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Control Plane Signalling L3
User Plane Information
Radio Resource Control (RRC)
Radio Link Control (RLC)
Logical channels
L2
Medium Access Control (MAC)
Transport channels
L1
Physical Layer
Figure 2 AS on the Air Interface SC2804/S4/v1.1
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1.4
Logical Channels
The MAC layer provides transfer services via a set of logical channels. A logical channel is defined for each different transfer requirement. Each logical channel relates to particular kinds of information that need to be transferred. Some relate to signalling information, and some to traffic information. The logical channels used for the transfer of signalling information in FDD mode are the Broadcast Control Channel (BCCH), Paging Control Channel (PCCH), Common Control Channel (CCCH) and Dedicated Control Channel (DCCH). The logical channels used for the transfer of user information in FDD mode are the Dedicated Traffic Channel (DTCH) and the Common Traffic Channel (CTCH). 1.4.1
Logical Channel Types
Broadcast Control Channel (BCCH) The BCCH is a downlink broadcast channel carrying system information. Paging Control Channel (PCCH) The PCCH is a downlink channel carrying paging messages. It is used when the network does not know the location cell of the UE, or the UE is using sleep mode procedures. Common Control Channel (CCCH) This is a bidirectional channel carrying control information between the network and the UE. It is used when the UE has no RRC connection with the network. Dedicated Control Channel (DCCH) This is a point-to-point bidirectional channel carrying dedicated control information between the network and the UE. It is used when a dedicated connection has been established through RRC connection set-up procedures. Dedicated Traffic Channel (DTCH) The DTCH is a dedicated point-to-point channel carrying user information between the network and the UE. It may be used in both the uplink and downlink directions. Common Traffic Channel (CTCH) The CTCH is a common point-to-multipoint downlink-only channel used for carrying broadcast or multicast user information.
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Control Channels from RLC BCCH PCCH CCCH DCCH
Traffic Channels from RLC DTCH
CTCH
Medium Access Control (MAC)
Figure 3 Logical Channel Types SC2804/S4/v1.1
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1.5
Transport Channels
Information is transferred from the MAC layer and mapped into the physical channels via a set of transport channels. Transport channels can be classified into two groups: common channels and dedicated channels. Information in common channels will require in-band identification of the UE. For dedicated channels the UE’s identity is associated with the channel allocation. The common transport channels for FDD mode are the Random Access Channel (RACH), Common Packet Channel (CPCH), Forward Access Channel (FACH), Downlink Shared Channel (DSCH), Broadcast Channel (BCH) and the Paging Channel (PCH). The dedicated transport channel for FDD mode is the Dedicated Channel (DCH). 1.5.1
Transport Formats
Each transport channel has an associated transport format. This is defined as a combination of encoding, interleaving, bit rate and mapping into physical channels. For some transport channels this may be variable within a set of transport formats.
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Common Channels from MAC RACH
CPCH
FACH DSCH
BCH PCH
Dedicated Channels from MAC DCH
Physical Layer
Figure 4 Transport Channel Type SC2804/S4/v1.1
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1.5.2
Transport Channel Types
Random Access Channel (RACH) A contention-based channel in the uplink direction, the RACH is used for initial access or non-real-time dedicated control or traffic data. Common Packet Channel (CPCH) This channel is only used in FDD mode. It is a contention-based channel used for the transmission of bursty traffic data in a shared mode. Fast power control is used. Forward Access Channel (FACH) The FACH is a common downlink channel without power control. It is used for control or traffic data. Downlink Shared Channel (DSCH) A downlink channel used in shared mode by several UEs, the DSCH is used to carry control or traffic data. Broadcast Channel (BCH) This is a downlink broadcast channel used to carry system information across a whole cell. Paging Channel (PCH) The PCH is a downlink broadcast channel used to carry paging and notification messages across a whole cell. Dedicated Channel (DCH) The DCH is used in the uplink or downlink direction to carry user information to or from the UE.
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Common Channels from MAC RACH
CPCH
FACH DSCH
BCH PCH
Dedicated Channels from MAC DCH
Physical Layer
Figure 4 (repeated) Transport Channel Type SC2804/S4/v1.1
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1.6
Downlink Physical Channels
In the downlink direction there are a number of channels carrying higher-layer information and a large number having control and synchronization functions associated with layer 1. 1.6.1
Physical Downlink Shared Channel (PDSCH)
This is a DL channel used to carry the DSCH. It is shared by multiple users by way of code multiplexing. The PDSCH is always associated with one or more DL Dedicated Physical Channels (DPCHs). 1.6.2
Secondary Common Control Physical Channel (SCCPCH)
The SCCPCH is used to carry the transport channels PCH and FACH in the DL direction. There may be one or more SCCPCHs, and if an SCCPCH is only carrying the FACH, it may be transmitted over only part of the cell using beam-forming antennas. 1.6.3
Primary Common Control Physical Channel (PCCPCH)
This is used in the downlink direction to broadcast the BCH across a cell. There will be only one of these on each cell. 1.6.4 Dedicated Physical Data Channel (DPDCH) and Dedicated Physical Control Channel (DPCCH) The DPDCH is a bidirectional channel used to carry higher-layer information from the transport channel DCH. It is multiplexed with the DPCCH that provides the layer 1 control and synchronization information. Once multiplexed, the two are referred to as a DPCH. One DPCCH may be associated with one or more DPDCHs 1.6.5
Paging Indicator Channel (PICH)
This DL channel is used to carry Paging Indicators (PI). These are used to enable discontinuous reception of the PCH being carried on an associated SCCPCH.
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Transport Channels Layer 2
DCH
BCH
DPDCH
PCCPCH
Layer 1
FACH
PCH
DSCH
Physical Channels
CD/ AP-AICH CSICH CA-ICH
AICH
CPICH
SCH
PICH
SCCPCH
PDSCH
DPCCH
DPCH
Figure 5 Downlink Physical Channels SC2804/S4/v1.1
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1.6.6
Synchronization Channel (SCH)
This is a downlink channel used during cell search. It consists of primary and secondary subchannels, and conveys information to the UE concerning the time alignment of a cell’s codes and frame structures. 1.6.7
Common Pilot Channel (CPICH)
This channel is used to provide the phase reference for the SCH, PCCPCH, AICH and the PICH. It may also be the default phase reference for all the other DL channels. There will be only one Primary CPICH in a cell. It is an option to have one or more Secondary CPICHs in a cell. If present, the Secondary CPICHs would act as the phase reference for SCCPCHs, and potentially DPCHs. 1.6.8
Acquisition Indicator Channel (AICH)
This downlink channel carries Acquisition Indicators (AI). These are used to acknowledge UE random access attempts, and grant permission for a UE to continue with its random access transmission. 1.6.9
Physical Channels for Common Packet Channel (CPCH) Access
These channels carry information used for the CPCH access procedure and do not carry transport channels. CPCH – Access Preamble Acquisition Indicator Channel (AP-AICH) This channel carries AP acquisition indicators that correspond with the AP signature transmitted by the UE. It is also used to acknowledge the random access preambles, which are then followed by a collision detection preamble. CPCH – Collision Detection/Channel Assignment Indicator Channel (CD/CA-ICH) The CD/CA-ICH is used to acknowledge the collision detection access preamble. CPCH – Status Indicator Channel (CSICH) The CSICH uses the unused part of the AICH channel to indicate CPCH physical channel availability so that access is only attempted on a free channel.
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Transport Channels Layer 2
DCH
BCH
DPDCH
PCCPCH
Layer 1
FACH
PCH
DSCH
Physical Channels
CD/ AP-AICH CSICH CA-ICH
AICH
CPICH
SCH
PICH
SCCPCH
PDSCH
DPCCH
DPCH
Figure 5 (repeated) Downlink Physical Channels SC2804/S4/v1.1
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1.7
Uplink Physical Channels
In the UL direction there are three types of physical channel: the Physical Random Access Channel (PRACH), Dedicated Physical Channel (DPCH) and the Physical Common Packet Channel (PCPCH) 1.7.1
Physical Random Access Channel (PRACH)
This UL channel is a contention-based channel used to carry higher-layer information in the form of the RACH. 1.7.2
Dedicated Physical Channel (DPCH)
The DPCH is ultimately used to carry the transport channel DCH. However, in addition to this it carries layer 1 information in the form of the pilot, Transmit Power Control (TPC), and Transport Format Combination Indication (TFCI) bits.The DPCH can therefore be considered as two subchannels: the DPDCH, which is used to carry DCH; and the DPCCH, which is used to carry the layer 1 information. These two subchannels are multiplexed together to form the DPCH. 1.7.3
Physical Common Packet Channel (PCPCH)
The PCPCH carries the common packet transport channel, which comprises access preambles, collision detection preamble, power control preamble and a message part.
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Transport Channels Layer 2 Layer 1
CPCH
DCH
RACH
Physical Channels DPCCH
PCPCH
DPDCH
DPCH
PRACH
Figure 6 Uplink Physical Channels SC2804/S4/v1.1
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1.8
Channel Mapping Options
1.8.1
Logical to Transport Channel Mapping
Before its transmission across the air interface, information presented in logical channels must be mapped into transport channels. This mapping process is very flexible, and for some logical channels there are several options, depending on the function and the type of information being transferred. 1.8.2
Transport to Physical Channel Mapping
The physical layer applies error protection and maps and multiplexes transport channels into physical channels. It should be noted that some unidirectional channels, i.e. PICH, CPICH and AICH, at the physical layer. These are referred to as physical signals. 1.8.3
Mapping for the Uu Interface
The directions of arrows shown in Figure 7 reflect the mapping process as seen from the UTRAN side. For the channels carrying broadcast information, mapping is direct from BCCH to BCH and from PCCH to PCH. For the other control- and traffic-carrying channels, mapping is more flexible. For example, downlink DCCH can be mapped either to FACH or to DSCH, depending on information requirements. In the uplink direction DCCH may take information from CPCH, RACH, USCH or DCH. The logical channel DTCH is similar, in that it has access to a range of transport channels. However, the CCCH is simple: it uses only RACH and FACH for bidirectional communication.
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BCCH
PCCH
DCCH
CCCH
CTCH
DTCH
Logical Channels
MAC
BCH
PCH
CPCH
RACH
FACH
DSCH
DCH
Transport Channels
Physical Layer PCCPCH
SCCPCH
PCPCH
PRACH
PDSCH
DPCH
Air Interface
Figure 7 Channel Mapping Options SC2804/S4/v1.1
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CELL CONFIGURATION Figure 8 shows how a typical UMTS cell may be configured in the uplink and downlink directions. 2.1
Example Downlink Channels
The cell will contain a single PCCPCH. This channel carries the BCH transport channel, which in turn carries system information messages. Phase synchronization for this physical channel is provided by the CPICH. These channels will always be scrambled by the cell-specific primary scrambling code. The two channels will be time-aligned in terms of scrambling code and frame structure, this timing being indicated by the Primary and Secondary SCH. This example cell contains one SCCPCH. This channel is being used to carry the FACH and PCH. These are variable-rate channels that, in the case of FACH, may contain a mixture of signalling and traffic. There are several types of physical channels with which a cell may be provisioned that carry only physical layer signalling. Two of these are shown in Figure 8: the AICH, which is used to acknowledge random access probes, and the PICH, which is used to support a discontinuous reception function for the PCH. There are likely to be multiple DPCHs and PDSCHs in operation on the cell. These are variable-rate channels that may carry signalling or traffic. In general, bursty packet-switched traffic is likely to be carried in the DSCH, while circuit-switched traffic must be carried in a DCH. 2.2
Example Uplink Channels
In the UL direction there are three physical channel types with slightly different code requirements. The PRACH and the PCPCH are always directed at a single cell; soft handover is not a feature of these channels. As a result, the codes used can be cellspecific. Up to 16 PRACH and up to 64 CPCH channels could be provisioned on a cell but the example cell has two PRACHs and four PCPCHs. The cell also has provision for uplink DPCHs to match those operating in the downlink direction. These channels can use soft handover and therefore the codes are not cell-specific.
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Downlink BCH PCH FACH DCH (multiple) DSCH (multiple)
SCH CPICH PCCPCH SCCPCH AICH PICH DPCH (multiple) PDSCH (multiple)
Uplink PRACH (multiple e.g. 2) PCPCH (multiple e.g. 4) DPCH (multiple)
RACH (multiple e.g. 2) CPCH (multiple e.g. 4) DCH (multiple)
Figure 8 Example Cell Configuration SC2804/S4/v1.1
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2.3
Configuration Options
There are many cell configuration options available to a UMTS operator. The most suitable configuration will depend on location and the likely traffic profile of users in the cell area. At rollout it is likely that all cells will be configured in one way only or perhaps in a limited set of default ways. Optimization can include consideration of channel provisioning on a cell. Given that different channels are suited to different traffic characteristics it is likely that the channel types available on a cell could be optimally matched to the local traffic requirements. For example, a cell being used for an indoor coverage solution is more likely to carry high-bit-rate packet-switched data. This means that more CPCHs and DSCHs may need to be provisioned. In addition to the changes to the site database this will also impact the Node B’s physical requirements for channel elements and terrestrial transmission bandwidth. Another possibility is to build a new cell to provide a specific function. For example, at a sports stadium or in a large public arena a cell could be used to stream audio and visual content, perhaps as a commentary of an event. This would require the CTCH, which is mapped into the FACH. This could be operating at a very high bit rate requiring the construction of a cell with its capacity predominantly dedicated to a SCCPCH carrying the FACH.
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In-building – more provision for packet data with CPCH and DSCH
Sports stadium – more provision for streamed audio/video with CTCH in FACH
Standard configuration
Figure 9 Configuration Options SC2804/S4/v1.1
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2.4
Using More than One Frequency
Most UMTS licence holders have enough radio spectrum for more than one FDD radio carrier pair. It is possible to add second or even third frequencies to a Node B. However, this concept is slightly different in UMTS than in GSM. A Node B can contain one or more cells. A typical arrangement would be to have three cells using appropriate directional antenna on a Node B site. All three cells would be using the same frequency. It would be possible to add more capacity to the Node B by adding a second frequency for each set of antennas. However, each new frequency added carries its own full set of control and traffic channels. This means that the second frequency must be considered as a new cell. Thus a three-cell Node B becomes a six-cell Node B even though only three sets of directional antennas are used. It is possible to use wideband power amplifiers so that a single power amplifier can amplify two frequencies. This would save cost because although the site has six cells, only three power amplifiers would be needed. However, this means that the power available to each cell is halved. If the cells are downlink limited then this will halve the capacity of the cells.
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F1 Cell 1
F1 Cell 3
F1 Cell 2
F1 Cell 1 F2 Cell 4
Three sets of antennas Three cells One frequency
F1 Cell 2 F2 Cell 5
F1 Cell 3 F2 Cell 6
Three sets of antennas Six cells Two frequencies
Figure 10 Using More than One Frequency SC2804/S4/v1.1
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CELL TRANSMIT POWER 3.1
Downlink Power
In the downlink direction the maximum transmit power available from the highly linear power amplifier can be considered constant. The total power available will depend on the vendor and on the type of Node B. For a macro cell product it could be expected to be in the range 20 to 45 W, for a micro cell or pico cell product it would be proportionally lower. The specifications1 require that the power amplifier has a total power dynamic range of at least 18 dB. Maximum transmit power is limited to 50 dBm (100 W). This power is shared between all downlink channels. Downlink power control is implemented through the adjustment of the weighted sum of the downlink channels. Broadcast and common control channels are likely to be allocated a fixed proportion of the power available. The remainder of power is then shared between users. The weighting may be used to vary proportions to each user dependent on path loss, interference and required quality of service. For closed loop power control the UE indicates the requested power step changes to the Node B. However, a limit will be set for the power proportion available to each channel type, so the Node B may not obey all power control commands. If the cell is operating at less than full load then the total power transmitted is less than the total power available. More power is required if: • there are more channels required • if users are distant from the Node B • if users request higher data rates • if users request a higher quality of service Thus the total power available in a cell ultimately limits downlink capacity and quality of service.
1
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3GPP TS 25.104 BS Radio Transmission and Reception (FDD).
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Channel 1 Dynamic range at least 18 dB
G1
Modulation and linear power amplifier
Channel 2
G2 Maximum transmit power Channel n
Gn
Total transmit power
Currently unused Distant user
Higher weighting
Nearby user Nearby user
Lower weighting
SCCPCH PCCPCH CPICH 0
Figure 11 Downlink Power SC2804/S4/v1.1
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3.2
Downlink Power Weightings
The power weighting allocated to downlink broadcast and common channels, along with the range for power control available to other channels, is an important part of a cell’s configuration. All downlink channels will be allocated a fraction of the maximum transmit power available on the cell. This fraction is referred to as the channel code power and can be in the range –3 dB to –28 dB relative to the maximum transmit power available. Thus the maximum proportion that can be allocated to any individual channel is 50% of the maximum power. The starting point and reference for all other channels in the CPICH. The standards allow for the code power in this channel to be set from –10 dBm to 50 dBm. However, the important consideration for this channel is the percentage allocated to it from the total power available for the cell. A typical value for this percentage is 10% of total transmit power. Nevertheless, the optimal value may depend on local conditions, so it should be an optimization task to refine this setting. All the other channels are then set as a power relative to the CPICH power. Figure 12 shows an example of power settings in a macro cell with a maximum transmit power capability of 46 dBm (40 W). The CPICH has been set at 10% (4 W) of the total power. The primary and secondary SCHs have been set at 6% of total power, but they are subject to a 10% duty cycle so they average a combined power of only 0.48 W. The primary and secondary CCPCHs have each been set at 5% (2 W), but it is worth noting that there may be multiple SCCPCHs and that the SCCPCH is potentially a variable rate channel. Higher data rates in the SCCPCH would require a higher power weighting. The PICH and AICH have been set at 1.5% (0.6 W) each, but again it should be noted that this example only shows a single AICH. There is a one-to-one mapping of AICHs to the number of RACH channels configured so there could be up to eight AICHs on a cell. The total power allocated to control channels on this example cell is 9.68 W, almost 25% of the total power available on the cell.
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Maximum transmit power for the cell is 46 dBm (40 W) Percentage of Total Power
Channel
Power (dBm)
Power (W)
CPICH
10%
36 dBm
4W
P&S SCH (inc. 10% duty cycle)
6% (x 2 x 0.1)
33.8 dBm
2.4 W (x 2 x 0.1)
PCCPCH
5%
33 dBm
2W
SCCPCH
5%
33 dBm
2W
PICH
1.5%
27.8 dBm
0.6 W
AICH
1.5%
27.8 dBm
0.6 W
24.2%
39.86 dBm
9.68 W
Total for control channels
Figure 12 Downlink Power Weightings SC2804/S4/v1.1
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3.3
Varying the CPICH Weighting
The nature of a CDMA channel is that a receiver must have accurate code phase alignment in order to successfully receive a channel. The required code phase alignment in the downlink direction in UMTS comes from the CPICH. This explains the need for a large power weighting on the CPICH channel. Although a typical value for this weighting might be 10% there is considerable scope for variation. Variation in this weighting can be used by the optimizer as a tool to influence the balance between coverage and capacity in a cell. A higher weighting extends cell range because it enables UEs that are further away to decode downlink channels. However, the larger power weighting uses more of the cell’s capacity. Additionally, a larger cell will tend to have a higher proportion of more distant UEs that also require larger power weightings, thus further reducing cell capacity. This setting may be more appropriate in rural areas. A lower power weighting reduces cell range because UEs need to be closer before they can decode downlink channels. The lower power weighting coupled with less distant UEs allows more capacity in the cell.
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CPICH
Higher weighting for CPICH increases cell radius but reduces capacity
CPICH
Lower weighting for CPICH reduces cell radius but increases capacity
Figure 13 Varying the CPICH Weighting SC2804/S4/v1.1
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3.4
Utilizing Soft Capacity and Dynamic CPICH Power
The maximum capacity available in a cell is governed in part by the amount of interference that can be tolerated. Yet the level of interference is not simply a product of the load on the cell itself. Interference is also contributed from neighbour cells. Thus a cell’s potential capacity at any moment is partly influenced by the load on its neighbour cells. This means that a cell can carry more traffic if its neighbours are carrying less and vice versa. In real networks the offered traffic is not evenly spread over the ground, therefore neighbouring cells will tend to carry different loads. A busy cell will be forced to transmit more power in the downlink direction because there are more established channels. This creates more interference to neighbour cells, limiting their potential capacity. It would be desirable to balance the load as far as possible between cells in order to distribute interference more evenly. This should lead to a higher total capacity. Load balancing can be achieved by varying the CPICH weightings among cells. Busier cells would be given lower CPICH weightings to reduce coverage area and load. Quieter cells would have higher CPICH weightings to increase coverage and capture more offered traffic. Varying the CPICH weightings can be performed as an optimization function by setting fixed values based on average conditions. However, traffic characteristics in real networks are variable, making it hard to find a truly optimal setting. Furthermore, without great care it would be easy to create coverage holes by setting values that are too low, or to increase the proportion of soft handovers with settings that are too high. Either way this would reduce rather than increase overall capacity. Some vendors may have features that enable the dynamic control CPICH power weighting. This uses an algorithm in the RNC to dynamically adjust power weighting on cells to suit current traffic conditions. The optimizer’s input would then relate to setting the triggers and constraints for the dynamic weighting control algorithm. They could, for example, influence whether coverage or capacity is the dominant factor. Simulations of such systems show useful gains in capacity. They also show considerable variation in optimal CPICH weightings as high as 60%. This again suggests that a optimal static value would be hard to find.
