LTE optimisation

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LTE Optimization GRAHAM WHYLEY 1

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Scheduler

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Scheduler It is the task of the scheduler to assign resource blocks to physical channels belonging to different users or for general system tasks. The job of the MAC layer

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Scheduler • It is the task of the scheduler to assign resource blocks to physical channels belonging to different users or for general system tasks

• If resources are still available after the GBR demands then different schedulers are available

• There are 4 main schedulers • Max SINR • Proportional Demand • Proportional Fair • Round Robin 4

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Scheduler Round Robin • The aim of this scheduler is to share the available/unused resources equally among the RT terminals (i.e. the terminals requesting RT services) in order to satisfy their RT-MBR demand.

Proportional Fair • The aim of this Scheduler is to allocate the available/unused resources as fairly as possible in such a way that, on average, each terminal gets the highest possible throughput achievable under the channel conditions.

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Scheduler Proportional Demand

• The aim of this scheduler is to allocate the remaining unused resources to RT terminals in proportion to their additional resource demands Application Layer

NAS Protocol(s) (Attach/TA Update/…)

Physical Uplink Shared Channel(PUSCH) Buffer Status Report

IP / TCP | UDP | … (E-)RRC (Radio Resource Control)

PDCP (Packet Data Convergence Protocol)

RLC (Radio Link Control)

RLC (Radio Link Control)

RLC (Radio Link Control)





Logical Channel

Medium Access Control (MAC)

Transport Channels

PDCP (Packet Data Convergence Protocol)

PDCP (Packet Data Convergence Protocol)

RLC (Radio Link Control)

RLC (Radio Link Control)

eNodeB

MAC Scheduler DL Physical Downlink Control Channel (PDCCH) Additional UL GRANT

Scheduling / Priority Handling HARQ

FDD | TDD - Layer 1 ( DL: OFDMA, UL: SC-FDMA )

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Scheduler Max SINR

• Terminals with higher bearer rates (and consequently higher SINR) are preferred over terminals with lower bearer rates (and consequently lower SINR). This means that resources are allocated first to those terminals with better SINR/channel conditions, thereby maximising the throughput. 6 BITS 2 BITS

64QAM

4 BITS 16 QAM QPSK

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Scheduler

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Frequency-selective Scheduler

Wideband CQI

Sub-band CQI, can be created by splitting the channel into several subbands

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The number of sub-bands depends on the channel bandwidth

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ASSET – LTE

There are 4 schedulers • Max SINR • Proportional Demand • Proportional Fair • Round Robin

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Using MU-MIMO

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Cell throughput. •MU-MIMO is used to increase the cells’ throughput. •This is achieved by co-scheduling terminals on the same Resource Blocks. •Applying MU-MIMO will make no obvious changes to a network unless it is overloaded. 12

What is CSSR?

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Are there any disadvantages of MUMIMO?

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MU-MIMO We can observe that when MU-MIMO is deployed everywhere, it provides small improvements close to the cell, large improvements close to the cell edge

RSRQ changes when MU-MIMO is deployed because the number of served terminals changes. 14

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MU-MIMO DL Cell throughput per carrier

Cell Throughout (per carrier) increases when MUMIMOis enabled. This is an effect of the eNodeB now being capable to serve a higher number of usersby scheduling them on the same resources. These users would be otherwise failing to connect. 15

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Strategy will be to use SM close to the eNodeBs to increase data rates Diversity further away from the eNodeB to increase coverage MU-MIMO for heavily loaded cells

SM close to the eNodeBs to increase data rates switches to Diversity

MU-MIMO for heavily loaded cells switches to Diversity

Switch Over is based on DLRS SNR – What happens if the load increases 16

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LOAD INCREASES- What happens to cell edge? Load increases – DLRS reduces

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Achievable DL Bearer without and with Diversity

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DL Data Rate Improvement with Diversity

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TTI bundling •TTI bundling can repeat the same data in multiple (up to four) TTIs •TTI bundling effectively increases the TTI length allowing the UE to transmit for a longer time. •A single transport block is coded and transmitted in a set of consecutive TTIs •The same hybrid ARQ process number is used in each of the bundled TTIs..

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TTI bundling •TTI bundling can repeat the same data in multiple (up to four) TTIs •TTI bundling effectively increases the TTI length allowing the UE to transmit for a longer time. •A single transport block is coded and transmitted in a set of consecutive TTIs •The same hybrid ARQ process number is used in each of the bundled TTIs..

