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Long Term Evolution (LTE) Radio Access Network Planning Guide
Long Term Evolution (LTE) Radio Access Network Planning Guide 1 How to Use This Guide
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1.1 Introduction ...............................................................................................................................................1 1.2 General Radio Network Planning Process ....................................................................................................1 1.3 Quick Guide to Content of Each Section .....................................................................................................2 2 LTE Fundamentals & Key Technologies .......................................................................................................3 2.1 Overview of Data Market as a Whole ..........................................................................................................3 2.2 3GPP Evolution and Market Expectation .....................................................................................................3 2.3 LTE Modulation Technology Highlight .........................................................................................................4 2.3.1 OFDM Fundamental .....................................................................................................................................................................5 2.3.2 SC-FDMA Fundamental ................................................................................................................................................................7
2.4 LTE Frame Structure ....................................................................................................................................8 2.5 LTE Resource Block Architecture ..................................................................................................................9 2.6 Reference Signal Structure ........................................................................................................................10 2.7 Timing and Sampling Architecture ............................................................................................................11 2.7.1 Normal and Extended Cyclic Prefix .............................................................................................................................................12 2.7.2 Synchronization Channel ............................................................................................................................................................13
2.8 Uplink Physical Channel Structure .............................................................................................................13 2.8.1 FDD Uplink Control, Sounding and Demodulation Reference Signal Structure
............................................................................14
2.9 Multiple Input Multiple Output (MIMO) ....................................................................................................15 2.9.1 3GPP MIMO Mode Definition .....................................................................................................................................................15 2.9.2 Open Loop MIMO ......................................................................................................................................................................16 2.9.3 Closed Loop MIMO ....................................................................................................................................................................17 2.9.4 Pre-coding Matrix ......................................................................................................................................................................18 2.9.5 Beam Forming
...........................................................................................................................................................................20
2.10 LTE FDD vs LTE TDD Main Features Comparison ......................................................................................21 2.11 LTE Channels Hierarchy Overview ............................................................................................................22 2.11.1 Physical Channel Modulation Schemes .....................................................................................................................................22 2.11.2 Downlink Channel Functionality Breakdown .............................................................................................................................23 2.11.3 Uplink Channel Functionality Breakdown .................................................................................................................................23 2.11.4 Channel Functionality Description in Detail ...............................................................................................................................23 2.11.5 Downlink Control Channel and RE Mapping Relationship .........................................................................................................25
2.12 Cell Search, Synchronization & Mobility–UE Call Flow View .....................................................................25 2.12.1 Cell Search and Synchronization ...............................................................................................................................................25 2.12.2 UE Procedure for Reporting Channel Quality Indication (CQI), Precoding Matrix indicator (PMI) and rank indication (RI) ...........26 2.12.3 System Information Bit Definition .............................................................................................................................................27 2.12.4 Mobility Management ..............................................................................................................................................................27
2.12.5 EUTRAN Hierarchy and Interface Overview ...............................................................................................................................27 2.12.6 Summary of Handover Call Flow – 3GPP Example TS36.300 .....................................................................................................28
2.13 Example of Peak Data Rate Calculation ...................................................................................................29 3 LTE Frequency and Spectrum Planning .....................................................................................................30 3.1 Frequency Spectrum Overview - FDD ........................................................................................................30 3.2 Frequency Spectrum Overview - TDD ........................................................................................................30 3.3 Channel Bandwidth and Subcarrier Allocation ...........................................................................................31 3.4 Channel Arrangement ...............................................................................................................................32 3.4.1 Channel Spacing ........................................................................................................................................................................32 3.4.2 Channel Raster ...........................................................................................................................................................................32 3.4.3 Carrier Frequency and EARFCN ...................................................................................................................................................33
3.5 Frequency Planning Recommendations .....................................................................................................34 3.5.1 Conventional Frequency Reuse Scheme 1*3*1 ............................................................................................................................34 3.5.2 SFR 1*3*1 – Downlink and Uplink ..............................................................................................................................................35 3.5.3 TDD Specific Frequency Planning Considerations ........................................................................................................................36 3.5.4 Frequency Band Selection ..........................................................................................................................................................37 3.5.5 Cyclic Prefix Planning
.................................................................................................................................................................38
3.5.6 Placing Multiple Technologies@Multiple Frequency Band ...........................................................................................................38
4 Link Budget and Coverage Planning .........................................................................................................40 4.1 Conventional Link Budget .........................................................................................................................41 4.2 Propagation Parameters ............................................................................................................................42 4.2.1 Channel Model ..........................................................................................................................................................................42 4.2.2 3GPP Value for Multipath and Doppler Effect .............................................................................................................................43 4.2.3 Propagation Model ....................................................................................................................................................................45 4.2.4 Penetration Loss .........................................................................................................................................................................50 4.2.5 Body Loss ...................................................................................................................................................................................51 4.2.6 Feeder Loss ................................................................................................................................................................................52 4.2.7 Background Noise ......................................................................................................................................................................53
4.3 Equipment-Related Parameters .................................................................................................................53 4.3.1 Transmit Power ..........................................................................................................................................................................53 4.3.2 Receiver Sensitivity .....................................................................................................................................................................54 4.3.3 Noise Figure ...............................................................................................................................................................................54 4.3.4 Antenna Gain .............................................................................................................................................................................54
4.4 LTE-Related Parameters .............................................................................................................................56 4.4.1 MIMO Gains ..............................................................................................................................................................................56 4.4.2 Cell Edge Rate ............................................................................................................................................................................57 4.4.3 Interference Margin ...................................................................................................................................................................59 4.4.4 Beam Forming
...........................................................................................................................................................................59
4.5 System Reliability ......................................................................................................................................60 4.5.1 Slow Fading Margin ...................................................................................................................................................................60 4.5.2 Effect of Earth Curvature ............................................................................................................................................................62 4.5.3 Absence of Fast Fading and Soft Handover Margin ....................................................................................................................62
4.6 Specific Factors in Link Budget Consideration ............................................................................................62 4.6.1 Features Overview ......................................................................................................................................................................62 4.6.2 TTI Bundling ...............................................................................................................................................................................63 4.6.3 Interference Rejection Combining ..............................................................................................................................................63 4.6.4 Reference Signal Power Boosting Gain .......................................................................................................................................65 4.6.5 Remote Radio Unit and eNodeB Portfolio ...................................................................................................................................65
4.7 Summary of Variables inside Link Budget Tools .........................................................................................65 5 Interference and Guard Band Analysis ......................................................................................................69 5.1 Overview ..................................................................................................................................................69 5.1.1 Basic Concepts ...........................................................................................................................................................................69 5.1.2 Analysis of Background Noise ....................................................................................................................................................73 5.1.3 Impact of Interference
...............................................................................................................................................................74
5.2 Interference Between TDD Systems ...........................................................................................................75 5.2.1 Interference between Different Carriers ......................................................................................................................................75 5.2.2 Interference within the Same Carrier ..........................................................................................................................................76 5.2.3 Theoretical Analysis of Interference under Site Sharing ...............................................................................................................77 5.2.4 Theoretical Analysis of Interference: Non Colocated eNodeB
.....................................................................................................78
5.3 Guard Band Requirement: LTE-FDD vs GSM/UMTS ....................................................................................79 5.4 GuardBand Requirement: LTE FDD vs LTE TDD ..........................................................................................79 5.5 Spectrum Refarming for LTE ......................................................................................................................80 5.5.1 Summary ...................................................................................................................................................................................80 5.5.2 GSM Spectrum Refarming ..........................................................................................................................................................80 5.5.3 Introduction of Buffer Zone ........................................................................................................................................................81
5.6 Radio Access Technologies Co-location Strategies .....................................................................................82 5.6.1 Overview ...................................................................................................................................................................................82 5.6.2 GSM-LTE Co-Location Examples .................................................................................................................................................82 5.6.3 LTE TDD and WiMAX Systems Co-Location .................................................................................................................................87
6 LTE Access Network Capacity Planning .....................................................................................................89 6.1 Definition of Capacity ...............................................................................................................................89 6.2 3GPP Services Classification ......................................................................................................................91 6.3 EUTRAN Capacity Limiting Factors ............................................................................................................91 6.3.1 Operating Frequency Band .........................................................................................................................................................92 6.3.2 RF coverage - RSRP ....................................................................................................................................................................93 6.3.3 Impact of Interference on Capacity ............................................................................................................................................93 6.3.4 Signal Interference Noise Ratio and Adaptive Coding .................................................................................................................94 6.3.5 Radio (Transmitter) Power Availability .........................................................................................................................................94 6.3.6 Spectrum Bandwidth Availability ................................................................................................................................................94 6.3.7 Base Band Channel Card Processing Capacity .............................................................................................................................94 6.3.8 S1/X2 Capacity ...........................................................................................................................................................................94 6.3.9 Application of Special Antenna Technologies (MIMO/BF/V MIMO) ..............................................................................................94 6.3.10 Scheduling Mode .....................................................................................................................................................................95 6.3.11 Actual Cell Site Placement in Relation to Traffic ........................................................................................................................96