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Unbalanced load
CPICH Power weightings can be used to balance load
Figure 14 Soft Capacity SC2804/S4/v1.1
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3.5
Pilot Pollution
Pilot pollution occurs in areas of overlapping coverage between multiple cells. Specifically, it is an area where signal strength is good but there is also a large number of non-dominant servers. Signal strength in this case can be considered to be the CPICH RSCP. In such an area the receiver is not able to decode the downlink channels because the multiple good servers interfere with each other to the extent that the signal-to-interference ratio for the CPICH, Ec/Io, is not good enough despite the high CPICH RSCP. In effect, this will result in a coverage hole, where UEs are not able to obtain service from the network. Most 3G planning tools will be able to plot and account for areas of pilot pollution. Nevertheless, planning tools are limited by the accuracy of the propagation model. This means that a key early optimization task may be to identify and rectify significant areas of pilot pollution in the built network. Adjustment of CPICH weightings is one option for dealing with pilot pollution. It can be used to create a dominant server in affected areas. Other techniques to consider may be antenna adjustments including orientation, downtilt and height.
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Pilot Pollution Multiple non-dominant servers CPICH RSCP is good CPICH Ec/lo is poor
Adjust CPICH power weightings Adjust antenna azimuth, downtilt or height
Figure 15 Pilot Pollution SC2804/S4/v1.1
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ANTENNA CONFIGURATIONS 4.1
Use of Downtilt
In the interests of controlling interference and coverage, it is common to use antennas whose vertical beams are tilted down towards the ground by a few degrees. Looking at Figure 16a, a directional antenna with 0° downtilt is illustrated. For comparison, Figure 16b shows a sector antenna with x° in-built (electrical) downtilt. Note that all lobes are downtilted. Figure 16c shows an antenna with 0º electrical downtilt which has been mechanically downtilted by x°. Note that the back lobe is tilted up and side lobes in the horizontal plane will not be fully downtilted. Accordingly, electrical downtilting is often preferred, although a combination of electrical and mechanical tilting is common, as shown in Figure 16d. Another technique involves mechanically uptilting an electrically downtilted antenna, as shown in Figure 16e. This can be used to create an antenna with a heavily depressed back lobe, which could be useful for interference rejection in some cases. A typical configuration for UMTS at rollout is to use minimal downtilt to maximize coverage. As the network matures, downtilt is applied when in-fill cells are built. Ideally, therefore, variable electrical downtilt antennas should be used to facilitate this. Some estimates are that an antenna’s downtilt could need changing between three and four times in the first five years of operation. This may mean that the most economical solution would be remotely adjustable downtilts. This would greatly reduce the number of site visits required. It would also allow for the potential dynamic adjustment of downtilts based on load conditions. Normally, omni antennas can only be tilted by electrical means, as shown in Figures 16f and 16g. (Mechanically tilted omni antennas are very rare indeed, but may be seen on steep hillside sites). Typical tilts in use vary from 0 degrees (no tilt) to over 10º. Electrical tilt angles of 0º, 2º, 4º and 8º are common, but others are available. In general, the majority of tilt should be achieved using electrical tilting, with final fine adjustment being mechanically made.
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Antenna (a) Sector antenna 0° tilt
x°
(b) Sector antenna x° electrical tilt
x°
(c) Sector antenna x° mechanical tilt
x°
(d) Sector antenna x° electrical tilt y° mechanical tilt
y° x°
(e) Sector antenna electrically downtilted mechanically uptilted (f) Omni antenna 0° tilt
x°
(g) Omni antenna x° electrical tilt
Figure 16 Antenna Downtilt SC2804/S4/v1.1
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4.2
Calculations for Beamtilt
Figure 17 illustrates how the range from an antenna for the main lobe, as well as the upper –3 dB and lower –3 dB beam extremities, can be calculated. The example shown assumes an antenna height of 15 m. The formulas used are: Radio horizon = Main lobe
=
Upper –3 dB =
4.12 h
kilometres
h 1000 tan α
kilometres
h kilometres 1000 tan (α–β/2)
Note: if α < β/2 this will be over the horizon Lower –3 dB =
h kilometres 1000 tan (β/2 + α)
In all cases: h = antenna height above average terrain in metres α = downtilt in degrees β = vertical beam width in degrees
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Horizon Antenna Height h
ß
– downtilt angle ß – vertical beamwidth
Lower –3 dB
Main Lobe
Upper –3 dB
Example: h = 15 metres, vertical beamwidth, ß = 10°, radio horizon = 15.95 kilometres Downtilt ° 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Main Lobe (km) 15.95 0.85 0.43 0.29 0.21 0.18 0.14 0.13 0.11 0.10 0.08 0.08 0.06 0.06 0.06 0.05
Lower –3 dB (km) 0.18 0.14 0.13 0.11 0.1 0.08 0.08 0.06 0.06 0.05 0.05 0.05 0.05 0.05 0.05 0.03
Upper –3 dB (km) over horizon over horizon over horizon over horizon over horizon over horizon 0.85 0.43 0.29 0.21 0.18 0.14 0.13 0.11 0.10 0.08
Figure 17 Results for Beamtilt SC2804/S4/v1.1
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4.3
Practical Antenna Types and Tilt Effects
It is important when considering downtilts to be aware of the effects likely to be produced in realistic environments. The specific effect will depend on the type of antenna and on the antenna characteristics. It is common for an operator to use a mix of vendors for similar types of antenna; often this is driven by cost or availability at the time of site build. However, the optimizer should be aware that specific characteristics, even for the same type of antenna, will differ with different vendors. These differences could be significant when making changes in UMTS. Thus any simulations performed to test the likely effects of a change should always be done with the correct vendor’s antenna data. The different characteristics of mechanical and electrical downtilt are illustrated in Figures 19a to 19d. Figure 18 shows the terrain and basic site characteristics for the simulations.
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Antenna at 15m PA Max 37 dBm
Figure 18 Antenna Characteristics Simulation Parameters SC2804/S4/v1.1
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4.3.1
Changing Antenna Type
Figures 19a to 19d show the differences between several common antenna types. All the antennas shown are designed for use with UMTS. Figure 19a is an omnidirectional antenna from Cellwave. It has a maximum gain of 7.65 dBi. Note the impact that terrain has on the omnidirectional radiation pattern. Figure 19b shows a 85º directional antenna from CSA Wireless. It has a maximum gain of 16 dBi, a horizontal beam width of 85º and a vertical beam width of 7º. It is fitted with 2º of electrical downtilt and is mounted with 0º of mechanical downtilt. Figure 19c shows a 65º directional antenna from CSA Wireless. It has a maximum gain of 17.5 dBi, a horizontal beam width of 65º and a vertical beam width of 9º. It is fitted with 2º of electrical downtilt and is mounted with 0º of mechanical downtilt. Figure 19d shows a 33º directional antenna from CSA Wireless. It has a maximum gain of 20 dBi, a horizontal beam width of 33º and a vertical beam width of 6.5º. It is fitted with 2º of electrical downtilt and is mounted with 0º of mechanical downtilt.
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Figure 19a Omni
Figure 19b 85 16 2elec 0mech SC2804/S4/v1.1
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Figure 19c 65 17 2elec 0mech
Figure 19d 33 20 2elec 0mech SC2804/S4/v1.1
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4.3.2
Applying Mechanical Downtilt
Figures 20a to 20f show the effect of applying mechanical downtilt up to 16º for the 65º directional antenna. Figure 20a shows 0º mechanical downtilt for the 65º antenna. Figure 20b shows 4º mechanical downtilt for the 65º antenna. Figure 20c shows 8º mechanical downtilt for the 65º antenna. Figure 20dshows 12º mechanical downtilt for the 65º antenna. Figure 20e shows 16º mechanical downtilt for the 65º antenna. Figure 20f shows the vertical and horizontal radiation patterns for the 65º antenna. Note the upper side lobes in the vertical radiation pattern and compare these with the coverage predictions
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Figure 20a 65 17 2elec 0mech
Figure 20b 65 17 2elec 4mech SC2804/S4/v1.1
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Figure 20c 65 17 2elec 8mech
Figure 20d 65 17 2elec 12mech 4.47
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Figure 20e 65 17 2elec 16mech
Vertical
Horizontal
Figure 20f Polar Plots for 65_17_2 SC2804/S4/v1.1
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4.3.3
Applying Electrical Downtilt
Figures 21a to 21c show the effect of applying electrical downtilt up to 6º for the 65v directional antenna. Figure 21a shows 0º electrical downtilt for the 65º antenna. Figure 21b shows 4º electrical downtilt for the 65º antenna. Figure 21c shows 8º electrical downtilt for the 65º antenna. Note that electrical downtilt applies in all azimuths whereas the mechanical downtilt applies predominantly in the bore sight of the antenna. Combinations of electrical and mechanical downtilt can be used to shape the coverage pattern with great effect.
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Figure 21a 65 17 2elec 0mech
Figure 21b 65 17 4elec 0mech SC2804/S4/v1.1
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Figure 21c 65 17 6elec 0mech SC2804/S4/v1.1
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4.4
Choice of Antenna
An integrated, multiband antenna offers a relatively compact and low-cost solution. Low visual impact, together with ease of installation and relatively low ongoing maintenance, make it a popular choice. Wind loading is also low. However, such antennas confine the operator to using the same azimuth bearing and downtilt for both GSM and UMTS (within most current antenna design limits). Separate upgrade and optimization cannot be carried out. These factors may persuade operators to deploy separate antennas for UMTS, although this is bound to be an unpopular choice for planning approval and so may be limited to specific cases.
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Consideration
Type of UMTS Antenna Separate
Integrated/Multiband (Broadband)
Low visual impact Low cost Low wind loading Low maintenance Rapid fitting Different azimuths Different downtilts Separately optimized Separately upgraded
Figure 22 Choice of Antennas SC2804/S4/v1.1
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5
RADIO PERFORMANCE 5.1
Minimum Coupling Loss
Maintaining acceptable noise levels at the Node B receiver for UMTS is very important if uplink capacity and coverage are to be maximized. In general this is the function of the closed loop power control process. As the UE approaches the Node B the power control process steps its power down. The UE is required1 to be able to step its power down to at least –50 dBm. Figure 23 shows a situation where the mobile’s power has been stepped down to –50 dBm. The service supported is 12.2 kbit/s speech. The Node B requirement2 for sensitivity level at this service bit rate is –121 dBm. This figure is quoted for static conditions and includes the processing gain and the receiver noise figure. Processing gain at 12.2 kbit/s is 25 dB, a required Eb/No of 5 dB is assumed and a 3 dB interference margin has been allowed for other traffic on the cell. The result is that uplink power control should be aiming for –104 dBm at the Node B. This gives a coupling loss (including antenna gains) of 54 dB. At this value of coupling loss the signal level is just sufficient to meet the bit error requirements in the channel. If the UE were to move closer in order to reduce the coupling loss it would be contributing more power at the receiver. Since its power cannot be reduced further it will have the effect of reducing capacity. Thus, in this example, 54 dB is the Minimum Coupling Loss (MCL). This is a fairly typical value. Assuming a typical combined antenna gain figure for the UE and the Node B of 16 dBi, the minimum path loss will be about 70 dB. This is only likely to occur with clear line of site over very short distances. For free space this would mean a distance of less than about 40 m.
1 2
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3GPP TS 25.104 User Equipment (UE) Radio transmission and reception (FDD). 3GPP TS 25.104 BS Radio transmission and reception (FDD).
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Coupling Loss
–50 dBm Coupling Loss = 54 dB
–104 dBm
–121 dBm –129 dBm Eb/No 5 dB IM 3 dB
Gp 25 dB
Figure 23 Minimum Coupling Loss SC2804/S4/v1.1
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5.2
Adjacent Channel Leakage Ratio (ACLR)
Limitations in design capability mean that a transmitter will always radiate some power outside the intended radio channel. Since there is very little space between adjacent channels in UMTS, and since UMTS is sensitive to interference, the Adjacent Channel Leakage Ratio (ACLR) is an important consideration. The specified performance requirements for ACLR are shown in Figure 24 for the UE and for the Node B (both in FDD mode).
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–33 dB
–33 dB
–43 dB fc – 10 MHz
–43 dB fc – 5 MHz
fc
fc + 5 MHz
fc + 10 MHz
UE ACLR Performance Requirement for FDD Mode
–45 dB
–45 dB
–55 dB fc – 10 MHz
–55 dB fc – 5 MHz
fc
fc + 5 MHz
fc + 10 MHz
Node B ACLR Performance Requirement for FDD Mode
Figure 24 Adjacent Channel Leakage Ratio (ACLR) SC2804/S4/v1.1
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5.3
Radio Carrier Spacing
Nominal channel spacing for UMTS radio carriers is 5 MHz, but the standards allow for the centre frequencies to be altered in 200 kHz steps. Each 200 kHz step is described with a UMTS Absolute Radio Frequency Channel Number (UARFCN). An operator may take advantage of this by creating larger guard bands between adjacent radio carriers. This is most likely to be the case where adjacent radio carriers belong to different operators, as shown in Figure 25. Nevertheless, there is no specified or technical restriction demanding that an operator uses consistent UARFCNs across their network. Thus, for example, an operator with space for three radio carriers could allow much larger guard bands in geographical areas where only two radio carriers are in use. This would increase the potential capacity of each of the individual radio carriers.
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Enhanced guard band
Operator A
Operator B
5.4 MHz
4.8 MHz 4.8 MHz
Enhanced guard bands
Operator A
Operator B
Figure 25 Radio Carrier Spacing SC2804/S4/v1.1
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5.4
Adjacent Channel Interference (ACI)
There are a number of scenarios where adjacent channel interference may have a significant bearing on system performance. The example illustrated in Figure 26 involves interference between two cell layers. Node B 1 is a micro cell and Node B 2 is a macro cell. Node B 2 is the serving cell for the UE. In this scenario the UE is very close to the micro cell, but on the edge of coverage for its serving cell. The result is that it is transmitting a large amount of power on an adjacent channel very close to the micro cell’s receiver. 5.4.1
Worst Case Assumption
Consider the worst case for the scenario illustrated in Figure 26. If the UE is class 4 it may be transmitting 21 dBm assuming a minimum coupling loss to the micro cell of 54 dB and ACLR performance for the UE in the adjacent channel of 33 dB. The interfering signal level at the input to the micro cell receiver will be: ACI level
= 21 – 54 – 33 = –66 dBm
Clearly, an interfering signal level signal level of –66 dBm will have a serious impact on the performance of the micro cell. However, it is important to appreciate that this is a worst-case scenario, and if the two cells belong to the same operator the situation should be avoidable. In this case a hard handover from the macro to the micro cell would seem appropriate. This situation would be most problematic when the cells belonged to different operators.
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Adjacent Channel Interference (ACI)
Node B 2 Macro cell Node B 1 Micro cell UE Transmitting 21 dBm ACI level = UE_TX_PWR – coupling loss – ACLR = 21 – 54 – 33 = –66 dBm
Figure 26 ACI Scenario SC2804/S4/v1.1
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5.5
Reducing ACI
The strategies used to combat ACI depend on the scenario. In particular, the range of options open to the optimizer differs greatly if the ACI is inter-operator. 5.5.1
Intra-Operator ACI
In this case the most likely options open to the optimizer will be to consider the distribution of traffic within hierarchical layers. A UE should never be allowed to get into a position as extreme as that shown in Figure 27 (assuming intra-operator Node Bs). The most likely course of action will be to address handover triggers and connected mode measurements. This should ensure that the UE is served by the micro cell rather than the macro cell. If the mobility of the UE makes the micro cell an inappropriate choice as the serving cell then the position of the micro cell or its antennas is possibly incorrect. The antenna should be repositioned to create a higher coupling loss for a UE in this position. 5.5.2
Inter-Operator ACI
When the Node Bs belong to different operators as shown in Figure 27 the optimizer does not have the option of handover from the macro to the micro cell. Therefore they should consider modifying the coupling loss between the interfering UE and the micro cell. The antenna position could be revised as above. However, this may not be the best solution where the antenna position is critical for intended coverage or where there are no alternative positions. Another possibility is the deliberate desensitization of the micro cell receiver. This will increase its tolerance to adjacent channel interference. Desensitization is generally regarded as undesirable since it will reduce cell range, but this is less likely to be a problem for a micro cell because it is more likely to be downlink limited. In addition, the optimizer could consider adjusting the UARFCN slightly to increase the guard band between the two operators’ radio carriers. It should be noted that where the two operators use similar architectures this situation is much less likely to occur. Thus more site sharing should reduce interoperator ACI problems.
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Inter-operator ACI
Operator B Macro cell Operator A Micro cell UE Operator B
Options to reduce inter-operator ACI: revise antenna position desensitize receiver revise UARFCN inter-operator cooperation
Figure 27 Inter-Operator ACI SC2804/S4/v1.1
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6
INTERACTION AND INTERFERENCE WITH GSM 6.1
Transmitter Noise and Spurious Emissions
On sites where GSM and UMTS base stations are co-located, out-of-band spurious emissions from one service must not be allowed to adversely affect the performance of the other. This means that the strength of any spurious emission must be strictly controlled such that, with normal antenna decoupling (>30 dB), the spurious signal is received at a level well below the sensitivity threshold. 6.1.1
Currently Specified Equipment
GSM 900/1800 equipment manufactured to the latest 3GPP specification must not produce spurious emissions stronger than –96 dBm in the UMTS bands. This falls to –126 dBm (assuming 30 dB isolation between antennas) at the input to the Node B, well below the reference sensitivity of –121 dBm (12.2 kbit/s channel). Similarly, any spurious output from a Node B must be limited to –98 dBm in GSM bands, producing a nominal –128 dBm at the GSM BTS input (reference sensitivity –104 dBm). Given also that reference sensitivity is quoted for the RX input and not BTS/Node B input, it can be seen that a 30 dB antenna isolation is perfectly adequate.
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Antenna System GSM UMTS Isolation = 30 dB
–96 dBm
–126 dBm
GSM BTS
UMTS Node B
Antenna System GSM UMTS Isolation = 30 dB
–128 dBm
–98 dBm
GSM BTS
UMTS Node B
Figure 28 Emission Limits for Current Equipment SC2804/S4/v1.1
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6.1.2
Older GSM Equipment
Co-locating a UMTS Node B and older GSM 900 or especially GSM 1800 equipment requires additional isolation measures. This is because older GSM equipment was not required to suppress out-of-band spurious emissions to the current degree. In the UMTS band, it is required only that spurious emissions be kept to less than –30 dBm. Unless additional isolation is provided, the normal 30 dB of antenna isolation would result in a –60 dBm input to the Node B, with disastrous desensitization resulting. The effective uplink range of the Node B would be severely reduced. Because at least 60 dB are required, it may be impractical to increase the isolation by antenna spacing alone. One solution would be to include an in-line bandpass filter with a steep roll-off characteristic in the output from the GSM BTS. Such filters will introduce a small (1 to 2 dB) additional downlink loss for the GSM cell. This will cause a small reduction in range unless more BTS output power can be obtained from the TRXs to maintain the Effective Isotropic Radiated Power (EIRP). This assumes the filter is placed in the GSM transmit branch, i.e. between the combiner and duplex filter (if fitted).
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Antenna System GSM UMTS Isolation = 30 dB
–30 dBm
older equipment
–60 dBm
GSM BTS
UMTS Node B
> 60 dB attenuation in UMTS band Bandpass Filter
Antenna System GSM UMTS Isolation = 30 dB
–30 dBm older equipment filter included only in GSM transmit branch
GSM BTS
Node B desensitized
8 dB attenuation Bandpass Filter GSM in UMTS 1800 band
Feeder/Connector loss = 3 dB
UMTS
< 0 dBm filter included only in GSM receive branch
GSM 1800 BTS
43 dBm
UMTS Node B
Figure 31 Receiver Blocking – UMTS and GSM 1800 SC2804/S4/v1.1
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SECTION 5
IDLE MODE AND SYSTEM ACCESS
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CONTENTS 1
PLMN and Cell Selection 1.1 Selection and Idle Mode Activities 1.2 PLMN Selection 1.3 Capturing Roaming Subscribers 1.4 Cell Selection
5.1 5.1 5.3 5.5 5.7
2
Exercise 1 – Cell Selection Scenarios
5.13
3
The Random Access Channel (RACH) 3.1 Applications for RACH 3.2 RACH Operation 3.3 RACH Control Parameters 3.4 RACH Optimization Considerations
5.15 5.15 5.17 5.19 5.21
4
Cell Reselection 4.1 Introduction 4.2 Basic Cell Reselection Process 4.3 Basic Inter-RAT Reselection 4.4 Reselection with Hierarchical Cell Structures (HCS) 4.5 Inter-RAT Reselection with HCS
5.23 5.23 5.23 5.27 5.29 5.39
5
Exercise 2 – Cell Reselection Scenarios
5.41
6
Radio Resource Control (RRC) Functions 6.1 Introduction 6.2 Cell Access Restrictions 6.3 Admission Control
5.43 5.43 5.45 5.49
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OBJECTIVES At the end of this section you will be able to: • • • • • • • • • •
describe the UMTS cell selection process characterize the effect of each parameter relating to cell selection describe the UMTS cell reselection process characterize the effect of each parameter relating to cell reselection describe the configurations and capabilities of the RACH describe the open loop power control process and controlling parameters analyze the effect of changing open loop power control parameters describe the interactions between UMTS and GSM/GPRS in respect of cell selection and reselection describe the functions of RRC in terms of admission control explain how traffic prioritization and mapping to appropriate channel types can be used to optimize capacity in the radio access network
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1
PLMN AND CELL SELECTION 1.1
Selection and Idle Mode Activities
At switch-on the UE has a number of tasks to perform to ensure that it is in a condition to obtain services through a network as required. The first of these is to perform PLMN selection. The selection process is performed by the NAS part of the protocol stack, and may involve input from the user. Having selected a Public Land Mobile Network (PLMN) the UE is required to select and camp-on a suitable cell belonging to the selected PLMN. Registration is then performed through the camped-on cell. After a successful registration the UE will assume the camped normal state and begin idle mode tasks. Idle mode tasks will involve neighbour cell measurements, cell reselection, system information monitoring and paging monitoring. The precise behaviour of the UE when performing these tasks will depend upon the camped on cell’s channel configuration and the setting of several related parameters in system information. These actions are fully defined in the UMTS standards.1
1
5.1
3GPP TS 25.304 UE procedures in idle mode and procedures for cell reselection in connected mode. 3GPP TS 23.122 NAS functions related to MS in idle mode.
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Automatic
Manual NAS in UE
NAS in UE
Select PLMN and RAT Selected Results PLMN/RAT (PLMNs/RATs found)
AS in UE Scan for radio carriers and identify PLMNs
Registration System information for idle mode Suitable cell found
AS in UE Scan for suitable cell on selected PLMN
AS in UE Scan neighbours as required by configuration and system information for cell reselection Camped normal (idle mode)
PLMN selection
Cell selection
Cell reselection
Figure 1 Switch-on to Idle Mode SC2804/S5/v1.1
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1.2
PLMN Selection
PLMN selection is a NAS function, but the AS provides the list of available PLMNs from which the selection is made. To compile this list, the UE is required to scan all carriers within its frequency capability (this may include FDD or TDD UMTS carriers or GSM carriers depending on the UE capability). On each carrier on which a signal is received the UE will synchronize to the cell with the strongest signal level and attempt to read the PLMN identity from the system information. The AS reports all successfully-read PLMN identities to the NAS, but they are divided into two groups: those that meet the high-quality criterion and those that do not. The high-quality criterion for FDD cells is a measured CPICH RSCP of –95 dBm or better (better than –84 dBm for TDD cells and Received Signal Strength Indicator (RSSI) better than –85 dBm for GSM cells). For PLMNs that do not meet the highquality criterion the AS reports the identity and the measured CPICH RSCP to the NAS such that they can be ranked. The standards allow for the optimization of this measuring and reporting process through the use of stored information in the UE regarding carrier frequencies and other cell parameters such as scrambling codes. 1.2.1
PLMN Prioritization
Once a suitable list of available PLMNs is compiled, it is up to the NAS to select a PLMN for registration. This may be done automatically or manually. In automatic mode the available PLMNs are listed in priority order and the highest priority PLMN is selected. In manual mode a list of the available PLMNs is presented to the user in priority order, but the user may select any PLMN from the list. Prioritization for both modes is as follows: 1
Home PLMN (HPLMN).
2
PLMNs in the ‘User Controlled PLMN Selector with Access Technology’ data field in the SIM in priority order.
3
PLMNs in the ‘Operator Controlled PLMN Selector with Access Technology’ data field in the SIM in priority order.
4
Other PLMNs that meet the high-quality criterion in random order.
5
Other PLMNs that do not meet the high-quality criterion in order of decreasing signal quality.
Once a PLMN is selected this is indicated to the AS along with the selected radio access technology.
5.3
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NAS Builds priority list from available PLMNs and selects a PLMN and RAT using either automatic or manual mode: automatic manual
Available PLMNs
SIM Read priorities
user PLMN list operator PLMN list
highest priority user selected
Selected PLMN and RAT
AS Scans for and measures available PLMNs. Supplies a list of all PLMNs successfully identified to NAS. May use stored information to optimize the process.
Figure 2 PLMN Selection SC2804/S5/v1.1
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1.3
Capturing Roaming Subscribers
An important aspect of optimization for PLMN selection is to try to capture roaming subscribers. The main focus for this will be international airports, but other national entry points may also be considered significant. In both automatic and manual mode PLMN selection is influenced by PLMN prioritization. Thus the aim of the optimizer should be to get their PLMN as near to the top of the priority list as possible in key geographical areas where roaming subscribers tend to switch their phones on. However, the PLMN selection process defined for UMTS leaves little scope for significant influence. In a roaming situation it can be assumed that the HPLMN is not available. Therefore the PLMNs most likely to be selected are those that appear in the user-controlled and operator-controlled data fields in the SIM. It is likely that, unless a subscriber has a strong preference, the user-controlled data field may be empty. The operatorcontrolled data field will be determined through commercial relationships between operators and is thus likely to be beyond the scope of an optimizer’s influence. The only real option for the optimizer is to try to ensure a good signal level in areas likely to capture roaming traffic. For example, the use of an indoor coverage solution may be considered in a baggage reclaim area at an airport. However, it should be noted that even the successful capture of a roaming subscriber in these areas does not guarantee retention of service for the duration of their visit. The UE may make subsequent attempts to capture a higher-priority PLMN list from the SIM data field.
5.5
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Priority List Assumed not available
1
HPLMN
2
User-defined priority from SIM
3
Operator-defined priority from SIM
4
PLMNs meeting high-quality criterion in random order
5
Likely to be empty unless a subscriber has a strong preference Driven by commercial relationships between operators
Best strategy is to provide a good signal level
Other PLMNs in order of signal quality
Figure 3 Optimizing PLMN Selection SC2804/S5/v1.1
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1.4
Cell Selection
Following PLMN selection, the AS will be provided with the selected PLMN and the radio access technology to use. It is possible that the selected radio access technology will be GSM, in which case the cell selection process will be as described in GSM specifications.1 Assuming the radio access technology to be used on the selected PLMN is UMTS, the UE will find the strongest cell on each carrier and test it for suitability. The UE will select the first suitable cell it finds. A suitable cell is defined in the following way: • it belongs to the selected PLMN or an equivalent PLMN • it is not barred • it is not in a forbidden location area for roaming • the cell selection criteria are fulfilled Equivalent PLMNs will be indicated in system information and their use allows infrastructure sharing between operators. Cell bar status is also indicated in system information. The use of forbidden location areas for roaming allows operators to provide national roaming on a regional basis. The cell selection criteria are shown in Figure 4. As indicated, a UMTS FDD cell is considered suitable if both Srxlev and Squal are greater than zero. The system parameters used in this calculation are broadcast in system information and they are as follows: • Qqualmin (–24 to 0 dB) • Qrxlevmin (–115 to –25 dBm in 2 dB steps) • UE_TXPWR_MAX_RACH (–50 to 33 dBm) Qqualmin and Qrxlevmin are minimum required levels for the cell. UE_TXPWR_MAX_RACH is the maximum power that a UE is allowed to use on the RACH in the cell. Potentially these parameters could be adjusted by an optimizer to influence a UE’s behaviour. However, the most likely way to influence cell selection would be through adjustment of radio characteristics such as transmit power or antenna tilt. The other parameters used in the calculation are Qqualmeas and Qrxlevmeas, which are a UE’s measured values of CPICH Ec/No and CPICH RSCP respectively for a cell. Additionally, P_MAX is the UE’s maximum transmit power capability. 1
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A cell is suitable if: it belongs to the selected PLMN or an equivalent PLMN it is not barred it is not in a forbidden location area for roaming the cell selection criteria are fulfilled
The cell selection criteria for UMTS FDD cells are fulfilled when Srxlev and Squal are both greater than zero. Where: Squal = Qqualmeas – Qqualmin Srxlev = Qrxlevmeas – Qrxlevmin – Pcompensation and Pcompensation = max(UE_TXPWR_MAX_RACH – P_MAX, 0)
Figure 4 Cell Selection SC2804/S5/v1.1
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1.4.1
Initial and Stored Cell Selection
The UE may use either initial or stored cell selection. For initial cell selection the UE will have no information on frequencies or scrambling codes used by the selected PLMN. For stored cell selection, the more typical case, the UE will have stored previously received information elements relating to frequencies and scrambling codes used, which may speed up the process.
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A cell is suitable if: it belongs to the selected PLMN or an equivalent PLMN it is not barred it is not in a forbidden location area for roaming the cell selection criteria are fulfilled
The cell selection criteria for UMTS FDD cells are fulfilled when Srxlev and Squal are both greater than zero. Where: Squal = Qqualmeas – Qqualmin Srxlev = Qrxlevmeas – Qrxlevmin – Pcompensation and Pcompensation = max(UE_TXPWR_MAX_RACH – P_MAX, 0)
Figure 4 (repeated) Cell Selection SC2804/S5/v1.1
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1.4.2
Cell Selection Criteria Example
In this example the UE has synchronized to the strongest cell on a radio carrier. It has identified that the cell belongs to the selected PLMN, that it is not barred and that it is not in a forbidden location area. Therefore the cell will be suitable if the cell selection criteria are fulfilled. Figure 5 shows the example parameters and calculation that the UE will perform in this circumstance. It can be seen that Squal and Srxlev have both returned positive values. This means that the cell is suitable, and that it will therfore be selected. Note that once a suitable cell has been found, the selection is made and the search stops. The UE will not continue to search for better cells as part of this process.
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Qqualmin = 1 – 2 dB Qrxlevmin = 9 – 7 dBm UE_TXPWR_MAX_RACH = 24 dBm Power Class 3 = 24 dBm Qqualmeas = 1 – 0 dB – 0 dBm Qrxlevmeas = 9
Squal = Qqualmeas – Qqualmin =1 – 0–1 – 2 =2 Srxlev = Qrxlevmeas – Qrxlevmin – Pcompensation = – 90 – 9 – 7 – max(24 – 24, 0) =7–0 =7 Squal and Srxlev are both greater than zero, therefore the cell is selected.
Figure 5 Cell Selection Criteria Example SC2804/S5/v1.1
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2
EXERCISE 1 – CELL SELECTION SCENARIOS Figure 6 shows a cell and the associated broadcast parameter values for Qqualmin, Qrxlevmin and UE_TXPWR_MAX_RACH. It also shows values for Qqualmeas and Qrxlevmeas for three positions in the cell, A, B and C. 1
For positions A, B and C calculate whether the cell selection criteria are fulfilled for a power class 4 UE (21 dBm).
2
For positions A, B and C calculate whether the cell selection criteria are fulfilled for a power class 3 UE (24 dBm).
3
What might you adjust to ensure that the cell appears suitable for both types of UE in all locations?
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Qqualmin = 1 – 2 dB Qrxlevmin = 9 – 5 dBm UE_TXPWR_MAX_RACH = 24 dBm
A Qqualmeas = 9 – dB Qrxlevmeas = 7 – 4 dBm
B Qqualmeas = 1 – 0 dB Qrxlevmeas = 8 – 9 dBm
C Qqualmeas = 1 – 1 dB Qrxlevmeas = 9 – 3 dBm
Figure 6 Exercise 1 – Cell Selection Scenarios SC2804/S5/v1.1
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3
THE RANDOM ACCESS CHANNEL (RACH) 3.1
Applications for RACH
Having selected a cell, the UE will have to perform a registration. This will involve a location update to the circuit-switched core network and a routing area update to the packet-switched core network. This may be performed as a combined or as separate procedures. In order to perform these signalling procedures an RRC connection must be established. This will involve at least initial access using the RACH, but optionally the whole procedure could be performed on the RACH. The RACH is an uplink-only contention-based channel utilizing open loop power control. Its use is mandatory for initial access, but it has several other optional functions. Here, initial access prior to a registration procedure is being described, but initial access is also required to initiate packet data activity while RRC connected in the CELL_FACH, CELL_PCH or URA_PCH sub-states. The RACH may optionally be used for ongoing exchange of signalling or packet data; for example, a complete exchange of all messages in a location update, or the transfer of small packet data such as SMS or telemetry information. Thus the RACH is a multipurpose channel whose activity rate is not limited to the initiation of signalling procedures. Given the significant amount of RACH activity that can be expected in a UMTS cell it is important that the parameters that control its operation are set with care. In the rollout phase, activity will be limited and therefore it should be acceptable to use the same parameter values on all cells. However, inappropriate values may result in excessive interference from RACH channel usage as traffic load increases. Therefore, as a system matures and cell traffic load rises, it will be necessary to optimize parameters on the most affected cells.
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Figure 7 Applications of RACH SC2804/S5/v1.1
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3.2
RACH Operation
The RACH is a transport channel and as such is mapped onto a physical channel called the PRACH. It is the operation of the physical channel that is of most interest to the optimizer. The principal concern is the setting of parameters that control the open loop power control required in this contention-based channel. Figure 8 illustrates the basic operation of the physical channel that carries the RACH. As shown, the timing structure for the RACH occupies frames of 20 ms. These are divided into 15 access slots, each with duration 5,120 chip periods. There may be up to 16 RACH channels available on a cell. The UE will determine from system information the configuration of RACH on the cell and if there are any applicable access restrictions to channels or access slots within channels. Assuming the UE has appropriate access rights it begins the procedure by calculating an initial power. It then randomly selects one of 16 signature codes and transmits this in a preamble part of duration 4,096 chip periods. It will then monitor the AICH associated with the RACH. The AICH can be used to provide either a positive or a negative response to the preamble. A negative response would cause the UE to abandon the procedure. However, it is most likely (and desirable) that after the first preamble there will be no response in the AICH. In this case, either three or four access slots after the start of the first preamble, the UE will transmit a second preamble at a slightly higher power and again monitor the AICH. The power step used and the delay before retransmission are parameters read from system information. If the UE still has no response the pattern will be repeated; a third transmission with another power step up. This will continue until the UE gets a response in the AICH or until it has reached the maximum allowable number of preambles. Assuming that the UE does get a positive response in the AICH, it will begin transmission of the message part. This starts either three or four access slots after the start of the successful preamble and is transmitted at the same power as the successful preamble (or with a parameter defined offset). The duration of the message part may be either 10 ms or 20 ms.
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No response
No response
Positive response
20 ms Access Frame 10 ms Radio Frame DL AICH
0
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Power step Power step
Message Part 10 ms or 20 ms
Initial power UL RACH
0
First preamble
1
2
3
Second preamble
4
5
6
7
8
9
10
11
12
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Third preamble
Figure 8 RACH Basic Operation SC2804/S5/v1.1
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3.3
RACH Control Parameters
There are many parameters that describe RACH configuration and access rules that must be read from system information 1 by the UE. Among other things these parameters will describe the number of RACH channels configured, the bit rates available, access class applicability to channels and slots, signatures available and scrambling codes to be used. The parameters most applicable to the radio optimizer are those relating to open loop power control and the access procedure itself. These parameters are listed in Figure 9. An important value is the transmit power used for the first preamble transmission. The UE uses the formula shown2 to calculate the initial power. Primary CPICH TX power, UL Interference and Constant value are all found in system information. The value of CPICH RSCP is measured by the UE. Other parameters include the Power Ramp Step, Preamble retrans max and Mmax. This final parameter determines the maximum number of preamble cycles in a RACH attempt, whereas Preamble retrans max determines the maximum number of preambles in a cycle. NB01min and NB01max are limits for a random backoff time between preamble cycles.
1
3GPP TS 25.331 Radio Resource Control (RRC) protocol specification.
2
3GPP TS 25.331 Radio Resource Control (RRC) protocol specification.
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1 – 15 to 2 – 5 dBm
1 – 0 to 50 dBm
Preamble_Initial_Power = Primary CPICH TX power – CPICH_RSCP – UL Interference + Constant value Power Ramp Step
1 – 10 to 7 – 0 dBm
3 – 5 to 10 dB
1 to 8
Time Preamble cycle
Backoff
Backoff
Preamble retrans max 1 to 64 Mmax 1 to 32 NB01min NB01max 0 to 50
Figure 9 RACH Power and Access Control Parameters SC2804/S5/v1.1
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3.4
RACH Optimization Considerations
Optimization activity for the RACH is likely to be focused on the operation of open loop power control. Since interference is a capacity-limiting factor in a UMTS cell it is desirable to limit the interference contribution from RACH activity as much as possible. Ultimately the aim is to set parameters that enable UEs to find the correct power level quickly and reliably from any location in the cell. The key parameter in this respect is the Constant value, which is used to calculate initial power. If initial power is too high, mobiles’ RACH transmissions will cause significant interference to other traffic and signalling channels in the cell. This in turn will lead to higher average power being set by closed loop power control processes and a resulting loss in capacity. If initial power is too low, mobiles may need to transmit a large number of preambles before reaching a successful power level. In extreme cases there may also be a large number of aborted RACH attempts. The cumulative noise from a larger number of preamble transmissions will also result in higher levels of interference and in degraded capacity. An ideal setting would result in the first preamble failing to get a response from the cell with a positive response subsequently being returned in the AICH after the second preamble. An appropriate value for this condition will depend on radio conditions in the cell and the amount of activity required on the RACH channel. The number of preambles transmitted before a positive response is seen in the AICH is a good indicator of performance. This could be observed with an appropriate drive test tool used in the cell of interest. It should be set to make a large number of shortduration calls. If the first preamble consistently gets a response then the initial power is probably too high. In this case a decrease in the parameter Constant value should be considered. The opposite would be true for a consistently large number of preambles before a positive response is received. It is important that any assessment of the number of preamble attempts is averaged across a representative geographical area for the cell.
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Required power Single preamble successful Initial power too high Decrease Constant value Time
Required power Multiple preambles required Initial power too low Increase Constant value Time
Required power
Ideal Time
Figure 10 Setting the Constant Value SC2804/S5/v1.1
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CELL RESELECTION 4.1
Introduction
Once a UE is registered with a PLMN on a suitable cell it will assume idle mode and begin a monitoring process that may lead to cell reselection. This in turn may result in a location update, a routing area update, an UTRAN registration area update or a cell update depending on RRC connection status. This is because the UMTS idle mode cell reselection process is also used in the context of connected mode for certain types of packet data or signalling transactions. This occurs when the UE is in the CELL_FACH, CELL_PCH or URA_PCH states. There are many options for the way in which cell reselection is managed in UMTS. The mechanism used will depend on the architecture of the network and operator preferences. 4.2
Basic Cell Reselection Process
At rollout a UMTS network is likely to contain only a single cell layer. All UMTS cells would therefore be on the same frequency. This simplifies the cell reselection process in two ways. Firstly, the UE only needs to make intra-frequency UMTS measurements; and secondly there is no need to consider the hierarchical the cell reselection criteria. Although many networks will need to support inter-RAT reselection at rollout, typically to and from GSM, it is possible to manage this without use of the hierarchical cell structures. 4.2.1
Basic Measurement Rules
It is possible to limit the amount of neighbour cell measurement performed by the UE when the service from the current serving cell is adequate. This is controlled by setting the parameters Sintrasearch, Sintersearch and SsearchRATm. For UMTS FDD these parameters are applied by the UE, as shown in Figure 11. These are optional parameters and if they are not included in system information the UE will perform measurements on all indicated neighbour cells irrespective of the condition of the serving cell.
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Sintrasearch (–32* to 20 in steps of 2) Qqualmeas (–25 to 0) Calculate
Squal (1 to 25) Compare
Sintersearch (–32* to 20 in steps of 2) SsearchRATm (–32* to 20 in steps of 2, value 20 is interpreted as absent) * Negative values are considered to be 0.
Serving Cell (UMTS FDD)
UE Only perform intra-frequency measurements if: Squal • Sintrasearch Only perform inter-frequency measurements if: Squal • Sintersearch Only perform inter-RAT measurements if: Squal • SsearchRATm
Figure 11 Basic Measurement Rules SC2804/S5/v1.1
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4.2.2
Basic Cell Reselection Criteria
The UE tests all measured cells, including the current serving cell, against the cell selection criteria. All cells that meet the cell selection criteria are then ranked using the cell-ranking criterion ‘R’. A new cell will be selected if it is ranked higher than the current cell for a time interval defined by the parameter Treselection, and if the UE has been camped on the current serving cell for more than one second. Note that the values of Qqualmin, Qrxlevmin and UE_TXPWR_MAX_RACH used to calculate the cell selection criterion S can differ for each neighbour cell. This could be used to influence whether a neighbour cell is judged good enough to consider for reselection. However, these values apply to the neighbour cell for all cell selection circumstances, so should not be changed when trying to optimize only one reselection scenario. The key parameters for optimizing the cell reselection process for a single scenario are as used to calculate the ranking criterion ‘R’. The way that these parameters are applied is shown in Figure 12. (Note, Figure 12 only covers the case where all neighbour cells are UMTS FDD). The parameters used in the cell ranking criterion and reselection are as follows: • Rs – calculated ranking value for the serving cell • Rn – calculated ranking value for a neighbour cell • Qmeas,s – can be set as either Q
qualmeas
or Qrxlevmeas (serving cell)
• Qmeas,n – can be set as either Q
qualmeas
or Qrxlevmeas (neighbour cell)
• Qhysts – hysteresis value, 0 to 40 dB is steps of 2 • Qoffsets,n – offset value, –50 to 50 • Treselection – timer value, 0 to 31 seconds Note that a single value of Qhysts is set for the serving cell, but the value of Qoffsets,n can be set independently for each listed neighbour cell.