6.3.12 UE Capability ............................................................................................................................................................................96 6.3.13 User Traffic Mix and Call Modelling ..........................................................................................................................................97 6.3.14 Time Slot Allocation for Uplink and Downlink – TDD specific ....................................................................................................97 6.3.15 Cyclical Prefix Allocation ...........................................................................................................................................................98
6.4 S1 Bandwidth Dimensioning Procedure .....................................................................................................98 6.5 X2 Bandwidth Dimensioning Procedure ....................................................................................................99 6.6 Impact of Latency of X2 on Cell Throughput ...........................................................................................100 6.7 Inter Radio Access Technology Handover Considerations ........................................................................100 7 U-Net Simulation and Operation ............................................................................................................103 7.1 Introduction ............................................................................................................................................103 7.2 Simulation Process ..................................................................................................................................103 7.3 Creating Project ......................................................................................................................................104 7.4 Geographical Information .......................................................................................................................104 7.4.1 Quick Import Function .............................................................................................................................................................104 7.4.2 Defining Coordinate Systems ....................................................................................................................................................105 7.4.3 Properties of Clutter Class ........................................................................................................................................................106
7.5 Equipment Parameter .............................................................................................................................107 7.5.1 Overview .................................................................................................................................................................................107 7.5.2 Network Settings .....................................................................................................................................................................107 7.5.3 Equipment ...............................................................................................................................................................................113 7.5.4 Site, Cell and Transmitter Listing
..............................................................................................................................................113
7.5.5 Viewing “Hidden” Parameters ..................................................................................................................................................115 7.5.6 Propagation Model Selection ...................................................................................................................................................115 7.5.7 Clutter Related Modelling ........................................................................................................................................................118 7.5.8 Impact of Parameter Setting on Prediction and Simulation .......................................................................................................118
7.6 Engineering Parameter ............................................................................................................................119 7.6.1 Power Setting ..........................................................................................................................................................................119 7.6.2 Load Setting
............................................................................................................................................................................121
7.6.3 Frequency Planning ..................................................................................................................................................................122 7.6.4 Scheduling Parameters .............................................................................................................................................................125 7.6.5 Antenna Property .....................................................................................................................................................................126 7.6.6 Properties of a Single Transmitter .............................................................................................................................................128 7.6.7 Properties of a eNodeB Template .............................................................................................................................................130
7.7 LTE Traffic Model Parameters ..................................................................................................................132 7.7.1 Overview .................................................................................................................................................................................132 7.7.2 Environments ...........................................................................................................................................................................132 7.7.3 User Profiles .............................................................................................................................................................................133 7.7.4 Terminals .................................................................................................................................................................................134 7.7.5 Mobility Types ..........................................................................................................................................................................135 7.7.6 Services ....................................................................................................................................................................................136 7.7.7 Traffic Map ..............................................................................................................................................................................137
7.8 Prediction and Simulation .......................................................................................................................141 7.8.1 Predictions ...............................................................................................................................................................................141
7.8.2 Simulation ................................................................................................................................................................................145
7.9 Point Analysis Tool ..................................................................................................................................151 7.9.1 Profile ......................................................................................................................................................................................151 7.9.2 Reception
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7.9.3 Signal Analysis .........................................................................................................................................................................152 7.9.4 Result
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7.10 RF Cell Planning Optimization ...............................................................................................................152 7.11 U-Net Planning Case .............................................................................................................................154 7.11.1 Overview of Planning Area .....................................................................................................................................................154 7.11.2 Site Distribution .....................................................................................................................................................................155 7.11.3 Parameter Configuration and General Assumption .................................................................................................................156 7.11.4 Network Coverage Predictions ...............................................................................................................................................157 7.11.5 Network Capacity Simulation .................................................................................................................................................160
8 LTE Network Key Performance Indicators ...............................................................................................163 8.1 KPI Measurement Methodology ..............................................................................................................163 8.2 KPI Acceptance Procedure ......................................................................................................................163 8.3 Service KPIs and Network KPIs ................................................................................................................164 8.4 Cluster and Test Route ............................................................................................................................164 8.5 Proposed Key Performance Indicators .....................................................................................................165 8.6 Proposed KPIs for Final Acceptance (Stability Acceptance, Optional) .......................................................165 9 Network Planning Checklist ...................................................................................................................166 9.1 Introduction ............................................................................................................................................166 9.2 Checklist Items Consideration .................................................................................................................166 9.2.1 Understanding Customer Spectrum Bandwidth Availability .......................................................................................................166 9.2.2 Actual Frequency Band Allocation for LTE .................................................................................................................................166 9.2.3 Frequency Band Refarming Requirement for LTE ......................................................................................................................167 9.2.4 Location of Customer Coverage Requirement ...........................................................................................................................167 9.2.5 Highway and Tunnel Coverage Requirement ............................................................................................................................168 9.2.6 Evaluate Existing Network Condition for InterRAT ....................................................................................................................168 9.2.7 Terrain and Clutter Database Availability and Accuracy .............................................................................................................168 9.2.8 Scheduler Selection ..................................................................................................................................................................169 9.2.9 Indoor Coverage Requirement
.................................................................................................................................................169
9.2.10 Cell Edge Throughput Requirement ........................................................................................................................................169 9.2.11 Call Model and SmartPhone Penetration Growth Considerations ............................................................................................169 9.2.12 Base Station Antenna and Other Co-siting Equipment Selection .............................................................................................170 9.2.13 Interference Protection and Isolation Requirement .................................................................................................................170 9.2.14 Radio Related Equipment Selection ........................................................................................................................................171 9.2.15 Network and Spectrum Evolution Consideration ....................................................................................................................171 9.2.16 MIMO and Beam Forming Implementation ............................................................................................................................171 9.2.17 Cyclic Prefix Planning
.............................................................................................................................................................171
9.2.18 Understanding of Current Transmission Backhaul Network Capability .....................................................................................172 9.2.19 UE Distribution and Channel Model : Pedestrian vs High Mobility ...........................................................................................172
9.2.20 TDD Specific Uplink and Downlink Configuration ...................................................................................................................172 9.2.21 Power Boosting Configuration ...............................................................................................................................................172
10 Appendix: RF Antenna Systems ..............................................................................................................174 10.1 Overview ..............................................................................................................................................174 10.2 Antenna Classification ..........................................................................................................................174 10.2.1 Frequency ..............................................................................................................................................................................174 10.2.2 Directivity ...............................................................................................................................................................................174
10.3 Main Specifications of Antenna
............................................................................................................174
10.3.1 Work Band .............................................................................................................................................................................175 10.3.2 Antenna Gain .........................................................................................................................................................................175 10.3.3 Antenna Pattern .....................................................................................................................................................................176 10.3.4 Beamwidth ............................................................................................................................................................................177 10.3.5 Relation between Beamwidth and Gain .................................................................................................................................177 10.3.6 Front-to-rear Ratio .................................................................................................................................................................178 10.3.7 Upper Side Lobe Suppression .................................................................................................................................................178 10.3.8 Polarization Mode ..................................................................................................................................................................179 10.3.9 Down Tilt ...............................................................................................................................................................................179 10.3.10 VSWR (Voltage Standing Wave Ratio) ...................................................................................................................................179 10.3.11 Port Isolation
.......................................................................................................................................................................180
10.3.12 Power Capacity ....................................................................................................................................................................180 10.3.13 Input Port of Antenna ..........................................................................................................................................................180 10.3.14 Passive Intermodulation (PIM) ..............................................................................................................................................180 10.3.15 Dimensions and Weight of Antenna .....................................................................................................................................181 10.3.16 Wind Load ...........................................................................................................................................................................181 10.3.17 Work Temperature and Humidity .........................................................................................................................................181 10.3.18 Lightning Protection .............................................................................................................................................................181 10.3.19 Three-proof Capability ..........................................................................................................................................................181 10.3.20 Camouflaged Antenna Scheme for Sites ...............................................................................................................................181 10.3.21 Customized Camouflage ......................................................................................................................................................182 10.3.22 Outlook Camouflage ............................................................................................................................................................183 10.3.23 Antenna Camouflage in Special Environment .......................................................................................................................183
11 References ...........................................................................................................................................184
1 How to Use This Guide 1.1 Introduction The purpose of this document is to provide systems engineers/planners with a set of guidelines and introductions to LTE deployment planning that may aid the design of a high quality Long Term Evolution (LTE) RF System. In general, most of the content provided in this planning guide can be applied to LTE system design with field implementation considerations. Specific RF planning information unique to Huawei’s LTE EUTRAN product is also provided. Although there are numerous and detailed references made to particular tools, it is not the purpose of this planning document to replace any product and tools' operating manual/instruction. Please refer to the official publications of the respective product/tool for their most up to date functionality.
1.2 General Radio Network Planning Process The flow diagram below shows one of the more common work procedures recommended by the Radio network planning team. It covers all the major area that requires technical attention from the conceptual beginning of a network design to the provisioning of final network parameters required for the deployment phases.
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1.3 Quick Guide to Content of Each Section The LTE RF Planning Guide is a collection of fairly independent chapters covering various aspects of LTE system RF design and implementation. The table below outlines the key features of each Chapter.
Table 1-1 Quick Guide Chapter #
Chapter Title
Detailed Description
1
How to Use this Guide
Understand the contents of this document.
2
LTE Fundamentals & Key Technologies
Learn LTE fundamental which includes PHY and MAC layer technology. Meanwhile, some key LTE technologies such as MIMO and FFR will be presented in this section.
3
Frequency and Spectrum Planning
Overview of LTE Spectrum definition as in 3GPP. Understanding the various reuse options available to LTE as well as band selection and combination overview
4
Link Budget and Coverage Planning
Understand the parameters that comprise the LTE RF Link Budget. Learn about some of the basic propagation models as well as critical features that affect link budget values.
5
Interference, Guard band and Refarming Analysis
Understand some basic concept for interference analyze such as ACS, ACLR, etc. Learn different interference between two different systems among serials TDD and FDD system.
6
LTE Access Network Capacity Planning
Overview of LTE capacity planning as well as highlight all the critical factors and considerations that will affect capacity for an LTE network.
7
U-Net Simulation and Operation
Understand U-Net operations. Learn definition of different parameters such as equipment parameters, engineering parameters, traffic model parameters, etc. High level view on how to predict and simulate based on U-Net.
8
LTE Network Key Performance Indicators
Provide LTE KPIs classification and KPI Acceptance Procedure
9
Network Planning Checklist
Provide a list of items that Planning engineers need to consider and ideally have answers from customer before performing any detail planning.
2
2 LTE Fundamentals & Key Technologies 2.1 Overview of Data Market as a Whole Challenges: Limited Investment but 500x Capacity Increment
2.2 3GPP Evolution and Market Expectation
Source: Global mobile Suppliers Association – October 2010 3
2.3 LTE Modulation Technology Highlight In Nov. 2004, 3GPP began a project to define the long-term evolution (LTE) of Universal Mobile Telecommunications System (UMTS) cellular technology. The main goal is to provide •• Higher throughput performance •• 100 Mbit/s peak downlink, 50 Mbit/s peak uplink •• 1G for LTE Advanced •• Higher cell edge performance •• Reduced latency in setup time. Shorter transfer delay, shorter handover latency and interruption time for better user experience •• Support of variable and scalable bandwidth (1.4, 3, 5, 10, 15 and 20 MHz) •• Backwards compatible with Existing 3G technologies •• Works with GSM/EDGE/UMTS systems •• Utilizes existing 2G and 3G spectrum and new spectrum •• Supports hand-over and roaming to existing mobile networks •• Quality of Service Support. •• Wide application •• TDD (unpaired) and FDD (paired) spectrum modes •• Mobility up to 450km/h •• Large range of terminals (phones and PCs to cameras) LTE employs Orthogonal Frequency Division Multiple Access (OFDMA) for downlink data transmission and Single Carrier FDMA (SC-FDMA) for uplink transmission. It is also important to remember that LTE systems operate in two separate domains, namely time and frequency as shown in the figure below for downlink.
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Figure below is the LTE uplink allocation structure from a time and frequency perspective.
2.3.1 OFDM Fundamental OFDM was selected for the downlink because it can •• Improved spectral efficiency •• Reduce ISI effect by multipath •• Provide better Protection against frequency selective fading OFDM is a scheme that offers good resistance to multipath and is now widely recognized as the method of choice for mitigating multipath for broadband wireless. It can be straightforwardly extended to a multi-access scheme called OFDMA, where each user is assigned a different set of subcarriers. I. Frequency Spectral Efficiency Improvement OFDM increases spectral efficiency by incorporating multiple carriers in the same frequency space as a single carrier.
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II. Reducing the Impact by Inter Symbol Interference (ISI) Improvement of frequency spectral efficiency requires the reduction of Inter symbol interference (ISI). This is achieved by tighter frequency roll off and alignment of nulls and peaks between different frequencies.
III. Better Protection Against Frequency Fading Smaller subcarrier and resource block bandwidth increase robustness against frequency related fading
With this smaller carrier bandwidth, the frequency coherence bandwidth is much smaller than 3G systems while and correlation factor is much higher. As a result, it will also be much easier to implement scheduling algorithm based on Frequency Selective Scheduling to improve system throughput in the manner shown below. 6
Frequency Selective Fading Resistance
2.3.2 SC-FDMA Fundamental Single Carrier-FDMA is a recently developed single carrier multiple access technique which has similar structure and performance to OFDMA. SC-FDMA can be viewed as a special OFDMA system with the user’s signal pre-encoded by discrete Fourier transform (DFT), hence also known as DFT-pre-coded OFDMA or DFT-spread OFDMA. One prominent advantage of SC-FDMA over OFDMA is the lower PAPR (peak-to-average power ratio) of the transmit waveform for loworder modulations like QPSK and BPSK, which benefits the mobile users in terms of battery life and power efficiency. OFDM signals have a higher peak-to-average ratio (PAR)—often called a peak-to-average power ratio (PAPR)—than single-carrier signals do. The reason is that in the time domain, a multicarrier signal is the sum of many narrowband signals. At some time instances, this sum is large and at other times is small, which means that the peak value of
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the signal is substantially larger than the average value. This high PAR is one of the most important implementation challenges that face OFDM, because it reduces the efficiency and hence increases the cost of the RF power amplifier, which is one of the most expensive components in the radio. The figure below shows the relationship between OFDM and SC-FDMA in LTE. The major difference between the downlink and uplink transmission scheme is that each subcarrier in the uplink carries information about each transmitted modulation symbol as shown in figure below, whereas in downlink each subcarrier only carries information related to one specific modulation symbol. As a result, the uplink power level due to SC-FDMA also need to be increased by 2~3dB to compensate for the extra noise due to more spreading.