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Neighbour Cell Measurements (Based on measurement rules)
UE applies cell selection criterion ‘S’ Squal > 0 and Srxlev > 0 where: Squal = Qqualmeas – Qqualmin Srxlev = Qrxlevmeas – Qrxlevmin – Pcompensation
Cells meeting the ‘S’ criterion are ranked using the ranking criterion ‘R’ Rs = Qmeas,s + Qhysts Rn = Qmeas,n + Qoffsets,n
A neighbour cell is reselected if: it is ranked higher than the serving cell for a time greater than Treselection the UE has been camped on the current serving cell for at least one second
Figure 12 Basic Cell Reselection Criterion SC2804/S5/v1.1
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4.3
Basic Inter-RAT Reselection
Even without using reselection procedures for HCS it is possible to allow reselection of inter-RAT cells from UMTS FDD. There are two main cases for this, reselection to a GSM cell and reselection to a UMTS TDD cell. 4.3.1
Reselection to GSM
GSM neighbour cell information is included in system information. The downlink measurement used for assessment of a GSM cell is RSSI indicated in dB. There is no specific quality measure for a GSM cell, so the cell selection criterion is met for a GSM cell if Srxlev is greater than or equal to zero. Appropriate setting of SsearchRATm can also be used to control reselection to GSM since it can prevent GSM cells being considered when not required. The value of Qrxlevmin, used to calculate Srxlev, is set for individual cells. Therefore for a GSM cell it can be set to provide a weighting for or against performing an interRAT reselection. Similarly, the value MS_TXPWR_MAX is used to calculate Pcompensation, which could also be used to influence the likelihood of an inter-RAT reselection. Assuming neighbour cell measurements are performed on a GSM cell and that it meets the cell selection criterion, it must be ranked before it can be reselected. When a GSM cell is ranked the value of Qmeas,n will also be RSSI. Again, the value of Qoffsets,n is set individually for each neighbour cell and can be used to weight the likelihood of a GSM cell being reselected. 4.3.2
UMTS TDD Reselection
For UMTS TDD mode the cell selection criterion ‘S’ is based only on Srxlev being greater than or equal to zero. As for GSM, appropriate values of maximum transmit power and Qrxlevmin can be set to influence the value of Srxlev. This may prevent the TDD cell even being considered for reselection. In respect of ranking, the value of Q meas,n will be P-CCPCH RSCP and an appropriate value of Qoffsets,n can be set to weight the likelihood of reselection.
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Measurements: Qqualmeas Qrxlevmeas
Serving Cell (UMTS FDD)
Measurement: RSSI
UE uses RSSI for Qrxlevmeas and Qmeas,n
Neighbour Cell (GSM)
SsearchRATm – used to control ne ighbour cell measurements Qrxlevmin and Pcompensation – used to control neighbour cell measurements and consideration for ranking Qoffsets,n – used to control ranking of the considered neighbour cell
Figure 13 Reselection to GSM without HCS SC2804/S5/v1.1
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4.4
Reselection with Hierarchical Cell Structures (HCS)
The UMTS standards1 allow for the cell reselection procedures to be modified to account for the use of hierarchical cell structures. This is done in such a way that reselection depends on the relationship between the cell layer to which the serving and neighbour cell belong as well as the mobility of the UE. Whether reselection procedures with HCS are to be used or not is indicated in system information by the parameter Use of HCS. 4.4.1
HCS Priority Levels
In more mature UMTS systems it is likely that an operator will use a hierarchical architecture. Typically this may involve macro, micro and pico cells. Each HCS layer is allocated a priority level. The standards allow for up to eight priority levels numbered 0 to 7. HCS priority level 0 is lowest and HCS priority level 7 is highest. The highest priority level should be allocated to the smallest or the overlaid cells. Thus the example shown in Figure 14 has pico cells allocated level 7, micro cells allocated level 6 and macro cells allocated level 5. Inter-RAT cells can also be treated as HCS cell layers. Therefore GSM or TDD cells would also be allocated an HCS priority level. It is likely that this would be a lower priority than UMTS FDD macro cells.
1
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HCS Level 5
HCS Level 5
HCS Level 7
HCS Level 7 HCS Level 7
HCS Level 6 Level 5 – Macro cell Level 6 – Micro cell Level 7 – Pico cell
HCS Level 7
HCS Level 6 HCS Level 6
Figure 14 HCS Priority Levels SC2804/S5/v1.1
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4.4.2
HCS Measurement Rules for UMTS
As can be seen in Figure 15, measurement rules are changed for HCS intra- and inter-frequency measurements. Measurements are only performed on all listed neighbour cells if both if both the standard Sintersearch threshold and the additional HCS search threshold are passed. The threshold for HCS is called SsearchHCS. It can be assigned values in the range –105 to 91 dB in steps of 2 and specifies a minimum value of Srxlev before all neighbour cells are measured. If the two thresholds SsearchHCS and Sintersearch have not been passed, and also if the Sintrasearch threshold has not been passed, then the UE will only measure cells with a higher HCS priority level. For example if the UE was camped on a macro cell it might only be measuring neighbour cells that were micro or pico cells. If, in the above case, Sintrasearch is passed then the UE measures cells with an equal as well as those with a higher HCS priority level.
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If Squal • Sintersearch HCS The UE measures all listed neighbour cells
or if Squal > Sintersearch or Srxlev > SsearchHCS and Squal > Sintrasearch The UE measures only cells with a higher HCS priority than the current serving cell
or if Squal > Sintersearch or Srxlev > SsearchHCS and Squal Sintrasearch The UE measures only cells with a equal or higher HCS priority than the current serving cell
Figure 15 HCS Measurement Rules for UMTS SC2804/S5/v1.1
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4.4.3
Consideration of UE Mobility
Normal HCS reselection rules assume the UE is in a low-mobility state. If the UE is in a high-mobility state then the last two mentioned conditions are overruled. This state is determined using the parameters TCRmax and NCR. These two parameters are used to consider the UE’s recent history in terms of the number of reselections performed in a given time period (TCRmax). If the number of reselections performed in this period exceeds NCR then the UE considers itself in a high-mobility state. If the UE is in a high-mobility state then unless both the Sintersearch and the SsearchHCS have been passed it will measure neighbour cells with an equal or lower HCS priority level. The example shown in Figure 16 illustrates this. Consider that the UE is currently camped on a micro cell and that neither the SsearchHCS or the Sintersearch thresholds have been passed. If the UE was in a low-mobility state then it would only measure cells of equal or higher HCS priority level: in this case other micro or pico cells. However, if the UE was fast moving, perhaps in a vehicle, it is likely that there would have been several recent cell reselections. In this case the UE may have passed the threshold for high mobility. If the UE is in this state it will only measure cells of equal or lower HCS priority level.
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UE is high-mobility if more than NCR reselections have happened in the last TCRmax seconds 7
5
6 7
6
7 UE B 6 5 7
7 UE A
Mobile B – high mobility, measures only lower HCR priority cells
Mobile A – low mobility, measures only higher HCR priority cells
Figure 16 Consideration of UE Mobility SC2804/S5/v1.1
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4.4.4
Cell Reselection Criteria with HCS
The UE tests all measured cells, including the current serving cell, against the cell selection criterion. All those cells that meet the cell selection criterion are then further tested against the ‘H’ criterion. This criterion is used to determine if HCS cell reselection rules are to be applied. If there are cells that meet the ‘H’ criterion then from that set only those with the highest HCS priority level will be ranked for reselection. If no cells meet the ‘H’ criterion then all cells meeting the ‘S’ criterion are ranked as if HCS did not apply. As shown in Figure 17 the ‘H’ criterion uses the parameters Qhcss, Qhcsn, TOn and Ln. Qhcss/n is specified in system information for each cell and is the quality threshold for applying HCS prioritized ranking. The value range for Qhcss/n depends on the type of cell being considered: for UMTS FDD it is either –25 to 0 or –115 to –26 (dependent on quality measure), for UMTS TDD it is –115 to –26, and for GSM it is –110 to –37. TO n is a temporary offset used to control short-duration reselections. It is set individually for each cell and can have the value 3, 6, 9, 12, 15, 18, 21 or infinite. TOn is applied for a time set by the parameter PENALTY_TIMEn, which may have the value 0, 10, 20, 30, 40, 50 or 60 seconds. The parameter Ln may take the value 0 or 1 and is dependent on the HCS priority of the cell being considered. It is set to 0 if the neighbour cell’s HCS priority is the same as that of the serving cell, otherwise it is set to 1. A new cell will be selected if it is ranked higher than the current cell for a time interval defined by the parameter Treselection, and if the UE has been camped on the current serving cell for more than one second.
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Neighbour Cell Measurements (Based on HSC measurement rules) UE applies cell selection criterion ‘S’ Squal > 0 and Srxlev > 0 where: Squal = Qqualmeas – Qqualmin Srxlev = Qrxlevmeas – Qrxlevmin – Pcompensation
Cells tested against the ‘H’ criterion to determine if HCS prioritized ranking should apply Hs = Qmeas,s + Qhcss Hn = Qmeas,n + Qhcsn – (TO n x Ln)
Cells meeting the ‘S’ and ‘H’ criterion and with the highest HCS priority level are ranked using the ranking criterion ‘R’ Rs = Qmeas,s + Qhysts Rn = Qmeas,n + Qoffsets,n – (TO n x (1 – L n ))
A neighbour cell is reselected if: it is ranked higher than the serving cell for a time greater than Treselection the UE has been camped on the current serving cell for at least one second
Figure 17 Cell Reselection Criteria with HCS SC2804/S5/v1.1
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Key parameters for optimizing the cell reselection process for a single scenario are those used to calculate the HCS priority criterion ‘H’ and the ranking criterion ‘R’. These parameters are as follows: • Hs – calculated HCS priority applicability for the serving cell • Hn – calculated HCS priority applicability for a neighbour cell • Rs – calculated ranking value for the serving cell • Rn – calculated ranking value for a neighbour cell • Qmeas,s – can be set as either Qqualmeas or Qrxlevmeas (serving cell) • Qmeas,n – can be set as either Qqualmeas or Qrxlevmeas (neighbour cell) • Qhcss – HCS priority applicability threshold for the serving cell • Qhcsn – HCS priority applicability threshold for a neighbour cell • TOn – Temorary Offset, 3, 6, 9, 12, 15, 18, 21 or infinite • Qhysts – hysteresis value, 0 to 40 dB in steps of 2 • Qoffsets,n – offset value, –50 to 50 • Treselection – timer value, 0 to 31 seconds Note that a single values of Qhcss and Qhysts are set for the serving cell, but the values of Qhcs n, TO n and Qoffset s,n can be set independently for each listed neighbour cell.
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Neighbour Cell Measurements (Based on HSC measurement rules) UE applies cell selection criterion ‘S’ Squal > 0 and Srxlev > 0 where: Squal = Qqualmeas – Qqualmin Srxlev = Qrxlevmeas – Qrxlevmin – Pcompensation
Cells tested against the ‘H’ criterion to determine if HCS prioritized ranking should apply Hs = Qmeas,s + Qhcss Hn = Qmeas,n + Qhcsn – (TO n x Ln)
Cells meeting the ‘S’ and ‘H’ criterion and with the highest HCS priority level are ranked using the ranking criterion ‘R’ Rs = Qmeas,s + Qhysts Rn = Qmeas,n + Qoffsets,n – (TO n x (1 – L n ))
A neighbour cell is reselected if: it is ranked higher than the serving cell for a time greater than Treselection the UE has been camped on the current serving cell for at least one second
Figure 17 (repeated) Cell Reselection Criteria with HCS SC2804/S5/v1.1
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Inter-RAT Reselection with HCS
When reselection with HCS is used for inter-RAT reselections the other RATs are treated as additional HCS layers. Typically they would be given priority levels lower than that of the UMTS macro cells. However, the strategy for allocating HCS priority to different cell types is not defined and for most operators this may depend on the type of geographical area. There are two main cases for this, reselections to GSM cells and reselections to UMTS TDD cells. 4.5.1
Reselection to GSM with HCS
The measurement rules for GSM neighbours when HCS Prioritization is used are similar to those for UMTS neighbours. The UE only measures all listed GSM neighbour cells if both Srxlevs is less than the parameter SHCS,RATm and Squal is less than SsreachRATm. If this condition is not met but Squal is greater than Slimit,SearchRATm then the UE will not measure any GSM cells. If none of these conditions are met then the UE will only measure GSM cells with an HCS priority equal to or higher than the current serving cell. Appropriate setting of SHCS,RATm and Slimit,SearchRATm (both in the range –32 to 20 dB in steps of 2) can be used to control reselection to GSM since they can prevent GSM cells being considered when not required. Careful consideration of the HCS priority level allocated to GSM cells is also important. For example, it may be sensible to allocate a higher HCS priority to a GSM micro cell than to a UMTS macro cell in a shopping street where the majority of traffic is found to be voice or text messages. Once the measurement rules have determined that a GSM cell should be considered for reselection then all the same cell specific parameters as used for UMTS FDD cells are applied, i.e. those used for the ‘H’ criterion and the ‘R’ criterion. Appropriate setting of these will control the likelihood of reselecting of GSM cells. 4.3.2
UMTS TDD Reselection With HCS
Reselection of a UMTS TDD cell is treated as an inter-RAT reselection in exactly the same way as a GSM cell. The measurement parameter for a UMTS TDD neighbour is PCCPCH RSCP as opposed to RSSI for a GSM cell. Apart from that, all other procedure variations and parameters are the same as for GSM reselection.
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HCS Priority Level n, n 1 – or n+1
HCS Priority Level n Measurements: Qqualmeas Qrxlevmeas
Serving Cell (UMTS FDD)
Measurement: RSSI
UE uses RSSI for Qrxlevmeas and Qmeas,n SsearchRATm – used to control neig hbour cell measurements SHCS,RATm – used to control neig hbour cell measurements Slimit,SearchRATm – used to control neig hbour cell measurements
Neighbour Cell (GSM)
Qrxlevmin and Pcompensation – used to control neighbour cell measurements and consideration for ranking Qhcsn – used to control applicati on of HCS rules for reselection TOn – temporary offset to stop short term reselection Qoffsets,n – used to control ranking of the considered neighbour cell
Figure 18 Reselection to GSM with HCS SC2804/S5/v1.1
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EXERCISE 2 – CELL RESELECTION SCENARIOS Figure 19 is a map showing an area in a major city. The large road running diagonally from the top to the bottom of the map is a main route through the city. There are two lanes of traffic in each direction. It is always busy, but in normal conditions the traffic is fairly free running with an average speed of 40 kph. To the right of the main road is a popular shopping area. The main shops are located on the ground floors of buildings A, B, C and D. A number of these front onto area E, which is a square. Part of this contains grass and trees, the rest is an open paved area often used by street performers. Building A is a large hotel used by tourists and by business travellers. To the left of the main road, buildings G, H and F are offices used by several large companies. The area in the map and the surrounding areas are currently served by a threesector UMTS macro cell located on the roof of building A. The effective radius of this macro cell was planned to be approximately 2 km. When first implemented the macro cell was intended to operate with a maximum load factor of 65%. At first, this provided adequate capacity, but lately the number of UMTS subscribers has significantly increased for this operator. The load factor limit is frequently reached, resulting in a large amount of blocked traffic. The operator intends to build two micro cells in the map area to carry localized traffic and reduce load on the macro cell. The intention is to build one micro cell in the shopping area to serve the square and the hotel. The second micro cell is to be built in the office area to pick up traffic from building G,H and F. Task 1
Your group should suggest appropriate positions for the micro cells. A powerlaw propagation model and load factor of 75% suggests a radius for the micro cells of about 100 m with good building penetration.
2
The operator would like to utilize HCS prioritization for these micro cells. Your group should identify the key parameters that will need to be introduced for HCS to function effectively. For each parameter identify the considerations for selecting a value and, if possible, suggest a value that could be used. The operator is keen that UEs in vehicles on the main road should continue to be handled by the Macro cell and you should take this into account.
3
Your group should also consider if it would be possible to introduce the micro cells without utilizing HCS parameters. What would be the advantages and disadvantages of adopting this strategy?
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G
H
F
A Hotel
E Open Square
B
C
Scale
0
50 m
100 m
D Shops
Figure 19 Exercise 2 – Cell Reselection Scenarios SC2804/S5/v1.1
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RADIO RESOURCE CONTROL (RRC) FUNCTIONS 6.1
Introduction
RRC provides control of most radio access functions. A key aspect of this is access control. This enables the UMTS user to connect to the UMTS network in order to use UMTS services. Access control can be broken down into two main parts: admission control and congestion control. 6.1.1
Admission Control
The admission control function is located at the Controlling RNC (CRNC) or Serving RNC (SRNC). This depends on the admission function being performed. CDMA networks operate on a soft capacity concept; this means that new calls increase the interference level for all other calls. This affects the quality of all calls. Admission control provides the ability to admit or deny new users, new RABs, or new radio links. The decisions are based on QoS requirements, interference, current load conditions and resource measurements. 6.1.2
Congestion Control
Congestion control will monitor, detect and control situations when overload conditions occur. Congestion occurs when the network has run out of or is running out of resources. The function of congestion control is to bring the system back to a stable state (as quickly as possible).
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Core Network
QoS New users
RNC Node B
Radio Access Bearers
Broadcast system information
Node B
Uplink interference
Node B Node B Node B Downlink power
Handover resources (radio links)
Figure 20 Key RRC Functions SC2804/S5/v1.1
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6.2
Cell Access Restrictions
One strategy for managing traffic load on a cell is to use access class restrictions. This is not intended for general use but rather for exceptional or possibly emergency conditions. A blanket restriction may be placed on a cell by indicating in system information that it is barred. If this is done a parameter called T barred is included. This second parameter specifies the minimum time (in seconds) that a UE must wait before rechecking if the barring on a cell has been lifted. More subtle restrictions can be applied by barring individual or groups of access classes. A UE’s access class is stored on the SIM card. All subscribers are randomly allocated an access class in the range 0 to 9. The aim is that 10% of the subscriber population will belong to each access class. Some particular types of subscriber may also be allocated one or more of the special access classes from 11 to 15. This is also stored as a parameter on the SIM card. These classes are intended for use by groups as indicated in Figure 21. This second special access class is only applicable when a subscriber is in their home network. Although no subscribers are allocated access class 10, it may be used by the operator to restrict access for emergency calls. If access class 10 is barred then subscribers with access class 0 to 9 and any subscribers without an IMSI (i.e. no SIM card) may not make emergency calls. Subscribers in access classes 11 to 15 can still make emergency call unless their access class is also specifically barred.
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Public utilities Security services Emergency services
For PLMN use
PLMN staff
Emergency calls Randomly allocated to all users Access classes 0 to 9
May be individually barred
10
11
12 13 14 15
May be individually barred Used to restrict emergency calls
Figure 21 Access Classes SC2804/S5/v1.1
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6.2.1
Considerations for the use of Reserved Cells
Typically, reserved cells may be used to test potential sites for new base stations or to test system performance in respect of the introduction of new services. Additionally, existing cells may be temporarily put into this state for operational reasons. There are two possibilities for cell reservation. System information may indicate that the cell is ‘reserved for operator use’ or it may indicate that the cell is ‘ reserved for future extension’. In the case of reserved for operator use, UEs of access classes 11 or 15 and those for whom this is the HPLMN will treat the cell as normal for all processes relating to cell selection and reselection. UEs with lower access classes and access classes 12 to 14 will treat the cell as barred. In the case of reserved for future extension, all UEs will treat the cell as barred. This function is useful for testing purposes and for the integration of new cells. However, caution must be exercised with its use because it may limit normal service provision in the vicinity of the reserved cell. Consider the UE shown in Figure 22. The cell in the example is marked as reserved for operator use. The UE has access class 9 so it cannot access the reserved cell. If the UE was to select another bordering cell on the same frequency as its serving cell there is a possibility that it could cause excessive interference to the reserved cell. To prevent this the intrafrequency cell reselection indicator may be set to ‘not allowed’. The UE will be forced to search for a neighbour cell on another frequency. There are two potential problems with this. Firstly another frequency implies another HCS layer, which may be inappropriate for the UE’s position or mobility state. Secondly, if the UE cannot find a suitable cell on another frequency it will camp on an acceptable cell and assume the limited service state. Thus service could be denied to the user until the UE moves to another position even though there is not a problem with coverage in the area.
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Reserved Cells may be: Reserved for operator use Reserved for future extension
Possible interference Serving Cell
Reserved Cell Reserved for operator use
UE access class 9
Figure 22 Use of Reserved Cells SC2804/S5/v1.1
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6.3
Admission Control
The aim of admission control is to meet the QoS requirements of services provided with new channel requests and at the same time to control the load in the UTRAN. Each new connection contributes to the noise rise in the serving cell and in neighbour cells. Admission control will need to estimate how much interference increase there will be for a new request and how this will impact other connections already established in the UTRAN. There are many aspects to this and the algorithms used are not defined in the UMTS standards. However, key parameters are: • service mapping to channel types • service mapping to QoS classes • total downlink transmit power • maximumm channel code power • maximum allowable uplink transmit power • maximum load factor • current load • UE capability • code availability (OVSF code tree) • channel element availability Each of these factors, and perhaps others, will need to be included in the admission control process when it is used to determine if a newly-requested connection can be supported. Some or all of these factors may be available to the optimizer as adjustable parameters, or as new features that could be implemented. However, care should be taken when changing any of these parameters since they are likely to have been an input to the planning process. Significant changes in these values will need extensive simulation to assess any likely wanted or unwanted effects.
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Connection request
service mapping to channel types service mapping to QoS classes total downlink transmit power maximum channel code power maximum allowable uplink transmit power maximum load factor current load UE capability code availability (OVSF code tree) channel element availability
Figure 23 Admission Control SC2804/S5/v1.1
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6.3.1
Call Admission Control (CAC) Prioritization
The way in which different traffic types are prioritized for consideration by admission control is an implementation issue. It may be subject to optimization, but this will be a collaborative process involving many aspects of system and service package design. However, once the requirements are set it will be a key aim of the overall radio network optimization process to see that these requirements are met for as much of the time as possible. 3GPP 25.9221 provides examples of how service classes may be utilized. One of these is shown in Figure 24. Allocation of resources would be prioritized according to the QoS requirements associated with the requested service. Typically it is assumed that connections for circuit-switched services will have priority over those for packetswitched services. In respect of packet-switched connections, those needing realtime delay constraints will have priority over those requesting non-real time. It is important to ensure that services are matched appropriately to available QoS classes. If this is not considered resource utilization may be inefficient. For example, it may be desirable from a user-experience point of view to provide a web browsing facility based on a guaranteed bit rate utilizing the conversational QoS class. In the early operational stages of a UMTS network this should not cause a problem since there will be ample capacity on most cells. Yet, as network load increases, new connection requests for the interactive class may be refused because a guaranteed bit rate cannot be provided within the limits set for load factor. As illustrated in Figure 24, if the web browsing service was mapped to the background or perhaps the interactive QoS class it may not be refused even at high traffic loads. This is because these classes will use spare capacity on the cell and resources are allocated dynamically without guaranteed bit rate or delay. For a service such as web browsing this would be acceptable.