2.4 LTE Frame Structure The figure below shows the frame structure for LTE under Time division mode (TDD) Type 2 and Frequency Division mode (FDD) Type 1. Main differences between the two modes are •• Frame 0 and frame 5 (always downlink in TDD) •• Frame 1 and frame 6 is always used as for synchronization in TDD •• Frame allocation for Uplink and Downlink is settable in TDD
The sampling rate in both FDD and TDD is the same and both technologies operate under a 1-ms sub-frame (TTITransmission Time Interval) and 0.5us timeslot definition. The first 3 configurations (0-2) for TDD can also be viewed as 5ms allocation due to repetition. The figure below shows a detailed relationship between rates and frame structure. 8
2.5 LTE Resource Block Architecture The building block of LTE is a physical resource block (PRB) and all of the allocation of physical resource blocks (PRBs) is handled by a scheduling function at the 3GPP base station (eNodeB). In summary, •• One frame is 10ms and it consists of 10 sub-frames •• One subframe is 1ms and contains 2 slots •• One slot is 0.5ms in time domain and each 0.5ms assignment can contain N resource blocks [6 < N < 110] depending on the bandwidth allocation and resource availability. •• One resource block is 0.5ms and contains 12 subcarriers for each OFDM symbol in frequency domain. •• There are 7 symbols (normal cyclic prefix) per time slot in the time domain or 6 symbols in long cyclic prefix.
Resource element is the smallest unit of resource assignment and its relationship to resource block is shown as below from both a timing and frequency perspective. 9
2.6 Reference Signal Structure Reference signal is the “UMTS Pilot” equivalent and it is used by UE to predict the likely coverage condition on offer for each of the eNodeB cell received. The figure below shows the locations of the reference signal within each sub-frame when transmit antennae are used by the cell.
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As LTE is a MIMO based technology, it can have more than two transmit antennae and in order to avoid reference signals from the same cell interfering with each other, different antennae will be transmitting reference signal at different time and frequency and how these are allocated are shown below. As defined in the standard for TDD operations, the channel-sounding mechanism involves the UE’s transmitting a deterministic signal that can be used by the eNodeB to estimate the UL channel from the UE. If the UL and DL channels are properly calibrated, the eNodeB can then use the UL channel as an estimate of the DL channel, due to channel reciprocity.
2.7 Timing and Sampling Architecture Sampling frequency varies under different bandwidth configuration in LTE and the table below summarizes the possible combinations.
A quick summary of all the physical layer information for LTE is shown below.
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2.7.1 Normal and Extended Cyclic Prefix
The key to making OFDM realizable in practice is the use of the FFT algorithm, which has low complexity. In order for the IFFT/FFT to create an ISI-free channel, the channel must appear to provide a circular convolution. Adding cyclic prefix to the transmitted signal to create a signal that appears to be just like circular convolution and this is done by copying the last part of each OFDM symbol to the front of each symbol with the length of a guard interval, to form a cyclic prefix (CP). Also, to prevent the guard interval from destroying the inter-sub-carrier orthogonality, the delay of each path should not exceed the guard interval where the number of waveforms within the integral time of the FFT is an integer
The cyclic prefix, although elegant and simple, is not entirely free. It comes with both a bandwidth and power penalty. Since redundant symbols are sent, the required bandwidth for OFDM also increases. Similarly, an additional symbol must be counted against the transmit-power budget. Hence, the cyclic prefix carries a power penalty of v dB in addition to the bandwidth penalty. In summary, the use of the cyclic prefix entails data rate and power losses. The “wasted” power has increased importance in an interference-limited wireless system, causing interference to neighboring users. 12
Where L is the power used for non CP transmission. In the case where there is a large delay spread, e.g. due to large cell radius, an extended CP option can be used.
2.7.2 Synchronization Channel The diagram below shows the relative position of Primary Synchronization (PSS) and Secondary Synchronization (SSS) within the radio frame in a FDD LTE system.
The figure below shows the location of PSS and SSS in LTE-TDD and the major difference from LTE FDD is that LTE TDD embedding the Primary Sync channel in the DwPTS so the location will not be affected by different DL/UL combination ratio
2.8 Uplink Physical Channel Structure It is worth mentioning the physical structure of uplink channel. One uplink Slot is as below.
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2.8.1 FDD Uplink Control, Sounding and Demodulation Reference Signal Structure The figure below shows the relative position of uplink control channels in the frequency domain in relation to the entire channel bandwidth. In summary, 1) PUCCH resources are located at the edges of the spectrum •• To maximize frequency diversity 2) Multiple UEs can share the same PUCCH resource block 3) PUCCH is never transmitted simultaneously with PUSCH from the same UE 4) Two consecutive PUCCH slots in Time-Frequency Hopping at the slot boundary
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The Figure below shows respective position of the uplink demodulation reference signal in FDD LTE uplink frame structure including sounding reference signal position.
For LTE TDD only, SRSs can be transmitted in an ordinary sub-frame or in UpPTS sub-frame to improve spectral efficiency. Normally, it uses UpPTS sub-frame.
2.9 Multiple Input Multiple Output (MIMO) MIMO and other transmit spatial diversity scheme is a newer application than receive diversity and has become widely implemented only in the early 2000s. As the signals sent from different transmit antennas interfere with one another, processing is required at both the transmitter and the receiver in order to achieve gain while removing or at least attenuating the spatial interference. By using multiple antenna to transmit multiple path of information to UEs, either better throughput or lower SINR requirement can be achieved and the frequency selective characteristics of LTE is perfect for the implementation of such technologies. In general there are two mode of MIMO, open and closed loop. Additionally, if the multiple antennae are already at the base station for uplink receive diversity, the incremental cost of using them for transmit diversity is very low. Multiple antennae transmit schemes—both transmit diversity and spatial multiplexing—are often categorized as either open loop or closed loop. A high level signal processing diagram is shown below.
2.9.1 3GPP MIMO Mode Definition The table below shows the 8 definition used by 3GPP for MIMO modes 15
2.9.2 Open Loop MIMO Open-loop systems do not require knowledge of the channel at the transmitter. As a result, open loop operations occur when the access network does not have information or feedback from the UE to do coding adjustment or signal is not good enough.
The figure below shows a possible N Antennae + M input layers setup in spatial multiplexing
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2.9.3 Closed Loop MIMO On the contrary, closed-loop systems require channel knowledge at the transmitter, thus necessitating either channel reciprocity—same uplink and downlink channel, possible in TDD—or more commonly a feedback channel from the receiver to the transmitter. Hence, unlike open loop, closed loop operations occur when the access network execute dynamic adjustment based on feedback from the UE. The figure below shows a functional view of closed loop MIMO.
As a result, a more accurate coding application can be applied to the communication with the UE. The figure below shows where the pre-coding function may exist in a N Antennae with M input layers
In mode 5 (Multi-user MIMO), different UEs are receiving downlink data from different antenna. As a result, the overall throughput per cell is increased. 17
2.9.4 Pre-coding Matrix 3GPP 36-211 defines the types of matrix need to be used when multiple antennae are to be used for different conditions. The following is a quick summary of some possible pre-coding matrix combination under different scenarios I. Spatial Multiplexing Matrix Using Two Antenna Ports with Cell-Specific Reference Signals Spatial multiplexing is where multiple independent streams are transmitted across multiple antennas. If the receiver also has multiple antennas, the streams can be separated out using spatial multiplexing. Instead of increasing diversity, multiple antennas in this case are used to increase the data rate or capacity of the system. In a rich multipath environment, the capacity of the system can theoretically be increased linearly with the number of antennas when performing spatial multiplexing. Even two appropriately spaced antennas appear to be sufficient to eliminate most deep fades, which paints a promising picture for the potential benefits of spatial diversity. One main advantage of spatial diversity relative to time and frequency diversity is that no additional bandwidth or power is needed in order to take advantage of spatial diversity. The cost of each additional antenna, its RF chain, and the associated signal processing required to modulate or demodulate multiple spatial streams may not be negligible, but this trade-off is often very attractive for a small number of antennas, However, unlike transmit diversity and beam-forming, spatial multiplexing works mainly under good SINR conditions. A 2 × 2 MIMO system doubles the peak throughput capability of LTE but this is unlikely to be possible for all users in the cell due to variation in SINR.The capacity, or maximum data rate, grows as when the SINR is large. When the SNR is high, spatial multiplexing is optimal. On the other hand, when the SINR is low, the capacity maximizing strategy is to send a single stream of data, using diversity pre-coding. Although capacity gain is much smaller than at high SINR, the capacity still grows approximately linearly with since capacity is linear with SINR in the low-SINR regime. If the mobile station has only one antenna, LTE can still support spatial multiplexing by coding across multiple users in the uplink. This is called Multi-User MIMO (MU-MIMO). The matrix used for two antennae spatial multiplexing is shown below.
18
II. Transmit Diversity Matrix Using Two Antenna Ports The following matrix applies to input x is and y is the resulting output using a two antenna output configuration.
III. Spatial Multiplexing Matrix Using Four Antenna Ports with Cell-Specific Reference Signals
19
IV. Transmit Diversity Matrix Using Four Antenna Ports The following matrix applies to input x is and y is the resulting output under a four antenna output configuration.
2.9.5 Beam Forming Multiple antennas in LTE may also be used to transmit the same signal appropriately weighted for each antenna element such that the effect is to focus the transmitted beam in the direction of the receiver and away from interference, thereby improving the received SINR. The beam-forming weight vector should increase the antenna gain in the direction of the desired user while simultaneously minimizing the gain in the directions of interferers. Beamforming can provide significant improvement in the coverage range, capacity, and reliability. To perform transmit beamforming, the transmitter needs to have accurate knowledge of the channel, which in the case of TDD is easily available owing to channel reciprocity but for FDD requires a feedback channel to learn the channel characteristics so it is not implemented in LTE Release 8 or 9 yet. As of today, beam forming is specific only to LTE TDD and can operate either under 4x4 or 8x2 configurations.
20
One popular beam-forming algorithm is based on Direction of Arrival where the incoming signals to a receiver may consist of desired energy and interference energy—for example, from other users or from multipath reflections. The various signals can be characterized in terms of the DOA or the angle of arrival (AOA) of each received signal. Each DOA can be estimated by using EUTRAN signal-processing techniques as requested in 3GPP-TS 36-214. From these acquired DOAs, a beam-former extracts a weighting vector for the antenna elements and uses it to transmit or receive the desired signal of a specific user while suppressing the undesired interference signals. Ideally, the beam-former has unity gain for the desired user and two nulls at the directions of two interferers and can place nulls in the directions of interferers. The DOA-based beam-former in this case is often called the null-steering beam-former. The null-steering beam-former can be designed to completely cancel out interfering signals only if the number of such signals is strictly less than the number of antenna elements. Typically, there exists a trade-off between interference null and desired gain lost. Thus far, we have assumed that the array response vectors of different users with corresponding AOAs are known. In practice, each resolvable multipath is likely to comprise several unresolved components coming from significantly different angles. In this case, it is not possible to associate a discrete AOA with a signal impinging the antenna array. Therefore, the DOA based beam-former is viable only in LOS environments or in environments with limited local scattering around the transmitter.
2.10 LTE FDD vs LTE TDD Main Features Comparison The following table summarizes the major similarity between LTE FDD and LTE TDD
The table below summarizes the difference between the two technologies.
21
2.11 LTE Channels Hierarchy Overview 2.11.1 Physical Channel Modulation Schemes Supported modulation schemes in LTE are: QPSK, 16QAM, 64QAM. Maximum information block size = 6144 bits and CRC-24 is used for error detection. Broadcast channel only uses QPSK and are shown below.
22
2.11.2 Downlink Channel Functionality Breakdown
2.11.3 Uplink Channel Functionality Breakdown
2.11.4 Channel Functionality Description in Detail Physical channels •• PDSCH: Physical Downlink Shared Channel •• PBCH: Physical broadcast channel •• PMCH: Physical multicast channel •• PDCCH: Physical Downlink Control Channel •• PCFICH: Physical control format indicator channel •• PHICH: Physical Hybrid ARQ Indicator Channel 23
Reference Signal (RS) •• Cell specific RS •• UE-specific RS •• MBSFN RS Synchronization Signal (SCH) •• Primary Synchronization Signal (P-SCH) •• Secondary Synchronization Signal (S-SCH) SCH used for: •• Symbol synchronization •• Frame synchronization •• Cell-ID determination
SCH/BCH each occupy 72 center subcarriers regardless of system bandwidth
BCH indicates: •• Basic L1/L2 system parameters •• Downlink system bandwidth •• Reference-signal transmit power •• Multi-media Broadcast over a Single Frequency Network (MBSFN)-related parameters •• Number of transmit antennas •• HARQ resource allocation Control region is 1-3 OFDM symbols at the beginning of each subframe, composed of control channel elements (CCEs) •• 4 Res = Resource element group (REG) •• 9 REGs = 1 CCE PCFICH – Physical Control Format Indicator Channel •• # of OFDM symbols of control region PHICH – Physical Hybrid ARQ Channel •• ACK/NACK signalling PDCCH – Physical Downlink Control Channel •• Scheduling •• UL power control 24
2.11.5 Downlink Control Channel and RE Mapping Relationship
2.12 Cell Search, Synchronization & Mobility–UE Call Flow View 2.12.1 Cell Search and Synchronization
25
2.12.2 UE Procedure for Reporting Channel Quality Indication (CQI), Precoding Matrix Indicator (PMI) and Rank Indication (RI) As stated in TS 36-213, the time and frequency resources that can be used by the UE to report CQI, PMI, and RI are controlled by the eNodeB. For spatial multiplexing, the UE shall determine a RI corresponding to the number of useful transmission layers. For transmit diversity RI is equal to one. A UE in transmission mode 8 is configured with PMI/RI reporting if the parameter PMI-RI-Report is configured by higher layer signaling; otherwise, it is configured without PMI/RI reporting. CQI, PMI, and RI reporting is periodic or a-periodic. A UE shall transmit periodic CQI/PMI, or RI reporting on PUCCH as defined hereafter in sub-frames with no PUSCH allocation. A UE shall transmit periodic CQI/PMI or RI reporting on PUSCH as defined hereafter in sub-frames with PUSCH allocation, where the UE shall use the same PUCCH-based periodic CQI/PMI or RI reporting format on PUSCH. A UE shall transmit a-periodic CQI/PMI, and RI reporting on PUSCH if the conditions specified hereafter are met. For a-periodic CQI reporting, RI reporting is transmitted only if configured CQI/PMI/RI feedback type supports RI reporting. Figure below shows which channels will be used for different CQI reporting scenario
26
2.12.3 System Information Bit Definition
2.12.4 Mobility Management
2.12.5 EUTRAN Hierarchy and Interface Overview
27
2.12.6 Summary of Handover Call Flow – 3GPP Example TS36.300
28
2.13 Example of Peak Data Rate Calculation
29
3 LTE Frequency and Spectrum Planning 3.1 Frequency Spectrum Overview - FDD 3rd Generation Partnership Project (3GPP) Release 8/9 (3GPP TS36.104-860 Table 5.5-1 E-UTRA frequency bands) has clearly defined LTE as a system that can operate in various frequency bands into order to suit the need of different operators in the world. The table below shows the actual frequency range listed per the specification for the Frequency Division Duplex (FDD) version.