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3GPP TS 25.922 Radio Resource Management Strategies.
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Example Service Type Mapping to QoS Class (3GPP 25.922) Type of Service QoS Class Premium Conversational Assured service Streaming Best effort Interactive/background
Delay Low Medium –
Guaranteed bit rate Yes Yes No
Example Service Mapping to Service Type and CAC Strategy (3GPP 25.922) Service Voice Web
Load factor
CN Domain CS PS PS PS
Type of Service Premium Premium Assured service Best effort
CAC performed Yes Yes Yes No
Target load factor Available for connections needing best effort
Time
Used by connections needing realtime guaranteed bit rates
Figure 24 Call Admission Control (CAC) SC2804/S5/v1.1
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SECTION 6
CONNECTED MODE AND RADIO LINK CONTROL
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SECTION CONTENTS 1
RRC CONNECTED MODE STATES 1.1 States for Circuit-Switched Operation 1.2 States for Packet-Switched Operation 1.3 UE Activity in Connected Mode
6.1 6.1 6.1 6.3
2
Channel Types 2.1 Channel Allocation and Dimensioning 2.2 RACH for Data Transfer 2.3 CPCH for Data Transfer 2.4 FACH for Data Transfer 2.5 DSCH for Data Transfer 2.6 DCH for Data Transfer
6.7 6.9 6.11 6.11 6.11 6.13 6.13
3
Power Control 3.1 Measurements for Power Control 3.2 Power Control for CPCH 3.3 Closed Loop Power Control 3.4 Outer Loop Power Control 3.5 Power Offsets 3.6 Transmit Power Control Headroom 3.7 Power Control in Soft Handover 3.8 Processing TPC Bits from a Single Physical Channel 3.9 Processing TPC Bits From Multiple Physical Channels
6.15 6.15 6.17 6.19 6.25 6.27 6.29 6.31 6.33 6.35
4
Exercise 1 – Power Control Scenarios
6.37
5
Soft Handovers 5.1 Measurements for Handover 5.2 Neighbour Cells for Soft Handover 5.3 Considerations for Active Set Size 5.4 Configuring the Measurement Message 5.5 Soft Handover Parameters and Triggers 5.6 Optimizing Soft Handover Regions 5.7 Inter-Cell Synchronization
6.39 6.39 6.41 6.43 6.45 6.47 6.55 6.71
6
Hard Handovers 6.1 Compressed Mode Measurements 6.2 Neighbour Cells for Hard Handover 6.3 Hard Handover Parameters and Triggers
6.73 6.73 6.79 6.81
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SECTION OBJECTIVES At the end of this section you will be able to: • • • • • • • • • • • •
list and describe the RRC states within RRC connected mode suggest appropriate channels for user traffic transfer with different QoS requirements describe User Equipment (UE) activity while in connected mode, both for packet data transfer and circuit-switched data transfer summarize the key characteristics, configurations, capabilities and capacity of each traffic-carrying channel available on a UMTS air interface describe the measurement process for assessing the requirement for power control and handover commands describe the operation and controlling parameters for closed loop power control characterize the effect of each parameter relating to closed loop power control describe the operation of soft and hard handovers in UMTS characterize the effect of each parameter relating to hard and soft handover control describe the operation of and options for compressed mode describe how the soft and hard handover processes may relate to the use of hierarchical cell layers describe the interactions between UMTS and GSM/GPRS in respect of hard handover
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RRC CONNECTED MODE STATES The UE will enter the RRC Connected state by executing the RRC Connection Procedure. This will place the UE in a connected mode state. One connected-mode state may be used for circuit-switched operation, i.e. the CELL_DCH state, while there are four possible connected mode states for packet-switched operation: CELL_DCH, CELL_FACH, CELL_PCH and URA_PCH. 1.1
States for Circuit-Switched Operation
For circuit-switched operation the UE will be placed in the CELL_DCH state. Here, the UE will be using DCCH or DTCH logical channels mapped to dedicated transport channels carried by dedicated physical channels. The UE will be known to the UTRAN at cell level according to its current active set. 1.2
States for Packet-Switched Operation
For packet-switched operation the UE may be placed in the CELL_FACH state. No dedicated channels will be assigned to the UE, but common transport channels such as RACH/FACH and CPCH/FACH may be used. The UE will be known to the UTRAN at cell level and will perform cell updates. The UE may be placed into the CELL_PCH state to avoid the need to constantly monitor the downlink FACH channel. This will allow the UE to use Discontinuous Reception (DRX), prolonging battery life. The only way the UTRAN can reach the UE is by paging it at a cell level. Therefore the UE must still perform cell updates involving the transition to the CELL_FACH state. To minimize the number of cell updates the UE may be placed into the URA_PCH state. Within a UTRAN Registration Area (URA) the UE may perform cell reselection without performing a cell update unless a cell belongs to another URA. This will invoke the URA Update procedure carried out in CELL_FACH state. To reach a UE the UTRAN will page across the URA. For large volumes of packet data the UE may be placed in the CELL_DCH state. The UE will be known to the UTRAN at a cell level, but the UTRAN will control which cells are to be used based upon the measurement information supplied by the UE.
6.1
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CELL_DCH
CELL_FACH
CELL_PCH
URA_PCH
Figure 1 RRC Connected Mode States SC2804/S6/v1.1
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1.3
UE Activity in Connected Mode
1.3.1
CELL_DCH
In the CELL_DCH state dedicated resources are allocated to the UE for circuitswitched data or packet-switched data. For packet-switched services the current uplink and downlink data rates are indicated by the Transport Format Combination Indicator (TFCI). If the allocated resources are insufficient to match the QoS requirement the network will initiate a reconfiguration of the transport format. In the uplink direction the UE can report the observed traffic volume to the network in order for the network to re-evaluate the current allocation of resources. In this way the UE connected state may change from CELL_DCH to CELL_FACH or vice versa. In the CELL_DCH state the UE performs measurements and transmits measurement reports based upon the measurement control information. Certain FDD UEs can read system information in the CELL_DCH state using FACH. 1.3.2
CELL_FACH
In this state no dedicated resources are allocated to the UE. Instead the UE monitors the downlink FACH continuously. The UE may be assigned a common transport channel, e.g. RACH or CPCH, which can be used at any time. Before data is transmitted in the uplink direction the UE reports the observed traffic volume to the network in order for the network to re-evaluate the current allocation of resources. A selection procedure then determines whether the information should be sent on a common transport channel or whether a transition to CELL_DCH is required. In the CELL_FACH state the position of the UE is known at cell level. The cell update procedure must be executed if the UE reselects a new cell. Data transmission in the downlink direction can begin without prior paging. The UE will monitor system information broadcasts on BCCH. The measurement control information broadcast on BCCH informs the UE about measurements and reporting.
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CELL_DCH dedicated resources for PS and CS data resources allocated to match QoS requirements UE reports observed traffic volume UE performs measurements and transmits reports certain UEs read system information on FACH
CELL_FACH no dedicated resources UE constantly monitors FACH UE reports observed traffic volume before data transmission UE known at cell level and performs cell updates UE reads system information on BCH
Figure 2 CELL_DCH and CELL_FACH States SC2804/S6/v1.1
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1.3.3
CELL_PCH
In the CELL_PCH state no resources are allocated to the UE. The UE may perform DRX and must be paged in the cell to initiate downlink data transfer. Uplink activity will trigger a move to the CELL_FACH state. The UE will be known to the UTRAN on a cell level, therefore the UE must perform the cell update procedure when reselecting a cell. This will be done in the CELL_FACH state. To reduce the number of cell updates the UE may be ordered to the URA_PCH state. This will be done while in the CELL_FACH state. It can be based on an inactivity timer, and optionally a counter. The UTRAN can order the UE to the URA_PCH state when the number of cell updates exceeds a certain threshold. The UE performs measurements and transmits measurement reports according to the measurement control information. The UE will also read system information broadcasts on the BCH. 1.3.4
URA_PCH
In the URA_PCH state no resources are allocated to the UE. For data transmission a transition to the CELL_FACH state is required. The UE may use DRX and must be paged across the URA to initiate downlink data transfer. Uplink activity will trigger a move to the CELL_FACH state. The UE will be known to the UTRAN at the URA level. If the UE selects a cell belonging to another URA it must perform the URA update procedure in the CELL_FACH state. The UE performs measurements and transmits measurement reports according to the measurement control information.
6.5
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CELL_PCH no dedicated resources allocated UE may implement DRX UE performs cell updates UE paged at cell level UE moves to CELL_FACH for data transfer UE performs measurements and transmits reports
URA_PCH no dedicated resources allocated UE may implement DRX UE paged over URA UE performs fewer cell updates UE moves to CELL_FACH for data transfer UE performs measurements and transmits reports
Figure 3 URA_PCH and CELL_PCH States SC2804/S6/v1.1
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2
CHANNEL TYPES There are a variety of channel types available for data transfer. The choice largely depends upon the QoS requirements. In the UMTS QoS concept four traffic service classes are defined: • conversational • streaming • interactive • background The different classes indicate the sensitivity to delay where conversational class is the most delay sensitive and background class is the least delay sensitive. Conversational and streaming classes are intended to carry Real-Time (RT) services and may be circuit switched or packet switched. Interactive and background classes are intended to carry Non-Real-Time (NRT) services over a packet-switched connection. The data channels for the UMTS air interface can be divided into Dedicated, Common and Shared. Dedicated Channels (DCH) resemble circuit-switched connections and are therefore most suitable for real-time services. Common Channels (RACH/FACH/CPCH) are contention based and are therefore most suitable for the bursty packet data found with some non-real-time services. The shared channels (DSCH) have similar properties to both common and dedicated channels but are likely to be used for non-real-time services.
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Sensitivity To Delay Conversational Class
Streaming Class
Interactive Class
Real-Time Services
Dedicated Channels
CS/PS
DCH Shared Channels
NonReal-Time Services
Background Class
PS
Traffic Service Class
Service Type
Common Channels
DSCH
RACH FACH CPCH
Transport Channels
Figure 4 Channel Types SC2804/S6/v1.1
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2.1
Channel Allocation and Dimensioning
The DSCH and the CPCH are optional channels leaving the RACH/FACH pair for non-real-time services. The RACH transport channel is carried uplink over the air interface in the PRACH. There are only 16 PRACH scrambling codes available per cell, which limits the number of RACH/FACH pairs that could be used for non-realtime services. The DCH may support real-time and non-real-time services. There are a large number of UE scrambling codes; the only limitation is the number of available spreading codes. There are few low spreading factor codes and a larger number of high spreading factor codes. The allocation of codes is the responsibility of the RNC. The selection of channel type is done by the RNC and is based upon: • QoS • data volume • traffic load • interference level • performance of the transport channel
6.9
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DSCH + CPCH optional Leaving
RACH/FACH non real-time services
DCH real-time services
16 RACH/FACH pairs
Large number of codes
? Choice performed by RNC based upon QoS data volume traffic load
interference level performance of transport channel
Figure 5 Channel Allocation and Dimensioning SC2804/S6/v1.1
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2.2
RACH for Data Transfer
The RACH is an uplink common transport channel paired with the FACH in the downlink direction. This would be used in the CELL_FACH state to transfer small volumes of packet data. The duration of transmission can be set to 10 ms or 20 ms depending upon cell size. For large cells the 20 ms setting corresponds to a bit rate of 8 kbit/s giving a good processing gain as is required for mobiles at the cell edge. However, open loop power control is used on RACH and a 20 ms burst will generate more noise in a cell compared to 10 ms. As well as the absence of fast power control, soft handover is not supported. There are also a limited number of fixed spreading factor codes for the RACH. However, the RACH offers short set-up times. 2.3
CPCH for Data Transfer
The CPCH is an uplink common channel paired with FACH in the downlink direction. This would be used in the CELL_FACH state to transfer small to medium volumes of packet data. The duration of transmission is controlled by the RNC and a number of fixed spreading codes are available in the cell. There is a limit on the number of Physical Common Packet Channel Channel scrambling codes of up to 64 in a cell. The CPCH supports fast power control and offers fast set up time, but does not support soft handover. It therefore only offers reasonable radio performance. 2.4
FACH for Data Transfer
The FACH is a downlink common channel that is usually paired with RACH or CPCH. It offers a fixed number of channelization codes and is best suited to carrying small volumes of bursty data. The absence of fast power control and soft handover means its radio performance is low.
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RACH
CPCH
FACH
DSCH
DCH
Channel Type
Common
Common
Common
Shared
Dedicated
UE State
CELL_FACH
CELL_FACH
CELL_FACH
CELL_DCH
CELL_DCH
Direction
UL
UL
DL
DL
DL/UL
Power Control
Open loop
Closed loop
Open loop
Closed loop
Closed loop
SHO
No
No
No
No
Yes
Data Volume
Small bursty
Small/medium bursty
Small bursty
Medium/high bursty
Medium/high prolonged
Set-up Time
Low
Low
Low
High
High
Radio Performance
Poor
Medium
Poor
Medium
Good
Figure 6 Channels for Data Transfer SC2804/S6/v1.1
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2.5
DSCH for Data Transfer
The DSCH would be used in the CELL_DCH state. Channelization codes would be shared between a number of users allowing a larger number of users to access highbit-rate services. The DSCH is paired with a DCH downlink offering fast power control with the UEs using a DCH uplink. The DSCH is best suited to medium to high data volumes that are bursty in nature, but it demands a longer set-up time. The absence of soft handover gives a medium radio performance. 2.6
DCH for Data Transfer
The DCH is used in the CELL_DCH state. The DCH is used in both uplink and downlink directions and supports a wide variety of bit rates. It supports fast power control and soft handover, and is ideal for carrying medium to large volumes of data. The DCH offers good radio performance but it is not suited to bursty data.
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RACH
CPCH
FACH
DSCH
DCH
Channel Type
Common
Common
Common
Shared
Dedicated
UE State
CELL_FACH
CELL_FACH
CELL_FACH
CELL_DCH
CELL_DCH
Direction
UL
UL
DL
DL
DL/UL
Power Control
Open loop
Closed loop
Open loop
Closed loop
Closed loop
SHO
No
No
No
No
Yes
Data Volume
Small bursty
Small/medium bursty
Small bursty
Medium/high bursty
Medium/high prolonged
Set-up Time
Low
Low
Low
High
High
Radio Performance
Poor
Medium
Poor
Medium
Good
Figure 6 (repeated) Channels for Data Transfer SC2804/S6/v1.1
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3
POWER CONTROL In a CDMA system, where all the users share a common frequency, interference control is of paramount importance. From an uplink point of view the near–far effect must be controlled to avoid weaker, distant users being drowned out by nearer, stronger users. From a downlink point of view, system capacity gains can be achieved by ensuring that downlink channels use the minimum required power. Power control in UMTS is divided into open loop power control and closed loop power control. The open loop mechanism is used to set the initial transmit powers on cell access. Closed loop power control (often called fast power control) dynamically changes the transmit power levels 1500 times a second. Closed loop power control consists of inner loop and outer loop processes. 3.1
Measurements for Power Control
There are two measurements required for power control. Firstly, the UE must measure and report the RSCP. measured on the CPICH. This is defined as the received power on one code measured on the PCPICH, the reference point being the antenna connector of the UE. Secondly, the Node B must measure the received total wideband power, which is the wideband power including noise generated in the receiver. The reference point is the receiver antenna connector.
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UMTS Power Control
Closed Loop
Open Loop
Sets initial transmit power levels
Outer Loop
Inner Loop
Sets SIRTargets
Perform fast power control
Figure 7 Power Control SC2804/S6/v1.1
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3.2
Power Control for CPCH
The CPCH is an uplink contention-based channel that uses open loop power control initially before the fast closed loop process takes over. The first part of the CPCH access procedure is identical to the RACH procedure. The UE first determines the Preamble_Initial_Power which will set the initial open loop power level for the first CPCH access preamble (PRACH = PCPCH). Successive access preambles will be transmitted at an increased power level determined by the power ramping factor Power_Ramp_Step, ΔP0, an integer value > 0. Once the UE has received a positive acknowledgement on the AICH, a collision detect preamble is transmitted at the same power level as the successful access preamble. If this is not positively acknowledged the CD-preamble power is ramped up by ΔP1 and this new transmit power level is used in place of Power_Ramp_Step (ΔP0) for a time period. On receipt of the Postive acknowledgement on the CD/CA-ICH channel the CPCH procedure may enter the power control preamble phase or begin transmitting the CPCH. In either case the transmit power level is increased by a power offset measured in dB. (Pmessage-control – Pcd). The purpose of the power control preamble is to rapidly adjust the transmit power level to the optimal setting using the fast power control algorithms with different step sizes. Only the control part of the PCPCH is affected. The normal step sizes will be used after the first eight slots or if the power control command reverses for the first time. Then both control and data parts will be power controlled by the normal closed loop process. After the first slot in the power control preamble, changes in the control part of PCPCH will be determined by: ΔPCPCH-CP = ΔTPC-init x TPC_cmd Using power control algorithm 1, ΔTPC-init is equal to the minimum value out of 3 dB and 2ΔTPC. Using power control algorithm 2, ΔTPC-init is equal to 2dB. TPC-cmd is derived according to algorithm 1 irrespective of which algorithm is used.
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Power ΔPp-m ΔP1 ΔP0 ΔP0
PRACH
Data Part Control Part
Access preamble
CD-preamble
CDpreamble in case of no ACK
Power control preamble
Time
0 or 8 slots PRACH = PCPCH = Initial open loop power level for first CPCH access preamble. ΔP0 = Power step size for each successive CPCH access preamble. ΔP1 = Power step size for each successive CPCH access preamble in case of no AICH response. Only valid for a time period before being replaced by ΔP0. ΔPP-M = Pmessage-control – Pcd, measured in dB. Power offset between CD-preamble and the initial transmit power of the CPCH power control preamble (or the control part if no power control preamble).
Figure 8 CPCH Power Control SC2804/S6/v1.1
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3.3
Closed Loop Power Control
Closed loop power control operates in both uplink and downlink directions. 3.3.1
Uplink Power Control
The initial uplink DPCCH transmit power is set by higher layers based upon the following expression: DPCCH_Initial_power = DPCCH_Power_offset – CPICH_RSCP Where CPICH_RSCP is measured and reported by DPCCH_Power_offset is calculated in the RNC from the following:
the
UE
and
DPCCH_Power_offset = CPICH_TX_power + UL interference + SIRDPCCH – 10log(SFDPCCH) Where the UL interference is the received total wideband power. SIRDPCCH is the SIR value determined by the RNC and SFDPCCH is the spreading factor for DPCCH. Because the spreading factor for DPCCH and DPDCH is not necessarily the same, gain factors are applied to each channel to balance the power allocated to each channel. βc is the gain factor for DPCCH and βd is the gain factor for DPDCH The gain factors can either be signalled to the UE from higher layers for a certain transport format combination or calculated by the UE.
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DPCCH_Initial_power = DPCCH_Power_offset - CPICH_RSCP where CPICH_RSCP is measured and reported by the UE and DPCCH_Power offset = CPICH_TX_power + UL interference + SIRDPCCH - 10log (SFDPCCH) where UL interference = total wideband power SIRDPCCH = SIR determined by RNC SFDPCCH = Spreading factor
Figure 9 UL Closed Loop Power Control SC2804/S6/v1.1
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3.3.2
Uplink Power Adjustment
The uplink inner loop power control adjusts the UEs transmit power to keep the received uplink SIR equal to the SIR target, SIRtarget. The SIRtarget is set by the RNC using the outerloop power control process and signalled to the Node B. Higher layer signalling will inform the UE of which power control algorithm should be used. Additionally higher layer signalling will indicate the TPC-Step Size which is used to set TPC. If the TPC-StepSize is ‘dB1’ then TPC is set to 1 dB. If the TPCStepSize is set to ‘dB2’, then TPC is set to 2 dB. After determining the TPC-cmd from the TPC bits transmitted downlink, the UE will alter its transmit power according to the following: ΔDPCCU = ΔTPC x TPC – cmd
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x TPC_cmd where TPC_cmd = +1, –1, 0 Figure 10 UL Closed Loop Power Control SC2804/S6/v1.1
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3.3.3
Downlink Power Control
The downlink inner loop power control adjusts the Node B’s transmit power in order to keep the downlink SIR equal to the SIRtarget. The outer loop process sets the SIRtarget which is given to the UE by higher-layer signalling. The UE estimates the received downlink power on the DPCH and estimates the level of interference in the cell. The UE can then determine the SIRest. If SIRest > SIRtarget set the TPC command to ‘0’ indicating power down. If the SIRest < SIRtarget set the TPC command to ‘1’ indicating power up. Within the Node B the TPC commands are interpreted depending upon the parameter DPC_MODE. If DPC_MODE is set to 0 the TPC commands from the UE will be estimated TPCest to be 0 or 1 and will change the power in every slot. If the DPC_MODE is set to 1 the Node B will estimate the commands over three slots to be 0 or 1 and will change power every three slots. The power control step size ΔTPC can be set to 0.5, 1, 1.5 or 2 dB. A value of 1 dB is mandatory.