Figure 3-1 LTE FDD Spectrum Allocation The most popular commercial LTE bands are 2.6GHz (Band 7), AWS (Band 4) and 700MHz (Band 12) while momentum is being built up also for 1800MHz (Band 3) as well as Public Safety spectrum (Band 14) According to 3GPP TS 36.104 V9.4.0 (2010-06), Band 6 is no longer applicable and Band 15 and Band 16 are listed as Reserved.
3.2 Frequency Spectrum Overview - TDD 3GPP Release 8/9 (3GPP TS36.104-860 Table 5.5-1 E-UTRA frequency bands) has also defined the operating frequency for Time Division Duplex (TDD) based LTE technology in various frequency bands in order to operate in different parts of the world. The table below shows the actual frequency range listed per the specification for the TDD version. 30
Figure 3-2 LTE TDD Spectrum Allocation
It is worth noting that around the 2.3GHz band (Band 40), there is a significant frequency spectrum overlap (100MHz) between LTE TDD with WiMAX. To many WiMAX operators currently in this frequency band, it is an ideal opportunity to evolve their network back into the mainstream LTE technologies.
3.3 Channel Bandwidth and Subcarrier Allocation According to 3GPP specification, Operators can assign different channel bandwidth to suit their particular needs per the figure below. The number of RB supported for each bandwidth is equal to number of sub-carriers divided by 12.
Figure 3-3 Transmission bandwidth configuration NRB in E-UTRA channel bandwidths
The channel edges are defined as the lowest and highest frequencies of the carrier separated by the channel bandwidth, i.e. at FC +/- BWChannel /2. 31
Figure 3-4 Definition of Channel Bandwidth and Transmission Bandwidth Configuration for one E-UTRA carrier
Figure 3-5 Visualizing the Relationship between Channel Bandwidth, NRB and Transmission Bandwidth Configuration
3.4 Channel Arrangement According to 3GPP specification, operators can assign different channel bandwidth to suit their particular needs per the table below.
3.4.1 Channel Spacing The spacing between carriers will depend on the deployment scenario, the size of the frequency block available and the channel bandwidths. The nominal channel spacing between two adjacent E-UTRA carriers is defined as following:
Nominal Channel spacing = (BWChannel(1) + BWChannel(2))/2
where BWChannel(1) and BWChannel(2) are the channel bandwidths of the two respective E-UTRA carriers. The channel spacing can be adjusted to optimize performance in a particular deployment scenario.
3.4.2 Channel Raster The channel raster is 100 kHz for all bands, which means that the carrier centre frequency must be an integer multiple of 100 kHz. 32
3.4.3 Carrier Frequency and EARFCN The carrier frequency in the uplink and downlink is designated by the E-UTRA Absolute Radio Frequency Channel Number (EARFCN) in the range 0 - 65535. The relation between EARFCN and carrier frequency in MHz for the downlink is given by the following equation, where FDL_low and NOffs-DL are given in table 5.7.3-1 and NDL is the downlink EARFCN.
FDL = FDL_low + 0.1(NDL – NOffs-DL)
The relation between EARFCN and carrier frequency in MHz for the uplink is given by the following equation where FUL_low and NOffs-UL are given in table 5.7.3-1 and NUL is the uplink EARFCN.
FUL = FUL_low + 0.1(NUL – NOffs-UL)
NOTE: The channel numbers that designate central carrier frequencies so close to the operating band edges that the carrier extends beyond the operating band edge shall not be used. This implies that the first 7, 15, 25, 50, 75 and 100 channel numbers at the lower operating band edge and the last 6, 14, 24, 49, 74 and 99 channel numbers at the upper operating band edge shall not be used for channel bandwidths of 1.4, 3, 5, 10, 15 and 20 MHz respectively because of the bandwidth requirement. For example, for a 20MHz carrier, using channel 99 as center frequency will extend the LTE carrier below the allocated spectrum (99*0.1 = 9.9MHz but actual requirement is 10MHz from lower edge) E-UTRA Operating Band 1
Downlink
Uplink
FDL_low [MHz]
NOffs-DL
Range of NDL
FUL_low [MHz]
NOffs-UL
Range of NUL
2110
0
0 - 599
1920
18000
18000 - 18599
2
1930
600
600 - 1199
1850
18600
18600 - 19199
3
1805
1200
1200 - 1949
1710
19200
19200 - 19949
4
2110
1950
1950 - 2399
1710
19950
19950 - 20399
5
869
2400
2400 - 2649
824
20400
20400 - 20649
6
875
2650
2650 - 2749
830
20650
20650 - 20749
7
2620
2750
2750 - 3449
2500
20750
20750 - 21449
8
925
3450
3450 - 3799
880
21450
21450 - 21799
9
1844.9
3800
3800 - 4149
1749.9
21800
21800 - 22149
10
2110
4150
4150 - 4749
1710
22150
22150 - 22749
11
1475.9
4750
4750 - 4949
1427.9
22750
22750 - 22949
12
729
5010
5010 - 5179
699
23010
23010 - 23179
13
746
5180
5180 - 5279
777
23180
23180 - 23279
14
758
5280
5280 - 5379
788
23280
23280 - 23379
734
5730
5730 - 5849
704
23730
23730 - 23849
… 17 18
860
5850
5850 - 5999
815
23850
23850 - 23999
19
875
6000
6000 - 6149
830
24000
24000 - 24149
20
791
6150
6150 - 6449
832
24150
24150 - 24449
21
1495.9
6450
6450 - 6599
1447.9
24450
24450 - 24599
33
1900
36000
36000 - 36199
1900
36000
36000 - 36199
34
2010
36200
36200 - 36349
2010
36200
36200 - 36349
…
33
E-UTRA Operating Band 35
Downlink
Uplink
FDL_low [MHz]
NOffs-DL
Range of NDL
FUL_low [MHz]
NOffs-UL
Range of NUL
1850
36350
36350 - 36949
1850
36350
36350 - 36949
36
1930
36950
36950 - 37549
1930
36950
36950 - 37549
37
1910
37550
37550 - 37749
1910
37550
37550 - 37749
38
2570
37750
37750 - 38249
2570
37750
37750 - 38249
39
1880
38250
38250 - 38649
1880
38250
38250 - 38649
40
2300
38650
38650 - 39649
2300
38650
38650 - 39649
41
2496
39650
39650 - 41589
2496
39650
39650 - 41589
42
3400
41590
41590 - 43589
3400
41590
41590 - 43589
43
3600
43590
43590 - 45589
3600
43590
43590 - 45589
NOTE: T he channel numbers that designate carrier frequencies so close to the operating band edges that the carrier extends beyond the operating band edge shall not be used. This implies that the first 7, 15, 25, 50, 75 and 100 channel numbers at the lower operating band edge and the last 6, 14, 24, 49, 74 and 99 channel numbers at the upper operating band edge shall not be used for channel bandwidths of 1.4, 3, 5, 10, 15 and 20 MHz respectively.
Figure 3-6 E-UTRA channel numbers – 3GPP 36104-A10 – Table 5.7.3-1
3.5 Frequency Planning Recommendations 3.5.1 Conventional Frequency Reuse Scheme 1*3*1 Under this scheme, a single frequency will be used for the entire system. Although it eliminates the need of any frequency planning considerations, it also opens the door for inter-site and inter-sector interference which is detrimental for urban LTE deployment due to the high site density.
Figure 3-7 Conventional 1*3*1 frequency planning scheme
Application scenario •• Limited application scenario in urban and suburban environment without impacting QoS/QoE. •• Possible application in highly isolated rural scenario where users are also highly scattered 34
Advantage •• High spectral efficiency and high throughput per site. •• Easy to deploy. •• No special scheduling algorithm required Disadvantage •• High level of interference especially on cell edge area •• Low throughput on cell boundary and lower QoS/QoE for users on boundary area. •• Coverage control of cells becomes an important factor in achieving a high throughput level
3.5.2 SFR 1*3*1 – Downlink and Uplink SFR (Soft Frequency reuse) is the recommended frequency reuse methodology. Both FDD and TDD can use this interference reduction method. The SFR concept is based on dividing the entire LTE carrier bandwidth into 3 sub-sections as shown below
Figure 3-8 SFR 1*3*1 Downlink frequency division scheme Under this configuration, each sector will only use one of the sub-sections, also known as the primary band, which “1/3” of the entire carrier bandwidth, to serve the cell edge users. As a result, the interference level between sectors can be reduced, thereby enhancing the throughput of those users. For those users location near the center of the cell, the other 2 sections, which is the remaining “2/3” of the carrier bandwidth, also known as the secondary band, will be used to serve these users. The figure below depicts the actual layout
Figure 3-9 SFR 1*3*1 Downlink frequency planning scheme 35
Application scenario •• Recommended configuration to satisfy high traffic and high site density requirement. •• Best results will require the introduction of Inter Cell Interference Coordination (ICIC) Advantage •• Reduce inter-cell interference under a high site density deployment. •• Improve cell edge user throughput and quality of experience.
3.5.3 TDD Specific Frequency Planning Considerations It is very common for telecom Operators within the TDD band of LTE have a wider unpaired spectrum than the bandwidth defined maximum carrier bandwidth of 20MHz. As a result, the selection of carrier bandwidth for multiple carrier condition is also more complex in TDD than FDD. Moreover, the coexistence of WiMAX within the same TDD spectrum is also very common and this has further complicated the carrier and bandwidth planning for LTE TDD network from a carrier planning perspective. Planning engineers need to take all these variations along with customer throughput and coverage requirement into account when it comes to TDD frequency planning. Besides, carrier bandwidth, co-frequency and time sharing nature between uplink and downlink in TDD also require careful selection of guard band and pilot time slot (DwPTS, GP and UpPTS). Failure to include enough separation will create a lot of co-channel interference which will degrade the throughput performance significantly
Figure 3-10 Uplink-Downlink Pilot Time Slot and Guard band Configuration Schemes
Lastly, for TDD to work properly, all cells must be operating in time synchronous mode to avoid any extra interference being introduced to the network. IEEE 1588v2 implementation is recommended and will help to ensure the integrity of time synchronization within the LTE TDD network. 36
Figure 3-11 Synchronization Solution based on IPclk or 1588v2
3.5.4 Frequency Band Selection As many Operators worldwide possess spectrum in various frequency bands, choosing which band to use for LTE is always an important consideration. Parameters that will affect the overall cell coverage will be discussed in the next chapter. However, it is important to remember many components on the radio path will have slightly different properties at different frequency bands which will modify the final cell coverage radius. For example, antenna gain, feeder loss, power amplifier output, propagation characteristics, cell edge user throughput and penetration loss are all dependent on the operating frequency chosen. Results shown below are typical comparison in coverage radius between different frequency bands. Final results are highly dependent on the actual parameters used for customer design.
Cell Range in Uplink Case -- Result
Figure 3-12 Cell Coverage Comparison (UL@128kbps) between various frequency bands 37
Cell Range in Downlink Case -- Result
Figure 3-13 Cell Coverage Comparison (DL@1024kbps) between various frequency bands
3.5.5 Cyclic Prefix Planning Although Cyclic Prefix is not directly related to frequency or spectrum allocation, it will impact the actual cell range that can be served from a logical and signal processing perspective. By carrying a smaller number of symbols (6), a bigger cyclic prefix is configured per cell to allow a bigger delay in propagation. This is also known as long CP. The figure below shows the difference in symbol configurations between the normal, 7 symbols configuration (norma lCP) against 6 symbols (long CP configuration)
Figure 3-14 Cyclic Prefix Comparison
3.5.6 Placing Multiple Technologies@Multiple Frequency Band Choosing which technologies for which spectrum is a major challenge for many Operators worldwide. It is highly dependent on what the Operator already owned and what is their future business plan. Typically, higher frequency bands are likely to deploy more data centric services for high density area (e.g. CBD). As a result, LTE is more likely the technology of choice for most Operators looking at launching data services in the higher 38
frequency band. The figure below just some 1 example of what customer may do with multiple technologies and their evolution in different frequency band. It is the responsibility of the radio planner and account managers to work with customer to determine the best combination to meet their interest.
Dual-band Network Deployment is a trend
Figure 3-15 Example of Multiple Technologies Deployment to Various Frequency Band
SingleSON Solution Benefits: •• SingleSON brings synergized automation for GSM, UMTS and LTE •• It can remarkably reduce operational cost and improve efficiency, better user experience.
39
4 Link Budget and Coverage Planning Operators are rightfully focused on the service quality of a system and coverage is an important part of the service quality of a system. The aim of radio network planning is to balance coverage, capacity, quality, and cost so none of these can be considered in isolation. Various factors must be considered during LTE system coverage planning and setting of these parameters will affect coverage radius and the quantity of base stations. Coverage and design requirement must be analyzed in choosing parameters within the following parameter groups: •• Propagation-related •• Equipment-related •• LTE-specific •• System Reliability •• Specific Considerations Achievable cell radius can be derived from the Excel based link budget tools. Network planning tool, GENEX U-net, will provide site deployment specific simulation analysis to obtain the number of required base stations in the target area. The coverage area offered by a 3 sector and Omni site along with coverage planning flow is shown below
Figure 4-1 Radio network coverage pre-planning flow 40
This chapter will focus on the RF link budget itself and radio transmission model. System simulation will be described in Chapter7.
4.1 Conventional Link Budget The purpose of link budget in LTE network planning is: •• To use such factors as building penetration loss, feeder loss, antenna gain, and the interference margin of radio links to calculate all gains and losses that will affect the final cell coverage •• To estimate the maximum link loss allowed based on the maximum transmit power of the terminal and eNodeB transmit power allocation. Coverage radius of a base station can be obtained according to the maximum link loss allowance under a certain propagation model. The radius can be used in subsequent design. Link budget parameters are grouped as follows: •• Propagation (Transmission) related parameters, such as the penetration loss, body loss, feeder loss, and background noise •• Equipment dependent parameters, such as the transmit power, receiver sensitivity, and antenna gain •• LTE-specific parameters, such as the pilot power boosting gain, Multiple Input Multiple Output (MIMO) gain, edge coverage rate, repeated coding gain, interference margin, and fast fading margin •• System reliability parameters, such as slow fading margin •• Specific features that will affect the final path gain The figure below shows factors that will affect the link budget calculation process.
41
Figure 4-2 Link budget model – Downlink and Uplink
4.2 Propagation Parameters Propagation-related parameters have no relationship with technical systems or equipment vendors. Propagation-related gains or losses are constant, and are related to the environment of radio wave transmission. Such parameters include the penetration loss, body loss, feeder loss, and background noise. To obtain an objective value when comparing the link budget information of two equipment vendors, you must set the propagation parameters to be the same values.