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SIRest
SIRtarget
Figure 11 DL Closed Loop Power Control SC2804/S6/v1.1
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3.4
Outer Loop Power Control
The uplink and downlink outer loop power control is executed by vendor-specific algorithms. The uplink procedure is executed in the SRNC and is responsible for setting the SIRtarget in the Node B for each individual closed loop power control process. The downlink procedure is performed in the UE for each transport channel. The SIRtarget changes as the UE speed and multipath propagation environment change. The greater the variation in received power level the greater the SIRtarget needs to be. The target is determined according to the estimated link quality, based upon BER or Block Error Rate (BLER). A Cyclic Redundancy Checksum (CRC) could be used to determine if the target should be increased or decreased, e.g. if the CRC is OK the target can be lowered, otherwise it is increased. Suggested values for the step size range from 0.1 to 1.0 dB
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TPC Transmits Power Control Commands Measurement reports layer 3 information
SIRtarget
TPC TPC Inner Loop
Measurement report required
Outer Loop
Outer loop power control sets Signal-to-Interference Ratio (SIRtarget) Inner loop power control in Layer 1 adjusts peer entity transmit power so that the measured SIR fulfills SIRtarget requirements
Figure 12 Closed Loop Power Control SC2804/S6/v1.1
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3.5
Power Offsets
When a UE is in soft handover it can be proven that allocating more power to the TPC bits in the downlink DPCCH relative to DPDCH can result in 0.4 to 0.6 dB reduction in UE transmit power. The reason for this is that allocating proportionally more power to the TPC bits improves the power control signalling quality. There are three power offsets that can be applied to the downlink DPCH: PO1 sets the offset between DPDCH and the TFCI bits, PO2 is the offset for the TPC bits, and PO3 sets the offset for the pilot bits.
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PO1 TFCI
PO2 TPC
PO3 Pilot
Data 1 DPDCH
Data 2 DPCCH
DPDCH
DPCCH
Figure 13 Power Offsets SC2804/S6/v1.1
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3.6
Transmit Power Control Headroom
When determining the maximum cell radius in a CDMA system the maximum uplink path loss is used. This allows the link budget to be set and a fast fade margin to be added. Fast power control in UMTS is able to follow the fast fading envelope, particularly at low terminal speeds. However, at the cell edge the mobile will be transmitting maximum power, i.e. with no headroom. For all other positions in the cell the mobile will be transmitting power levels lower than the maximum so there will be varying degrees of headroom. This simplistic view does not reveal one of the hidden benefits of fast power control. Simulations have shown that for a speech service with a BLER of 1% without fast power control the required Eb/No for a pedestrian at 3 km/h is 11.3 dB. With fast power control the required Eb/No is 5.5 dB. The difference of 5.8 dB is known as the ‘fast power control gain’ or ‘power control headroom’. This can be translated into a 3.6 dB reduction in transmitted power. This gain diminishes with increasing speed because of the inability to follow the fading profile. A better definition of transmit power control headroom is: TPC headroom = average required received Eb/No without fast power control – average required received Eb/No with fast power control
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Max power =TX power
Node B
Max power
Max power Headroom
Headroom TX power
TX power
Figure 14 Power Control Headroom SC2804/S6/v1.1
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3.7
Power Control in Soft Handover
When a UE is in soft handover TPC bits are sent uplink and read by all the Node Bs engaged in the soft handover. If there are signalling errors on the air interface, one Node B may interpret the TPC bits differently to another Node B. This may result in one Node B powering up and another powering down. The resulting difference in power levels is known as ‘power drifting’. This has the effect of degrading the soft handover gain. One way of combating power drifting is for the Node Bs to average the transmission code power levels of the connections engaged in soft handover and pass them to the RNC. The time used for the averaging process is the measurement reporting period, typically set to 500 ms. From these measurements the RNC calculates a reference power level which is sent to all the cells concerned. The Node Bs will then use this to calculate a small power adjustment towards the reference value, thereby reducing the power drifting.
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Average transmission code power TPC commands
TPC commands
Node B powers up
Power drifting
Calculate power reference levels sent to all Node Bs involved in soft handover
Node B powers down
Average transmission code power
Figure 15 Power Drifting SC2804/S6/v1.1
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3.8
Processing TPC Bits from a Single Physical Channel
When the UE is not engaged in a soft handover it will be required to process only one set of TPC bits in each slot. The TPC bits must be processed in order to derive a value for TPC_cmd. There are two algorithms defined for this purpose. The UTRAN will indicate in higher-layer signalling which of the two algorithms should be used. 3.8.1
Algorithm 1 for Single Channels
In this case, the TPC bits are interpreted as either 1 or 0. These values are then directly mapped to values for TPC_cmd of +1 and –1 respectively. 3.8.2
Algorithm 2 for Single Channels
In this case, the algorithm represents an amalgam of five consecutive slots. For the first four received slots the value of TPC_cmd is set at zero, irrespective of TPC bit values. On receipt of the fifth slot, the five consecutive slots are considered together. TPC_cmd will only be set as +1 or –1 if all five slots are 1 or 0 respectively, otherwise TPC_cmd remains set at zero. Thus a power control command is implemented only three times in each frame.
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0
TPC
1
2
3
TPC
4
TPC
5
TPC
6
13
TPC
14
TPC_cmd
11111
TPC_cmd = +1
00000
TPC_cmd = –1
XXXXX
TPC_cmd = 0
Figure 16 Algorithm 2 for Single Channels SC2804/S6/v1.1
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3.9
Processing TPC Bits From Multiple Physical Channels
When the UE is engaged in a soft handover it will be receiving TPC bits from more than one Node B. These TPC bits will need to be combined in order to derive a value for TPC_cmd. The two defined algorithms are also used for this purpose. 3.9.1
Algorithm 1 for Multiple Channels
In each slot period, soft decision decoding is used to read the TPC bits from each Node B in the active set. The value of TPC_cmd will be set to 1 only if all the Node B TPC bits are 1; otherwise, the value of TPC_cmd will be set to –1. 3.9.2
Algorithm 2 for Multiple Channels
This process is outlined in Figure 15. In each slot period the TPC bits are decoded for each of the active set Node Bs. This is repeated for five consecutive slot periods. On reception of the fifth slot all five slots are considered such that for each Node B a value of TPC_tempi is determined. The value of TPC_tempi will be set at +1 or –1 only if the five consecutive slots are all 1 or all 0 respectively; otherwise, it will take the value 0. The second step is for the UE to combine the values for TPC_tempi into one value for TPC_cmd. This is done using the relationship shown in Figure 15. The whole process is repeated for each group of five slots, i.e. three times in each frame.
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0
1
2
3
4
5
6
7
TPCi TPC TPC TPC TPC From Node B1
8
9
10
11
12
13
14
TPC_temp 1
i=1 TPCi TPC TPC TPC TPC From Node B2 i=2
TPC_temp 2
.. .. .
TPCN TPC TPC TPC TPC From Node BN
TPC_cmd
TPC_temp N
i=N
1 1 1 1 1 – TPC_temp = +1 0 0 0 0 0 – TPC_temp = –1 X X X X X – TPC_temp = 0
TPC_cmd is set to 1 if TPC_cmd is set to –1 if
N
1 x N
i=1
1 x N
Σ
Σ N
TPC_tempi > 0.5 TPC_tempi < –0.5
i=1
Otherwise, TPC_cmd is set to 0 N = Number of cells
Figure 17 Algorithm 2 for Multiple Channels SC2804/S6/v1.1
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EXERCISE 1 – POWER CONTROL SCENARIOS Power Control Exercise A class 4 UE establishes a connection in a cell. Based upon the following parameters determine the uplink transmit power after 60 seconds of operation. At the end of this time period there is an excess of power down commands equivalent to 0.05% of the total number of commands. Class 4 UE CPICH Tx Power Target SIR UL Interference Spreading Factor CPICH RSCP Power Step Size
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= 21 dBm = 33 dBm = 9 dB = –98 dBm = 64 = –91 dBm = 1 dB
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5
SOFT HANDOVERS In a CDMA system the base stations are likely to be on the same frequency. It is therefore possible for a UE to set up communication links with a number of base stations simultaneously and be power controlled by all the base stations. This is known as a Soft Handover (SHO) and facilitates the movement of a connected mobile. There are two types of SHO defined for UMTS, a soft handover and a softer handover. The former is when the UE is communication with two or more Node Bs and signalling and traffic is sent to the RNC for combining. This combining process is known as macrodiversity. A softer handover occurs when a UE is in soft handover between cells controlled by the same Node B. In this case the signalling and traffic is combined in the Node B and is known as microdiversity. As far as the UE is concerned there is no distinction between a soft or softer handover. 5.1
Measurements for Handover
The measurement process for UMTS is more flexible than for GSM. When a UE is in the CELL_FACH, CELL_PCH or URA_PCH states it will use the information broadcast as system information for measurements. These measurements will largely be for cell reselection purposes. The only measurement to be reported will be traffic volume measurements and these will be sent in the CELL_FACH state. When a UE is in the CELL_DCH state it will be told precisely what to measure and when to report the measurement data using the Measurement Control message delivered to the UE via DCCH signalling. The Measurement Control message includes: • measurement identity number, a reference used by the UTRAN • measurement command, used to start, modify or suspend measurements • measurement objects – neighbour cell information • measurement quantity – what to measure, e.g. RSCP or RSSI • measurement reporting quantities – what quantities to report • measurement reporting criteria, which allow for the setting of triggers • reporting mode – acknowledged or unacknowledged mode of RLC • one of seven measurement types
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UTRAN Measurement Control Message
measurement identity number measurement command measurement objects measurement quantity measurement reporting quantities measurement reporting criteria reporting mode measurement type – intra-frequency – inter-frequency – inter-system – traffic volume – quality – internal – location measurements
Figure 18 Measurements for Handover SC2804/S6/v1.1
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5.2
Neighbour Cells for Soft Handover
For each defined cell in the RNC there will be a neighbour list. Potentially three neighbour lists may be defined: • intra-frequency list of up to 32 cells on the same frequency as the server • inter-frequency list of up to 32 cells on a different UMTS frequency • inter-system list of up to 32 GSM frequencies The neighbour lists are broadcast as part of system information, but a UE in connected mode can receive details using dedicated signalling via the DCCH. To identify a UMTS neighbour the list must include the following information: • Global RNC Id (MCC + MNC and RNC Id) • Cell Identifier (CI) • Location Area Code (LAC) • Routing Area Code (RAC) • UARFCN • Scrambling code for the PCPICH For a GSM cell the following information will be required: • Cell Global Identity (CGI) • BCCH frequency • Base Station Identity Code (BSIC)
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For each defined UMTS cell Neighbour List Intra-frequency list of up to 32 cells Inter-frequency list of up to 32 cells Inter-system list of up to 32 cells
Gobal RNC Id Cell Id LAC RAC UARFCN Scrambling code for PCPICH
UMTS neighbour list
GSM neighbour list
CGI BCCH frequency BSIC
Figure 19 Neighbour Cells for Soft Handover SC2804/S6/v1.1
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5.3
Considerations for Active Set Size
The cells that are engaged in soft handover are known as the active set of cells. The UE will be told which neighbours to perform measurements on as well as triggers for reporting. If the measurements on a neighbour satisfy the reporting criteria, a measurement result is sent to the RNC. If resources are available in the target cell the active set update procedure will be activated. The new cell is added to the active set and the UE is engaged in a soft hand over with the new cell. This is known as a Mobile Evaluated Hand Over (MEHO). However, decisions about handovers are still made by the RNC. There are a number of considerations for determining the size of the active set. Firstly, each cell in the active set will require a Radio Link (RL). RLs are added for each cell in the active set. The maximum number of RLs is eight. 3GPP 25.133 specifies a minimum of six. Secondly, a UE can only engage in soft handover with another base station if it has a spare finger in the rake receiver. The maximum number of fingers in a UE rake receiver is not specified and is manufacturer dependent. The soft handover probability target set in the radio network planning should be kept below 30–40% for the following reasons: Mobiles engaged in soft handover will consume more downlink spreading codes than single link connections. Reserving spreading codes in a Node B for soft handover will impact capacity. Each RL that is established will also require resources on the Iub interfaces. A 40% probability of soft handover demands 40% extra backhaul capacity. For UEs engaged in softer handover there will be no impact on backhaul capacity because signalling and traffic will be combined locally in the Node B. The benefit of soft handover is soft handover gain. A UE can combine a number of downlink signals using the rake receiver and get a net improvement in performance of as much as 3 or 4 dB compared to a single link connection. This can be taken into account favourably when determining the link budget. However, a UE in soft handover will also be power controlled by all the Node Bs concerned. If the path loss is the same for all the Node Bs the soft handover gain will be optimal. But if there is a small difference in path loss figures of a few decibels then the UE is likely to be powered up diminishing the soft handover gain. The subsequent noise rise will diminish capacity. Simulations (3GPP 25.942) have shown that in a planned area only 1% of locations require SHO to seven or more cells. Additionally, the SHO gain is minimal when more than three cells are in the active set. The conclusion is that the UE does not have to support more than four to six cells in the active set. 6.43
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Soft Handover Gain
Number of Rake Fingers
Considerations for Active Set Size
Spreading Code Capacity
Backhaul Capacity
Figure 20 Considerations for Active Set Size SC2804/S6/v1.1
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5.4
Configuring the Measurement Message
The configuration of the measurement message depends on what was specified in the downlink Measurement Control message. The reporting procedure is initiated by the UE when the reporting criteria are met, which can be based upon triggers or timers. The measurement message is used in both the CELL_DCH and the CELL_FACH states. However, in the CELL_FACH state only traffic volume measurements are reported. UEs in the URA_PCH or the CELL_PCH state will transit to the CELL_FACH state before transferring traffic volume information. The measurement report will contain the measured results of the quantity indicated in the Measurement Control message. The list will be ordered from best cell to worst cell (where the best cell is the one with the highest Ec/No value or smallest path loss). The measured results include: • intra-frequency measured results list • inter-frequency measured results list • inter-RAT measured results list • traffic volume measured results list • quality measured results list • UE internal measured results • UE positioning measured results Details about these can be found in 3GPP 25.331, but as an example the intrafrequency measured results list contains: • the scrambling code on the PCPICH • CPICH Ec/No or CPICH RSCP or pathloss • optionally, the cell ID, the SFN-SFN observed time difference and cell synchronization information
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Message Type Integrity Check Info Measurement Identity Measured Results Measured Results on RACH OP Additional Measured Results
OP
> Measured Results
MP
Event Results
OP
Measurement Report AM or UM RLC DCCH
Intra-frequency Measured Results List Inter-frequency Measured Results List Inter-RAT Measured Results List
Intra-frequency Measured Results List
OP
> Cell Measured Results
MP
Intra-frequency Measured Results List
Traffic Volume Measured Results List Quality Measured Results List UE Internal Measured Results List UE Positioning Measured Results List
Cell Identity OP SFN-SFN Observed Time Difference OP Cell Synchronization Information OP PCPICH Info
MP CPICH Ec/No OP CPICH RSCP OP Path loss
OP Cell Measured Results
Measured Results
Figure 21 Configuring the Measurement Message SC2804/S6/v1.1
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5.5
Soft Handover Parameters and Triggers
In UMTS there are a number of triggers associated with the measurement process. The 3GPP specifications do not give actual parameters for the thresholds, but equipment manufacturers are likely to implement thresholds as parameters within the UTRAN. The triggers can be grouped into six categories that tie in with six classes of measurement reports, and include: • intra-frequency measurements • inter-frequency measurements • inter-system measurements • traffic volume measurements • quality measurements • UE internal measurements 5.5.1
Intra-frequency Measurements
The measurement quantity that can be used to evaluate an intra-frequency event includes the Ec/No measured on the PCPICH, RSCP after despreading or the downlink path loss calculated as: Path loss in dB = PCPICH TX Power – PCPICH RSCP Path loss values will be rounded up to the nearest integer value and range between 46 and 158 dB. Six different events may trigger an intra-frequency measurement report. However, reports can be sent periodically if no new cells have been added to the active set. The six events are: • Event 1a: a PCPICH enters the reporting range • Event 1b: a PCPICH leaves the reporting range • Event 1c: a non-active PCPICH becomes better than an active PCPICH • Event 1d: a change of best cell • Event 1e: a PCPICH becomes better than the absolute threshold • Event 1f: a PCPICH becomes worse than the absolute threshold
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May be used to trigger handover
Intra-frequency measurements Inter-frequency measurements Inter-system measurements Traffic volume measurements Quality measurements UE internal measurements
Figure 22 Measurements to Trigger Handover SC2804/S6/v1.1
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5.5.2
Event 1a: A PCPICH Enters the Reporting Range
A report will be triggered if the following is fulfilled: Triggering condition for path loss ⎛ NA ⎞ 10LogM New ≤ W10Log⎜⎜ 1/ ∑ (1/Mi ) ⎟⎟ + (1 − W)10LogM Best + (R 1a − H1a /2) ⎝ i=1 ⎠
Triggering condition for all the other measurement quantities ⎛ NA ⎞ 10LogM New ≥ W10Log⎜⎜ ∑ Mi ⎟⎟ + (1 − W)10LogM Best − (R 1a − H1a /2) ⎝ i=1 ⎠
Where: MNew
is the measurement result of the cell entering the reporting range.
Mi
is a measurement result of a cell in the active set.
NA
is the number of cells in the current active set.
For path loss: MBest
is the measurement result of the cell in the active set with the lowest measurement result.
For other measurements quantities: MBest
is the measurement result of the cell in the active set with the highest measurement result.
W
is a parameter sent from the UTRAN to the UE.
R1a
is the reporting range constant.
H1a
is the hysteresis parameter for the event 1a.
If the measurement results are path loss or CPICH Ec/No then MNew, Mi and MBest are expressed as ratios. If the measurement result is CPICH-RSCP then MNew, Mi and MBest are expressed in milliwatts.
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PCPICH Ec/No
CPICH 1 Reporting Range
CPICH 2 Absolute Threshold
Time Event 1a
Figure 23 Event 1a: PCPICH Enters Reporting Range SC2804/S6/v1.1
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5.5.3
Event 1b: A PCPICH Leaves the Reporting Range
A report will be triggered when the following equation is satisfied: Leaving triggering condition for path loss ⎛ NA ⎞ 10LogM New > W10Log⎜⎜1/ ∑ (1/Mi ) ⎟⎟ + (1 − W)10LogM Best + (R 1a + H1a /2) ⎝ i=1 ⎠
Leaving triggering condition for all the other measurement quantities ⎛ NA ⎞ 10LogM New < W10Log⎜⎜ ∑ Mi ⎟⎟ + (1 − W)10LogM Best − (R 1a + H1a /2) ⎝ i=1 ⎠
Where: MOld
is the measurement result of the cell leaving the reporting range.
Mi
is a measurement result of a cell in the active set.
NA
is the number of cells in the current active set.
For path loss: MBest
is the measurement result of the cell in the active set with the lowest measurement result.
for other measurements quantities: MBest
is the measurement result of the cell in the active set with the highest measurement result.
W
is a parameter sent from the UTRAN to the UE.
R1b
is the reporting range constant.
H1b
is the hysteresis parameter for the event 1b.
If the measurement results are path loss or CPICH Ec/No then MNew, Mi and MBest are expressed as ratios. If the measurement result is CPICH-RSCP then MNew, Mi and MBest are expressed in milliwatt.
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PCPICH Ec/No
CPICH 1 Reporting Range
CPICH 2 Absolute Threshold
Time Event 1b
Figure 24 Event 1b: A PCPICH Leaves the Reporting Range SC2804/S6/v1.1
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5.5.4
Event 1c: A Non-Active PCPICH Becomes Better than an Active One
This report will be triggered when a PCPICH that is not in the active set is better than the worst PCPICH in the active set when the active set is full. This simply replaces one (the worst) PCPICH for another. A hysteresis known as the ‘replacement window’ is applied to this, meaning the new cell has to be better than the old by this hysteresis value. 5.5.5
Other Events
Event 1d: Change of Best Cell This report will be triggered when any PCPICH in the reporting range becomes better than the current serving cell plus a hysteresis value. Event 1e A PCPICH becomes better than the absolute threshold plus a hysteresis value. Event 1f A PCPICH becomes worse than the absolute threshold minus a hysteresis value.
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PCPICH Ec/No
CPICH 4 Hysteresis
CPICH 2
CPICH 3 Replacement Window
Hysteresis
Absolute Threshold
Hysteresis CPICH 1 Time Event 1e
Event 1c
Event 1f Event 1d
Figure 25 Events 1c, 1d, 1e and 1f SC2804/S6/v1.1
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5.6
Optimizing Soft Handover Regions
There are a number of parameters that could be used to optimize the network in respect of handovers, including: • active set size • reporting range • absolute threshold • hysteresis values • time to trigger values • offset values 5.6.1
Active Set Size
If the active set size is made too large it will result in unnecessary radio links being established, resulting in more signalling being required to add more cells to the active set. The soft handover margin will be eroded, resulting in increased UE and Node B transmit powers. The net result will be a reduction in downlink and uplink capacity. However, the impact of choosing too large a value can be controlled by the other soft handover parameters controlling the addition of cells to the active set. If the active set is made too small, frequent signalling and delayed handover will result, degrading performance in both uplink and downlink directions. Consequently, that the UE and Node B power levels will need to increase giving more interference and reduced capacity.
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Too small
Too large Active set size
Yes
Prevents necessary SHO
Little impact
SHO parameters set correctly ?
No
Unnecessary radio links
Increased TX power
Increased SHO
Degraded BLER UL/DL
Increased TX power
Increased call drop rate
Reduced UL/DL capacity
Figure 26 Active Set Size SC2804/S6/v1.1
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5.6.2
The Reporting Range
The measurements on a cell must be within the reporting range before Event 1a takes place and the cell is subsequently added to the active set. The size of the reporting range is made relative to the serving cell and is influenced by a large number of parameters. A hysteresis value H1a along with a reporting range constant R1a define the Addition Window. Neighbour cells that enter the addition window will be added to the active set. The reporting range is also used to identify cells that should be removed from the active set (Event 1b). A different hysteresis value, H1b, and the reporting range constant R1b may be used to define a Drop Window. This would be used to remove cells from the active set. If a non-active PCPICH enters the reporting range when the active set is full and is found to be better than an active one, Event 1c may take place. Here, the non-active PCPICH replaces the worst PCPICH. A hysteresis value can be applied, known as the replacement window.
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PCPICH Ec/No
Addition Window Drop Window Replacement Window
Absolute Threshold
Time
Event 1a
Figure 27 Reporting Range SC2804/S6/v1.1
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5.6.3
Addition Window
The addition window determines which cells are to be included in the active set relative to the serving cell. If this window is too large the SHO area will be too large, resulting in too many soft handovers. The soft handover gain will diminish, resulting in increased transmit power levels, which will reduce uplink and downlink capacity. Downlink capacity may also suffer as a result of the availability of spreading codes. Exceeding the 30–40% SHO probability will also demand greater backhaul capacity. Making the SHO area too small may result in frequent active set updates, which will place a burden on signalling mechanisms. Fewer cells in SHO will reduce the SHO gain therefore demanding greater transmit power, which will reduce uplink and downlink capacity.
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Too small
Too large Addition window
SHO region too small
SHO region too large
Frequent active set updates
Few cells in SHO
Signalling burden
Increased transmit power
Reduced DL capacity
Diminished SHO gain
Reduced UL/DL capacity
Increased demand on backhaul
Increased transmit power
Too many SHO
Reduced UL/DL capacity
Figure 28 Addition Window SC2804/S6/v1.1
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5.6.4
Drop Window
The drop window is set relative to the addition window, but the hysteresis values are set to make the drop window larger by a couple of dBs. If the window is set too large then the wrong cells will be in the active set, making the soft handover regions larger. The net result of this will be increased transmit powers and reduced uplink and downlink capacity. If the drop window is too small there will be fewer cells in the active set, which will reduce the SHO gain and result in higher transmit powers and a reduction in uplink and downlink capacity. Depending upon the terminal speed there may also be frequent handovers taking place, which will put a burden on signalling mechanisms.
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Too small
Too large Drop Window
Fewer cells in active set
Wrong cells in active set
Reduced SHO gain
Large SHO regions
Increased transmit power
Increased transmit power
Reduced UL/DL capacity
Reduced UL/DL capacity
Figure 29 Drop Window SC2804/S6/v1.1
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5.6.5
Replacement Window
The replacement window is used when a new PCPICH is found and the active set is full. The weakest PCPICH is replaced by the new PCPICH if it exceeds the replacement window. If the replacement window is too large there will be fewer replacements, making the active set less optimal. This will result in increased transmit powers and ultimately a reduction in downlink and uplink capacity. If the window is too small there will be excessive replacements and a ping-pong effect will occur.
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Too small
Too large Replacement window
Excessive replacements
Fewer replacements
Ping-pong effects
Nonoptimal
Excessive signalling burden
Increased transmit power
Reduced UL/DL capacity
Figure 30 Replacement Window SC2804/S6/v1.1
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5.6.6
Absolute Threshold
The absolute threshold guarantees the minimum quality for a cell. It is used with reporting Events 1e and 1f. For Event 1e the trigger is defined by: MNew ≤ T1e – H1e/2 for path loss or MNew ≥ T1e + H1e/2 for all other measurements Where: MNew
is the measurement result of a cell that becomes better than an absolute threshold
T1e
is the absolute threshold
H1e
is the hysteresis value
For Event 1f the trigger is defined by: MNew ≥ T1f + H1f/2 for pathloss or MNew ≤ T1f – H1f/2 for all other measurements Where:
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is the measurement result of the cell that becomes worse than the absolute threshold
T1f
is the absolute threshold
H1f
is the hysteresis value
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PCPICH Ec/No CPICH 3
CPICH 2
Absolute Threshold CPICH 1 Time Event 1e
Event 1f
Figure 31 Absolute Threshold SC2804/S6/v1.1
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5.6.7
Hysteresis Values
Hysteresis values can be used to limit the number of event-triggered reports. The value of the hysteresis is given to the UE in the Measurement Control message. In the case of Event 1d, where there is a change of best cell, this will not be reported until the difference is equal to the hysteresis value. Choosing a large value for the hysteresis will make the change less likely. A smaller value would make the change more likely, possibly resulting in a ping-pong effect. The hysteresis values ranging from 0 to 7.5 dB in steps of 0.5 dB. 5.6.8
Time-to-Trigger
To minimize the number of signalling messages being transmitted by the UE; a timeto-trigger parameter can be given to the UE in the Measurement Control message. The effect of this trigger is to ensure that a report is only triggered if the measured results are consistent and stable. The time-to-trigger values are integer values of 0, 10, 20, 40, 60, 80, 100, 120, 160, 200, 240, 320, 640, 1280, 2560, and 5000 milliseconds.
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PCPICH Ec/No
Hysteresis
Time Event 1d
Figure 32 Hysteresis Values
PCPICH Ec/No Reporting Range
Event 1a Time-to-Trigger Time
Figure 33 Time-to-Trigger SC2804/S6/v1.1
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5.6.9
Offset Values
For each cell in the monitored set an offset can be applied by the UE. This offset value may be positive or negative and is given to the mobile for each cell concerned in the Measurement Control message. By applying a positive offset the UE will send a measurement report as if the PCPICH Ec/No value was x dB greater than it actually is. This will allow the handover algorithm to include the cell in the active set earlier than without an offset. Applying a negative offset will cause the UE to subtract x dB before checking to see if the cell is in the reported range. This will make the cell less attractive for handover purposes. 5.6.10 Forbidding a PCPICH to Affect the Reporting Range The reporting range is defined as a function of all of the PCPICHs in the active set. If the weighting parameter, W, is set to zero, the reporting range is defined relative to the best PCPICH. If there was a PCPICH in an area which was intermittently strong, i.e. became the best PCPICH intermittently, then the reporting range would become unstable. To prevent this occurring it is possible to bar the offending PCPICH from the evaluation of the reporting range.
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PCPICH Ec/No Reporting Range
CPICH 1
CPICH 2
Offset
CPICH 3 Time Event 1b
Event 1a
Figure 34 Offset Values
PCPICH Ec/No CPICH 1
Reporting Range
CPICH 2 CPICH 3
Time
Figure 35 Forbidding a PCPICH SC2804/S6/v1.1
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5.7
Inter-Cell Synchronization
A Node B can support one or more cells. When a Node B creates three cells it is important that the radio frames transmitted over the air interface are not aligned. If they are, the synchronization channels in each cell will be aligned and cause excessive noise. Instead, an offset known as T_cell is applied to each cell to stagger the transmission of the radio frames. The parameter T_cell has a resolution of 256 chips with values between 0 and 9. T_cell is applied to the Node B Frame Number (BFN) in the Node B to calculate the cell System Frame Number (SFN) as follows: SFN = BFN adjusted with T_cell
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Cell 1
Cell 3
Cell 2
SCH 10 ms radio frame Cell 1
Cell 2 T_cell Cell 3 T_cell SFN = BFN adjusted with T_cell where T_cell = 0
9 x 256 chips
Figure 36 Inter-Cell Synchronization SC2804/S6/v1.1
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6
HARD HANDOVERS A hard handover involves changing frequency. It is a break-before-make process as in GSM. Hard handovers require the UE to change from one UMTS frequency to another, e.g. when moving between hierarchical cells. This is also known as an interfrequency handover. Hard handovers are also performed when moving from a UMTS frequency to a GSM frequency. This is known as an inter-system handover. Hard handovers in UMTS are Mobile Evaluated Hand Overs (MEHO). The UE is required to perform measurements on neighbours and report measurement results. The reporting process may be periodic or based upon triggers. However, because the neighbours are on different frequencies and the UE is using CDMA on the air interface, it is necessary to use compressed mode. 6.1
Compressed Mode Measurements
The UE is able to perform a single measurement type in one transmission gap pattern sequence. A transmission gap pattern sequence can consist of alternating transmission gap patterns 1 and 2. Each of these patterns in turn consists of one or two transmission gaps. The following parameters apply: • Transmission Gap Starting Slot Number (TGSN) • Transmission Gap Start Distance (TGD) • Transmission Gap Length 1 (TGL1) • Transmission Gap Length 2 (TGL2 • Transmission Gap Pattern Length (TGPL) • Transmission Gap Pattern Repetition Count (TGPRC) The TGSN is the slot number of the start of the first transmission gap in the first transmission gap pattern. The time to the start of the second transmission gap is given by the TGD. The duration of each transmission gap is given by TGL1 and TGL2. Note that it is possible that only one transmission gap will be requested. The length of the pattern containing the two transmission gaps is defined by TGPL1. Note that a second pattern length, TGLP2, may also be used. The sequence defined by TGPL1 and TGPL2 continues for a total number of frames defined by TGPRC.
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1 TG pattern 1
3
TG pattern 2
TG pattern 1
TG pattern 1
Transmission gap 1
4 TG pattern 2
5
nth=TGPRC
TG pattern 1
TG pattern 2
TG pattern 2
Transmission gap 2
TGSN
TGSN
TGL1
TGL1
TGL2
TGL2 TGD
TGD TGPL1
TGPL2
Figure 37 Compressed Mode Parameters SC2804/S6/v1.1
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6.1.1
FDD Inter-Frequency Measurements
In order for the UE to perform measurements on other FDD carriers the RNC must provide a transmission gap pattern sequence using the parameters TGL1, TGL2, TGD and Max TGPL.
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TGL1 (slots) TGL2 (Slots) 7 14 10 7 14
7 14
TGD (Slots)
Max TGPL (Frames)
undefined undefined undefined 15…269 45…269
18 36 24 18 + ceiling(TGD/15) 36 + ceiling(TGD/15)
Figure 38 FDD Inter-Frequency Measurements SC2804/S6/v1.1
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6.1.2
GSM Carrier RSSI
For a UE to be able to perform RSSI measurements on a GSM carrier the RNC must provide a transmission gap pattern sequence using the parameters TGL1, TGL2, and TGD. See Figure 39a. The Transmission Gap Length will dictate how many RSSI samples can be taken on a GSM carrier. To meet the measurement accuracy requirements stated in 3GPP TS 45.0081 the measurement time should allow the UE to take three RSSI samples per GSM carrier in the monitored set. This will require a TGL value no smaller than five slots. See Figure 39b. Figure 39c shows the combinations of TGL1, TGL2 and TGD, which will be used if the UE is also required to perform BSIC verification.
1
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TGL1 (Slots) TGL2 (Slots) TGD (Slots) 3 4 5 7 10 14 3 4 5 7 10 14
3 4 5 7 10 14
undefined undefined undefined undefined undefined undefined 15…269 15…269 15…269 15…269 15…269 15…269
Figure 39a RSSI Measurements
TGL
Number of GSM carrier RSSI samples in each gap.
3 4 5 7 10 14
1 2 3 6 10 15
Figure 39b RSSI Samples Per Carrier
TGL1 (Slots) TGL2 (Slots) TGD (Slots) 5 7 10 14 5 7 10 14
5 7 10 14
undefined undefined undefined undefined 15…269 15…269 15…269 15…269
Figure 39c BSIC Verification SC2804/S6/v1.1
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6.2
Neighbour Cells for Hard Handover
In the CELL_DCH and CELL_FACH states the UE can monitor up to 32 interfrequency cells, including FDD cells using up to two FDD carriers and, depending on UE capability, 32 GSM cells using up to 32 carriers. In the CELL_DCH state, when the compressed mode of operation is supported, the UE continuously measures on identified inter-frequency cells and searches for new cells as indicated in the Measurement Control message. The neighbour lists are broadcast as part of system information, but a UE in connected mode can receive details using dedicated signalling via the DCCH. To identify a UMTS neighbour the list must include the following information: • Global RNC Id (MCC + MNC and RNC Id) • Cell Identifier (CI) • Location Area Code (LAC) • Routing Area Code (RAC) • UARFCN • Scrambling code for PCPICH For a GSM cell the following information will be required: • Cell Global Identity (CGI) • BCCH frequency • Base Station Identity Code (BSIC)
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In CELL_DCH and CELL_FACH states the UE can monitor up to: 32 inter-frequency cells, including FDD cells using up to two FDD carriers depending on UE capability, 32 GSM cells using up to 32 carriers To identify a UMTS neighbour the list must include the following information: Global RNC Id (MCC + MNC and RNC Id) Cell Identifier (CI) Location Area Code (LAC) Routing Area Code (RAC) UARFCN scrambling code for PCPICH For a GSM cell the following information will be required: Cell Glocal Identity (CGI) BCCH frequency Base Station Identity Code (BSIC)
Figure 40 Neighbour Cells for Hard Handover SC2804/S6/v1.1
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6.3
Hard Handover Parameters and Triggers
6.3.1
Inter-Frequency Handovers
Quality estimates are performed on interfrequency measurements according to the following:
Q carrier j = 10LogM carrier j
⎞ ⎛ NA j = W j 10Log⎜⎜ ∑ Mi j ⎟⎟ + (1 − W j )10LogM Best j − H ⎠ ⎝ i=1
Where: Qcarrier j is the estimated quality of the active set on frequency j. Mcarrier j is the estimated quality of the active set on frequency j. Mi j is a measurement result of cell i in the active set on frequency j. NA j is the number of cells in the active set on frequency j. MBest j is the measurement result of the strongest cell in the active set on frequency j. Wj is a parameter sent from UTRAN to UE and used for frequency j. H is the hysteresis parameter. The measurement control message notifies the UE about which events to use to trigger a measurement report. The triggers are: • Event 2a – change of best frequency • Event 2b – estimated quality of a currently-used frequency is below a threshold, and the estimated quality of a non-used frequency is above a threshold • Event 2c – the estimated quality of a non-used frequency is above a threshold • Event 2d – the estimated quality of the currently-used frequency is below a threshold • Event 2e – the estimated quality of a non-used frequency is below a threshold • Event 2f – the estimated quality of the currently-used frequency is above a threshold
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Event 2a – change of best frequency Event 2b – estimated quality of a currently-used frequency is below a threshold, and the estimated quality of a non-used frequency is above a threshold Event 2c – the estimated quality of a non-used frequency is above a threshold Event 2d – the estimated quality of the currently-used frequency is below a threshold Event 2e – the estimated quality of a non-used frequency is below a threshold Event 2f – the estimated quality of the currently-used frequency is above a threshold
Figure 41 Inter-Frequency Events SC2804/S6/v1.1
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Event 2a If the quality estimate of a non-used frequency exceeds the quality estimate for the currently-used cell and Event 2a has been requested by the RNC, a report will be triggered. A hysteresis value and time-to-trigger will also apply to this event. Event 2b The UE will be given two threshold values in the Measurement Control message. There is a threshold for the used frequency and a threshold for the non-used frequency. If the estimated quality on a used frequency falls below its defined threshold and the estimated quality for a non-used frequency exceeds its defined threshold a report is triggered. Event 2c If ordered by the RNC, the UE will send a report if the estimated quality on a nonused frequency exceeds a predefined threshold. A hysteresis value and time-totrigger also apply to this event. Event 2d When ordered by the RNC, the UE will report when the estimated quality on a currently-used frequency falls below a predefined threshold. A hysteresis and timeto-trigger applies to this event. Event 2e The UE will send a report to the RNC when the estimated quality on a non-used frequency is below a threshold after applying a hysteresis and time-to-trigger value Event 2f The UE sends a report to the RNC when the estimated quality of a currently-used frequency is above a threshold taking into account hysteresis and time-to-trigger values.
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Estimated Quality
Threshold Cell 1
Threshold Cell 2 Cell 1
Cell 2
Time 2f
2c
2b + 2d
2e 2a
Figure 42 Inter-Frequency Triggers SC2804/S6/v1.1
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6.3.2
Inter-System Handovers
⎛ NA ⎞ Q UTRAN = 10LogM UTRAN = W10Log ⎜⎜ ∑ Mi ⎟⎟ + (1 − W)10LogM Best ⎝ i=1 ⎠ Where: QUTRAN is the estimated quality of the active set on the currently-used UTRAN frequency. MUTRAN is the estimated quality of the active set on currently-used UTRAN frequency expressed in a unit other than that used on the current UTRAN. Mi is a measurement result of cell i in the active set. NA is the number of cells in the active set. MBest is the measurement result of the strongest cell in the active set. W is a parameter sent from the UTRAN to the UE. The measurement control message notifies the UE about which events to use to trigger a measurement report. The triggers are: • Event 3a – the estimated quality of the currently-used UTRAN frequency is below a certain threshold and the estimated quality of the other system is above a certain threshold • Event 3b – the estimated quality of the other system is below a certain threshold • Event 3c – the estimated quality of the other system is above a certain threshold • Event 3d – change of best cell in other system
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Event 3a – the estimated quality of the currently-used UTRAN frequency is below a certain threshold and the estimated quality of the other system is above a certain threshold Event 3b – the estimated quality of the other system is below a certain threshold Event 3c – the estimated quality of the other system is above a certain threshold Event 3d – change of best cell in other system
Figure 43 Inter-System Events SC2804/S6/v1.1
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Event 3a If the estimated quality on the currently used frequency falls below the Threshold Own System and the estimated quality of the other system is above the Threshold Other System a report will be triggered. Hysteresis and time-to-trigger values are applied to both sets of comparisons. Event 3b If the estimated quality on the other system falls below the Threshold Other System a report will be triggered. Hysteresis and time-to-trigger values are also applied. Event 3c When the estimated quality on the other system exceeds the Threshold Other System a report will be triggered. Hysteresis and time-to-trigger values are applied. Event 3d If the quality estimates for a cell in the other system exceed the quality estimate for the best cell in the other system a report will be triggered. Hysteresis and time-totrigger values are also applied.
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Estimated Quality
Own Cell
Threshold Own System Other Cell Threshold Other System
Time 3a
3c
3b
Figure 44 Inter-System Triggers SC2804/S6/v1.1
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SECTION 7
UMTS FEATURES AND TECHNIQUES
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CONTENTS 1
Current Optional Features 1.1 Introduction 1.2 Site Selection Diversity Transmit (SSDT) 1.3 Transmit Diversity Options 1.4 Open Loop Mode 1.5 Closed Loop Mode 1.6 Impact of Transmit Diversity 1.7 Multi-User Detection (MUD) 1.8 Advanced Antennas
7.1 7.1 7.1 7.3 7.3 7.7 7.7 7.9 7.11
2
Future Optimal Features 2.1 Introduction 2.2 High Speed Downlink Packet Access (HSDPA) 2.3 New Transport and Physical Channels 2.4 Channel Adaptation 2.5 Implementation of the HS-DSCH 2.6 HARQ and Adaptive Channels 2.7 Adaptive Modulation 2.8 Multiple Input Multiple Output (MIMO) Antennas 2.9 Interworking with Wireless LANs (WLAN)
7.17 7.17 7.17 7.17 7.17 7.19 7.21 7.21 7.23 7.25
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OBJECTIVES At the end of this section you will be able to: • •
identify the optimal features that may be applied to UMTS systems as they mature assess the likely effects and benefits of introducing a range of optimal features
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CURRENT OPTIONAL FEATURES 1.1
Introduction
A number of features may be employed in UMTS to optimize the network. All of the features are mandatory for UEs but are optional on the network side. These optional features include: • Site Selection Diversity Transmit (SSDT) • transmit diversity • Multi-User Detection (MUD) • smart antennas 1.2
Site Selection Diversity Transmit (SSDT)
Site Selection Diversity Transmit (SSDT) is a technique that can be applied when a UE is engaged in a soft handover to reduce downlink interference. The principle of SSDT is that the UE will dynamically indicate the best cell from its current active set based on the downlink reception level measurement of the CPICH. Each cell in the active set is given a temporary ID within the set; the UE indicates the primary cell using part of the Feedback Information (FBI) field in the uplink DPCCH. The nominated primary cell then transmits both DPDCH and DPCCH. All other cells in the active set are selected as non-primary and only transmit DPCCH. The UE continues to monitor the DPCCH for all cells in the active set. If a nonprimary cell is judged to be better than the nominated primary, the UE indicates this change to the RNC. The new primary will then begin to transmit DPDCH and the old primary will continue with the transmission of DPCCH only. The lack of downlink DPDCHs will reduce the soft handover gain experienced by the UE. There will be no loss of gain in the uplink direction because all Node Bs engaged in soft handover will be listening to both DPDCH and DPCCH from the UE. It has been suggested that there may be a loss of capacity as a result of SSDT. It has been shown through simulations that although the capacity gain is large for high-bitrate services, it is small for low-bit-rate services such as speech. In order to improve the performance of SSDT, the use of enhanced SSDT has been discussed in RAN WG1.
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SRNC
Iub
Iub
DPDCH/ DPCCH
1
Iub
DPCCH only 2 DPCCH only
UE nominates Node B 1 as Primary 3
Figure 1 SSDT SC2804/S7/v1.1
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1.3
Transmit Diversity Options
There is no requirement for a 3G handset to support receive diversity, but it is possible to improve downlink performance using transmit diversity techniques. There are two ways of implementing transmit diversity: closed loop, where the UE reports the performance back to the Node B; and open loop, where no reporting is necessary. 1.4
Open Loop Mode
In open loop mode there are two types of transmit diversity available: Space Time Transmit Diversity (STTD) and Time Switched Transmit Diversity (TSTD). 1.4.1
Space Time Transmit Diversity (STTD)
In STTD two signals are transmitted from two transmission antennas simultaneously, as illustrated in Figure 2. The two signals are received by the UE on the same propagation paths but with uncorrelated fading characteristics. This provides space diversity. Time diversity is achieved by passing the data through an orthogonal block-encoding process prior to transmission. Due to the orthogonality of the block-encoding scheme over a sequence of 4 bits (2-Phase Quadrature Phase Shift Keying (2-QPSK)) symbols in the downlink) it is possible for the UE to separate the two signal components from the separate antennas (so long as the radio path remains time invariant over an interval corresponding to 2-QPSK symbols) and perform optimum combining. In addition to data and control signals, pilot signals can also be transmitted using STTD. For a detailed description of STTD encoding refer to 3GPP TS 25.211.