4.2.1 Channel Model Channel models used for LTE are defined in 3GPP TS 36.101 where the test condition was specified. Items covered include multi-path conditions, fading, and the terminal motion speed of the channel. Common models include speed at 3km/h, 30km, 60km and 120km. If required, different speed/condition can also be introduced and simulated according to specific needs. EPA3 and ETU3 are applicable to fixed services or pedestrian speed services. ETU30, ETU60, ETU120, EVA30, EVA60 and EVA 120 are applicable to vehicular services. Common channel models in LTE systems include EPA (Extended Pedestrian A), EVA (Extended Vehicular Model A) and ETU3 (Extended Typical Urban Model at 3km/hr).
42
Table 4-1 Typical Propagation Channel Models used for LTE
PDP
Extended Pedestrian A model
Extended Vehicular A model
Extended Typical Urban model
# of Paths
7
9
9
Relative Path Power (dB)
Delay (ns)
0.0
0
0.0
0
-1.0
0
-1.0
30
-1.5
30
-1.0
50
-2.0
70
-1.4
150
-1.0
120
-3.0
90
-3.6
310
0.0
200
-8.0
110
-0.6
370
0.0
230
-17.2
190
-9.1
710
0.0
500
-20.8
410
-7.0
1090
-3.0
1600
-12.0
1730
-5.0
2300
-16.9
2510
-7.0
5000
4.2.2 3GPP Value for Multipath and Doppler Effect Table below shows some of the propagation conditions that are used for performance measurements in multi-path fading environment at low, medium & high Doppler frequencies Model
Maximum Doppler frequency
EPA 5Hz
5 Hz
EVA 5Hz
5 Hz
EVA 70Hz
70 Hz
ETU 70Hz
70 Hz
ETU 300Hz
300 Hz
Table below shows possible variation of received power in multi-path fading environment under the various extended delay spread conditions listed below Extended Pedestrian A Model - EPA Excess tap delay [ns]
Relative power [dB]
0
0.0
30
-1.0
70
-2.0
90
-3.0
110
-8.0
190
-17.2
410
-20.8
Extended Typical Urban Model – ETU
43
Excess tap delay [ns]
Relative power [dB]
0
-1.0
50
-1.0
Excess tap delay [ns]
Relative power [dB]
120
-1.0
200
0.0
230
0.0
500
0.0
1600
-3.0
2300
-5.0
5000
-7.0
Extended Vehicular A Model – EVA Excess tap delay [ns]
Relative power [dB]
0
0.0
30
-1.5
150
-1.4
310
-3.6
370
-0.6
710
-9.1
1090
-7.0
1730
-12.0
2510
-16.9
A separate high speed train model is also defined and the Doppler shift trajectory is shown in the diagram below.
The assumption for this model is where Ds/2 is the initial distance of the train from eNodeB, and Dmin is eNodeB Railway track distance, both in meters; V is the velocity of the train in m/seconds. 44
4.2.3 Propagation Model The radio propagation model plays a key role in the link budget. The coverage radius of a base station is obtained based on the maximum propagation loss allowance in the link budget. Radio propagation models are classified into outdoor and indoor propagation models. These two types of propagation models involve different factors. In an outdoor environment, landforms and obstructions on the propagation path, such as buildings and trees, must be considered. Signals fade at varying rates in different environments. Propagation in free space gives the lowest fade rate. The fading of signals is larger than free space when radio waves propagate in open areas/suburban areas and fading rate is the largest in urban/dense urban areas. Indoor propagation model features low RF transmit power, a short coverage distance and complicated environmental changes. Although every planning tool will use slightly different method in their propagation calculation, the propagation models are generally based around modifying the following K factors.
K1-los K2-los K1-nlos K2-nlos
Indicate K1 and K2 in the line-of-sight condition. Indicate K1 and K2 in the none-line-of-sight condition.
K3
Indicates a coefficient related to Effective height of Transmitter.
K4
Indicates a coefficient related to diffraction loss.
Method
Method of calculating diffraction includes. •• 0-No Diffraction •• Do not count the diffraction loss. •• 1-Deygout •• This diffraction algorithm calculates the diffraction of a maximum of three obstacles. •• 2-Epstein-Peterson •• This calculation method is the same as Deygout, except that the method for calculating the height of obstacles is different. •• 3-Deygout with correction •• Correct the distance based on the Deygout calculation method. •• 4-Millington •• This diffraction algorithm calculates the diffraction of only one obstacle. Other parameters
K5
Indicates a coefficient related to the propagation distance and the effective height of the transmitter.
K6
Indicates a coefficient related to the receiver height.
Kclutter
Indicates a coefficient related to clutter loss.
This section describes the common propagation models in LTE planning. 45
I. Free Space Model Free space indicates an ideal, even, and isotropic medium of space. When electromagnetic waves are transmitted in this medium, no reflection, refraction, scattering, or absorption occurs. Propagation losses are caused only by the energy spread of electromagnetic waves. Satellite communication and microwave line-of-sight (LOS) communication are typical examples of free space propagation. In certain conditions, the antennae of the base station and terminal can be mounted at any height. In this case, LOS communication between the base station and the terminal is implemented. If a clear line of sight (CLOS) exists between the transmit antenna and receive antenna, then path loss complies with the free space model. The propagation losses in the free space model are as follows:
PL = 32.4 + 20log(d ) + 20log( f )
Where, d indicates the distance between the terminal and the base station. The unit is km. f indicates the carrier frequency. The unit is MHz. The preceding formula does not consider the impact of ground reflection, and thus often underestimates propagation loss. This model is applicable to the scenario when the antennas of the base station and terminal are mounted at considerable height and CLOS exists between the base station and the terminal. II. Cost231-Hata Model Cost231-Hata model can be used in macro cells as the propagation model. The application range is as follows: Frequency band: 1500 MHz to 2000 MHz Base station height: 30 meters to 200 meters. The base station must be higher than the surrounding buildings. Terminal antenna height: 1 meter to 10 meters Distance between the transmitter and receiver: 1 km to 20 km The Cost231-Hata model can be expressed by the following formula: Total = L - a(Hss) + Cm L = 46.3 + 33.9 × lg( f ) - 13.82 × lg(HBS) + (44.9 - 6.55 × lg(HBS)) × lg(d ) Where, f indicates the working frequency of the system. The unit is MHz. HBS indicates the height of the base station antenna. The unit is m. HSS indicates the height of the terminal antenna. The unit is m. d indicates the distance between the terminal and the base station. The unit is km. a(hss) indicates the terminal gain function. This function is related to the antenna height and working frequency of the terminal and the environment. The value of Cm depends on the terrain type. The values of Cm in the standard Cost231-Hata are as follows: In large cities: Cm = 3 (as defined in Urban - large city in the related protocol)
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In medium-sized cities: Cm = 0 (as defined in Urban – small city in the related protocol) In suburban areas: Cm = -2(log( f /28))2 - 5.4dB (as defined in Urban – Suburban in the related protocol) In rural open areas: Cm = -4.78 × (lg( f ))2 + 18.33 × lg( f ) -40.94 (As defined in Rural (open) – desert in the related protocol) In highways: Cm = -4.78 × (lg( f ))2 + 18.33 × lg( f ) -35.94 (As defined in Rural (quasi-open) – countryside where the terminal is unobstructed for 100 meters in the front in the related protocol) Since some of the working frequencies of the LTE networks are 2.3 GHz and 2.6 GHz have exceeded the band range of the standard Cost 231-Hata model, that is, 150 MHz to 2000 MHz. Therefore, in the actual LTE system design, the standard Cost231-Hata model must be corrected based on the CW test result. According to the planning experience and actual CW test results in multiple scenarios, a set of Cm has been created in the experienced model. III. Standard Propagation Model (SPM) The standard propagation model is a model (deduced from the Hata formula) particularly suitable for predication in the 150MHz~3500MHz band over long distance (1Km
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