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Antenna 1
b0
b1
b2
Channel bits
Antenna 2
b0
b1
b2
b3
Antenna 1
–b
b3
b0
–b
Antenna 2
b3
2
STTD orthogonally encoded bits over
1
2 QPSK symbols
Figure 2 Transmit Diversity SC2804/S7/v1.1
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1.4.2
Time Switched Transmit Diversity (TSTD)
TSTD can be applied to the SCH. With reference to Figure 3, in even-number slots both the Primary SCH (P-SCH) and the Secondary SCH (S-SCH) are transmitted on antenna one, and in odd-number slots both P-SCH and S-SCH are transmitted on antenna two.
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Slot 0
Gp Antenna 1
Slot 1
Slot 2
Slot 14
P-SCH
TX off
P-SCH
P-SCH
S-SCH
TX off
S-SCH
S-SCH
P-SCH
S-SCH
Slot 0
Gs
Antenna 2
Slot 1
Slot 2
Slot 14
TX off
P-SCH
TX off
TX off
TX off
S-SCH
TX off
TX off
G – Gain Control
Figure 3 TSTD SC2804/S7/v1.1
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1.5
Closed Loop Mode
Closed loop mode is only applicable for use in dedicated channels. In closed loop mode the UE monitors the CPICH transmissions from the serving cell to assess the performance of transmit diversity. It then uses defined algorithms to calculate the optimal settings required by the Node B for best link performance. The requirements are conveyed to the Node B in the D field of the FBI bits within the DPCCH. There are two closed loop modes: closed loop mode 1 and closed loop mode 2. With reference to Figure 4, the DPCH for transmission is applied to both transmit antenna branches and weighted with antenna-specific weighting factors W1 and W2. In closed loop mode 1 the phase of one antenna is adjusted relative to the other with an accuracy of 1 bit per timeslot. In closed loop mode 2 both the relative phase and amplitude are adjusted with an accuracy of 1 bit. 1.6
Impact of Transmit Diversity
The additional multipaths created by transmit diversity may result in the loss of downlink orthogonality for the spreading codes. This would degrade the downlink performance, in particular for terminals moving at speed through a macro cellular environment. Simulation results have shown that the greatest benefit is achieved when transmit diversity is introduced into cells that have little multipath, such as micro cells. Both open loop and closed loop transmit diversity offer the benefit of a reduced Eb/No requirement. Simulations of a 12.2 kbit/s service with a BLER target of 1% have shown a 0.5 to 3 dB reduction in the Eb/No requirement for open loop mode. But it depends on terminal speed and environment. This can be improved by a further 0.5 dB using closed loop mode 1. These improvements impact upon downlink system capacity and the downlink coverage area. Improving the downlink coverage area is particularly important in micro cells, where the Node B transmit power is relatively small. Simulations have also shown that capacity gains of up to 70% may be achieved in a micro cell using closed loop mode 1.
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CPICH
Antenna 1
DPCH
Weight (W1)
Weight (W2)
Antenna 2
CPICH
Weight Generation
FBI D Field Extraction
DPCCH
Figure 4 Closed Loop Mode SC2804/S7/v1.1
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1.7
Multi-User Detection (MUD)
Multi-User Detection (MUD) is a form of noise suppression process. In active noise suppression systems, the ambient noise is sampled, inverted and transmitted back into the environment in real time to cancel the noise. In a CDMA system, the noise on a wanted channel is largely the interference of other users in the cell. Although noise-like in nature, it is determinable as they are also being decoded at the base station for the other users in their channels. A conceptual diagram of a base station with MUD is shown in Figure 5. The outputs of rake receivers 2 and 3 are weighted and fed back to receiver 1.
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Weighted Correction
RF down conversion and A to D conversion
Rake Receiver
User 1
Rake Receiver
User 2
Rake Receiver
User 3
etc.
Figure 5 Conceptual MUD Receiver SC2804/S7/v1.1
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1.8
Advanced Antennas
Traditional cellular antennas have fixed radiation and reception patterns and the gain of the antenna is proportional to its size. Research into advanced antennas has been driven by the need for range extension and controlling interference. Controlling interference will bring about capacity gains and together with range extension improvements in coverage will be possible. The technology behind advanced antennas includes switched beam arrays and adaptive (smart) antennas. 1.8.1
Switched Beam Arrays
A switched beam array comprises a phased array antenna and some logic to switch a radio connection from one beam to the next. A phased array antenna comprises a number of fixed antenna elements into which power is delivered with different relative phases. The signals are summed coherently in a specific Direction of Arrival (DoA). In the uniform phased array antenna the phase shift relative to one antenna element increases linearly from element to element. The phase shift is a function of the element spacing, d, DoA, θ, and wavelength, λ. The simplest switched beam array uses the Butler Matrix to define the phase shifts associated with each beam. A user would be switched from beam to beam very much like moving from cell to cell using conventional antennas. Allocated to each beam there will be a secondary CPICH to serve as a phase reference and to be used for measurement purposes. Based upon the mobile’s reported measurements the RNC can switch the mobile from beam to beam by performing handovers. In addition, secondary cell scrambling codes can be allocated to the individual beams, allowing the reuse of spreading codes.
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DoA θ
1
d
2
3
4
Phase Shift
Figure 6 Uniform Linear Array SC2804/S7/v1.1
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Beam
Antenna Elements 1
2
3
4
1
0º
1 – 35º
2 – 70º
–405º
2
0º
–45º
–90º
1 – 35º
3
0º
45º
90º
135º
4
0º
135º
270º
405º
Figure 7 Butler Matrix 7.13
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Conventional cell
degrees 6 – 0
–40
2 – 0
0
20
40
60
Figure 8 Beam Pattern SC2804/S7/v1.1
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1.8.2
Adaptive (Smart) Antennas
Adaptive (or smart) antennas use the same phased array as switched beam antennas but require more sophisticated logic. This logic will provide Spatial Filtering Interference Rejection (SFIR) and Spatial Division Multiple Access (SDMA). SFIR is a process where the array of elements provides a composite coverage pattern with a null in the direction of an interferer and dynamically steering the null as the interference moves. SDMA includes the SFIR technique but is also able to steer the lobe serving a mobile and dynamically adjust the power in that lobe. Using multiple lobes will allow a number of mobiles to be served in the same area. Smart antennas increase the complexity of a system and may not support transmit diversity. There will also be a major impact on the radio resource management function in the RNC. Consequently, smart antennas may not find an application in 3G. 1.8.3
Impact of Advanced Antennas
Simulation results comparing a four-beam array with polarization diversity with conventional two antenna diversity are very favourable. In the uplink direction the reduction in required Eb/No ranged from 5–6.5 dB depending upon terminal speed and operating environment. In the downlink direction, comparison of a four-beam array with a single transmit antenna in a macro cell gave a 4.5 dB reduction in the required Eb/No value.
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a)
Interference
B
Azimuth and radiated power of beam(s) may be dynamically adjusted to account for traffic distribution and interference sources
b)
Interference
B
Figure 9 Adaptive (Smart) Antennas SC2804/S7/v1.1
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2
FUTURE OPTIMAL FEATURES 2.1
Introduction
UMTS is evolving, and subsequent releases of the 3GPP specification, namely Releases 4, 5 and 6, will bring new features which may be used to optimize network performance. Some of these are: • High Speed Downlink Packet Access (HSDPA) • Hybrid ARQ (HARQ) and adaptive channels • Multiple Input Multiple Output (MIMO) antennas • interworking with Wireless LANs (WLAN) 2.2
High Speed Downlink Packet Access (HSDPA)
A number of advanced techniques have been put forward to achieve high bit rates on the air interface under the general heading of HSDPA. Downlink data rates of 8–10 Mbit/s are possible, increasing to 20 Mbit/s in the future. Details of these techniques are included in Release 5 and Release 6 of the UMTS specifications. 2.3
New Transport and Physical Channels
New transport channels have been defined: the High Speed Downlink Shared Channel (HS-DSCH) and the High Speed Shared Control Channel (HS-SCCH). 2.4
Channel Adaptation
Adaptive Modulation and Coding (AMC) is a mechanism whereby the modulation schemes of four-state Quadrature Phase Shift Keying (QPSK) and 16-state Quadrature Amplitude Modulation (16QAM) can be chosen dynamically according to the suitability of the radio environment. This also includes adjusting the Forward Error Correction (FEC) and puncturing rates. Once the FEC checks have been made, Hybrid Automatic Repeat Request (ARQ) provides soft combining of all retransmissions. Release 6 will include Multiple Input Multiple Output (MIMO), whereby several transmit antennas can be employed at the base station along with several antennas at the receiver. This allows downlink bit rates to achieve 20 Mbit/s.
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High Speed Downlink Packet Access (HSDPA)
HS-DSCH DL Transport HS-SCCH DL Physical HS-PDSCH DL Physical HS-DPCCH UL Physical 8–10 Mbit/s QPSK 16QAM
20 Mbit/s
HARQ
MIMO
Figure 10 HSDPA SC2804/S7/v1.1
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2.5
Implementation of the HS-DSCH
The HS-DSCH is implemented at the MAC layer. Under normal circumstances dedicated and shared transport channels would be implemented in MAC-d and MAC-c/sh located in the RNC. However, because of the need for high-speed data transfer with error correction, a new MAC entity, MAC-hs, will be implemented in the Node B. This will eliminate the need for retransmission of erroneous data over the Iub interface, reducing delays. MAC-hs is responsible for handling the data transmitted on the HS-DSCH. Additionally, it is responsible for the management of the physical resources allocated to HSDPA. MAC-hs is composed of four different functional entities: • flow control • scheduling/priority handling • HARQ • Transport Format Resource Combination (TFRC) selection Flow control is used to reduce discarded and retransmitted data as a result of the HS-DSCH congestion. The scheduling/priority handling manages the HS-DSCH resources between HARQ entities and data flows, according to their priority. The HARQ handles the control of errors whilst TFRC is responsible for the selection of the appropriate transport format for the data to be transmitted.
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Node B
Iub
RNC DTCH DTCH
MAC-hs
Scheduling/ priority handling HARQ
MAC-c/sh
MAC-d
TFRC selection
HS-DSCH
Figure 11 Implementation of the HS-DSCH SC2804/S7/v1.1
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2.6
HARQ and Adaptive Channels
In conventional ARQ schemes, frame errors at the receiving end are detected using a Cyclic Redundancy Checksum (CRC). If a frame is received in error a Negative Acknowledgemet (NACK) is returned prompting a retransmission of the erroneous frame. Otherwise, an Acknowlegement (ACK) positively acknowledges the received frame. User data and the CRC bits may be additionally encoded by an error correcting code, which increases the probability of successful transmission. Such schemes are known as Hybrid ARQ (HARQ) schemes. A measure of ARQ protocol efficiency is throughput, defined as the average number of user bits accepted at the receiving end in a given time. The more redundant bits transmitted, the lower the efficiency. In mobile environments the Incremental Redundancy (IR) HARQ scheme exhibits higher throughput efficiency by adapting the error correcting code redundancy to different channel conditions. A block of user data is sent with a CRC and parity bits. If the CRC checksum fails in the receiver a NACK is returned to the transmitter, which transmits additional parity bits only. These bits are combined with the first in a second attempt to correct the error. If the CRC checksum fails again, additional parity bits are transmitted until the receiver can decode the information successfully. By only retransmitting parity bits the throughput will be greatly improved. 2.7
Adaptive Modulation
High-order modulation schemes such as 16QAM provide high spectral efficiency in terms of bit/s/Hz compared to QPSK, and can provide much higher peak data rates. However, the air interface is a hostile environment and 16QAM is not as robust as QPSK. One way in which 16QAM could be used advantageously would be by allocating proportionally more power to that channel.
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Transmitter
Receiver
Data and parity and CRC
ACK
Data and parity and CRC
CRC fails
NACK
Parity
CRC fails
NACK
Parity
CRC OK
ACK
Figure 12 Hybrid ARQ SC2804/S7/v1.1
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2.8
Multiple Input Multiple Output (MIMO) Antennas
MIMO is a technique for increasing data rates over a radio interface. In 1998, Bell Labs demonstrated a new technique to greatly increase the capacity of a radio link which has become known as Multiple Input Multiple Output (MIMO). Researchers at Bell Labs used temporal and multi-antenna spatial diversity techniques in an architecture called BLAST (Bell labs LAyered Space-Time). In conventional wireless systems, multipath propagation is problematic because components arrive at the receiver at slightly different times, giving rise to fast fading and time dispersion. However, MIMO techniques exploit multipath to enhance performance by treating the multiple components as separate parallel subchannels. This is achieved by splitting a single user’s data stream into multiple substreams and using an array of antennas to simultaneously transmit the parallel substreams. Since the user’s data is being sent in parallel via multiple antennas, the effective bit rate is increased roughly in proportion to the number of antennas. At the receiver, an array of antennas is used to pick up the multiple substreams and their multipath components. Each antenna ‘sees’ all of the transmitted substreams superimposed. However, if there is sufficient multipath scattering, the multiple substreams are all scattered slightly differently, since they originate from different transmit antennas that are located at different points in space. Using sophisticated signal processing, these slight differences in scattering allow the substreams to be identified and recovered. The signal processing algorithms used at the receiver are central to the technique. At the bank of receiving antennas, high-speed signal processors look at the signals from all the receiver antennas simultaneously, first extracting the strongest substream then proceeding with the remaining weaker signals. It is anticipated that using MIMO within the UTRAN will allow transmitted bit rates to be increased five fold. Potentially, a downlink bit rate of 20 Mbit/s could be achieved if MIMO was used with HSDPA.
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Transmitter
Receiver
Figure 13 MIMO SC2804/S7/v1.1
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2.9
Interworking with Wireless LANs (WLAN)
Wireless LANs (WLAN) offer broadband-style access to the public Internet. Most WLANs conform to IEEE 802.11b or 802.11g standards. 802.11b has existed for a number of years and there are numerous of ‘WiFi® hotspots’ in public places such as fast food restaurants, bars and shops. 802.11b offers Internet access at bit rates up to 11 Mbit/s. The more recent 802.11g standard is backwards-compatible with 802.11b, but offers bit rates up to 54 Mbit/s. WLANs are commonly used on corporate intranets to simplify network design and implementation. The purpose of interworking UMTS with WLANs is to extend the UMTS services and functionality to the WLAN environment. The interworking system will provide bearer services for connecting a UMTS subscriber via a WLAN to access IP-based services compatible with those offered via the packet-switched domain. The interworking specification (3GPP 23.234) defines two procedures for the UMTS system. The first is WLAN Access, Authentication and Authorization, which will allow access to the WLAN and the locally connected IP network. Authentication and Authorization is done by the UMTS system and access to the locally connected network is known as WLAN Direct IP Access. Secondly, WLAN 3GPP IP Access will allow WLAN UEs to establish a connection to 3GPP IP-based services or the Internet via the UMTS network. 2.9.1
Interworking Network Elements
The WLAN UE is the user equipment equipped with a UMTS Integrated Service Card (UICC) card. The UE may be capable of WLAN access only, or both WLAN and UMTS operation. The 3GPP AAA server deals with Authentication, Authorization and Accounting for individual WLAN UEs accessing the system. The 3GPP AAA Server will be implemented in the 3GPP network as a proxy server. The packet data gateway will allow services on the 3GPP packet-switched network to be accessed by the WLAN UE.
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Internet/ intranet
3GPP network 3GPP AAA Server 802.11 WLAN Packet Data Gateway WLAN UE
WLAN 3GPP IP access
3GPP PS services and Internet access
Figure 14 Interworking with Wireless LANs SC2804/S7/v1.1
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Glossary of Terms
INTRODUCTION TO UMTS OPTIMIZATION
GLOSSARY OF TERMS
© Wray Castle Limited
i
Glossary of Terms
II
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Glossary of Terms
2G 2QPSK 3G 3GPP 4QPSK 16QAM
Second Generation 2-state Quadrature Phase Shift Keying Third Generation 3rd Generation Partnership Project 4-state Quadrature Phase Shift Keying 16-state Quadrature Amplitude Modulation
AAA ACI ACK ACLR AI AICH AM AMC AMR AP-AICH ARQ AS
Authentication, Authorization and Accounting Adjacent Channel Interference Acknowledgement Adjacent Channel Leakage Ratio Acquisition Indicator Acquisition Indicator Channel Acknowledged Mode Adaptive Modulation and Coding Adaptive Multi Rate Access Preamble Acquisition Indicator Channel Automatic Repeat Request Access Stratum
BCCH BCH BFN BLAST BLER BSIC BTS
Broadcast Control Channel Broadcast Channel Node B Frame Number Bell labs LAyered Space-Time Architecture Block Error Rate Base Station Identity Code Base Transceiver Station
CAC CCCH CD/CA-ICH CDMA CGI CI CPCH CPICH CRC CRNC CSICH CTCH CW
Call Admission Control Common Control Channel Collision Detection/Channel Assignment Indicator Channel Code Division Multiple Access Cell Global Identity Cell Identifier Common Packet Channel Common Pilot Channel Cyclic Redundancy Checksum Controlling Radio Network Controller CPICH Status Indicator Channel Common Traffic Channel Carrier Wave
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G.1
Glossary of Terms
DCCH DCH DL DoA DPCCH DPCH DPDCH DRX DSCH DTCH
Dedicated Control Channel Dedicated Channel Downlink Direction of Arrival Dedicated Physical Control Channel Dedicated Physical Channel Dedicated Physical Data Channel Discontinuous Reception Downlink Shared Channel Dedicated Traffic Channel
EFR EIRP
Enhanced Full Rate Effective Isotropic Radiated Power
FACH FBI FDD FEC
Forward Access Channel Feedback Information Frequency Division Duplex Forward Error Correction
GPS GSM
Global Positioning System Global System for Mobile Communications
HARQ HCS HSDPA HS-DPCCH HS-DSCH HS-PDSCH HS-SCCH
Hybrid Automatic Repeat Request Hierarchical Cell Structure High Speed Downlink Packet Access High Speed Dedicated Physical Control Channel High Speed Downlink Shared Channel High Speed Physical Downlink Shared Channel High Speed Shared Control Channel
IEEE IM IP IPDL IS IR
Institute of Electrical and Electronics Engineers Interference Margin Internet Protocol Idle Period Downlink Interim Standard Incremental Redundancy
KPI
Key Performance Indicator
LAC LCS LLC LMU LNA LOP
Location Area Code Location Services Logical Link Control Location Management Unit Low Noise Amplifier Line of Position
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Glossary of Terms
MAC MCL MEHO MHA MIMO MSC MUD
Medium Access Control Minimum Coupling Loss Mobile Evaluated Hand Over Mast Head Amplifier Multiple Input Multiple Output Mobile-services Switching Centre Multi-User Detection
NACK NAS NF NMC NRT
Negative Acknowledgement Non-Access Stratum Noise Figure Network Management Centre Non-Real Time
O&M OMC OTDOA OTSR OVSF
Operations and Maintenance Operations and Maintenance Centre Observed Time Difference of Arrival Omni Transmit Sector Receive Orthogonal Variable Spreading Factor
PA PCCH PCCPCH PCF PCH PCPCH PDA PDC PDF PDSCH PI PICH PLMN PO PRACH P-SCH
Power Amplifier Paging Control Channel Primary Common Control Physical Channel Position Calculation Function Paging Channel Physical Common Packet Channel Personal Digital Assistant Personal Digital Cellular Probability Distribution Function Physical Downlink Shared Channel Paging Indicator Paging Indicator Channel Public Land Mobile Network Power Offset Physical Random Access Channel Primary Synchronization Channel
QoS
Quality of Service
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G.3
Glossary of Terms
RAB RAC RACH RAT RL RLC RNC RNS RRC RSCP RSSI RT RTD RTT
Radio Access Bearer Routing Area Code Random Access Channel Radio Access Technology Radio Link Radio Link Control Radio Network Controller Radio Network Subsystem Radio Resource Control Received Signal Code Power Received Signal Strength Indication Real Time Real Time Difference Round Trip Time
SCCPCH SCH SDMA SFIR SFN SHO SID SIR SRNC S-SCH SSDT STTD
Secondary Common Control Physical Channel Synchronization Channel Spatial Division Multiple Access Spatial Filtering Interference Rejection System Frame Number Soft Handover Silence Description Signal to Interference Serving Radio Network Controller Secondary Synchronization Channel Site Selection Diversity Transmit Space Time Transmit Diversity
TDD TFCI TFRC TGD TGL TGPL TGPRC TGSN TMA TPC TSTD
Time Division Duplex Transport Format Combination Indicator Transport Format Resource Combination Transmission Gap start Distance Transmission Gap Length Transmission Gap Pattern Length Transmission Gap Pattern Repetition Count Transmission Gap Starting Slot Number Tower Mounted Amplifier Transmit Power Control Time Switched Transmit Diversity
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Glossary of Terms
UARFCN UE UHF UICC UL UM UMTS URA USIM UTRAN
UMTS Absolute Radio Frequency Channel Number User Equipment Ultra High Frequency UMTS Integrated Circuit Card Uplink Unacknowledged Mode Universal Mobile Telecommunications System UTRAN Registration Area UMTS Subscriber Identity Module UMTS Terrestrial Radio Access Network
WCDMA WLAN
Wideband Code Division Multiple Access Wireless Local Area Network
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Glossary of Terms
G.6
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