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Number of TTIs bundled

1

4

Transmission bandwidth

360 kHz

360 kHz

Required SNR (dB)

-4

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Reducing other cell interference

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SINR SINR ave =

S I+N

I = Iown + Iother

What is N? SNR =

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S N

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Interference – Own Cell The LTE uplink is orthogonal, which is to say there is, at least in the ideal case, no interference between users in the same cell.

Closer a terminal is to a neighbouring cell the stronger the interference

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Cell-edge performance SINR ave =

S I+N

I = Iown + Iother

Most trial networks only contain a few base stations.

Some people believe that the out-of-cell interference is not important if it originates from cells that are physically far away from the centre cell LTE supporting Cell Not supporting LTE

Reduced coverage may arise due to interference

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Soft Frequency Reuse in LTE

Frequency Reuse is a well known concept that has been applied to wireless systems over the past two decades e.g. in GSM systems.

Frequency Reuse implies using the same frequencies over different geographical areas.

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Different Carrier /Different Frequency

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DLRS SINR= 10.32 dB

DLRS SNR = 13.3 db SNR = S/N

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Cell Loads

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Load (%)

Interference Margin (dB)

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1

40

1.3

50

1.8

60

2.4

70

2.9

80

3.3

90

3.7

100

4.2

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inter-cell interference control (ICIC).

ICIC can allocate different RB frequencies to cell-edge users in different cells

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inter-cell interference control (ICIC). Proactive schemes: Here an eNodeB informs its neighboring eNodeBs how it plans to schedule its users in the future (i.e. sending announcements), so that the neighboring eNodeB can take this information into account. Proactive schemes are supported via standardized signaling between eNodeBs over the X2 interface. ICIC schemes are primarily designed for improving the performance of the uplink and downlink shared data channel (PDSCH and PUSCH 31

PDSCH

eNB

X2 PDSCH eNB

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inter-cell interference control (ICIC). The following ICIC schemes are supported in ASSET: • Reuse 1 (Prioritisation) • Soft Frequency Reuse • Reuse Partitioning Fundamental to each of these methods is a division of the network into two areas in relation to the cell coverage, i.e. Cell Centre Users (CCUs) and Cell Edge Users (CEUs).

Cell Edge Thresholds defined per cell in the Site Database 32

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inter-cell interference control (ICIC).

The available thresholds are “RSRP” and “Relative RSRP”. RSRP is self explanatory while the latter is defined in dBs and can be expressed as the difference between the RSRPs of the serving and the strongest interfering cell

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inter-cell interference control (ICIC).

RSRPs of the serving and the strongest interfering cell

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Defining cell centre

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Global Editor

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Carriers

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The following ICIC schemes are supported in ASSET: • Reuse 1 (Prioritisation) • Soft Frequency Reuse • Reuse Partitioning

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REUSE 1(PRIORITISATION) 15 Mhz Carrier 1 A 1 A 1 5 Mh z

Number of Partitions = 3

A 3

A Carrier 1 3

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A 2 Carrier 1

A 2

The simplest way to minimize ICI within a Frequency Reuse 1 (FR 1) scenario is by prioritisation of resources. Reuse 1 (Prioritisation) scheme prioritises certain portions of the carrier bandwidth (i.e., number of RBs) in each cell according to a set plan. The whole bandwidth is still available for transmission in all cells, but the concept is that each cell uses its prioritised RBs more often than its non-prioritised RBs, so that it minimises the interference that it may cause to other cells.

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Coordination factor The improvement of Traffic & Control SINR with the deployment of Prioritisation is dependent on the Cell Loading and on the coordination factor. coordination factor of 0 assumes no coordination at all. No dB improvement. No ICI coordination factor of 1 means perfect coordination. Recommended 0.7

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REUSE 1(PRIORITISATION)

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Achievable DL bearer without and with ICIC (Reuse-1, Prioritisation)

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DL Data Rate without and with ICIC (Reuse-1, Prioritisation)

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Soft Frequency Reuse in LTE .

In Soft Frequency Reuse (SFR) the cell area is divided into two regions; a central region where all of the frequency band is available and a cell edge area where only a small fraction of the spectrum is available.

The spectrum dedicated for the cell edge may also be used in the central region if it is not being used at the cell edge.

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Soft Frequency Reuse in LTE .

The lack of spectrum at the cell edge may result in much reduced Shannon Capacity for that region. This is overcome by allocating high power carriers to the users in this region thus improving the SINR and the Shannon Capacity.

Note: 1. The Signal to Interference and Noise Ratio is given as: SINR=Signal Power/(Intercell Interference+Intracell Interference+AWGN Noise)

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Soft Frequency Reuse

Soft Frequency Reuse Scheme (Power Ratio 50%, Bandwidth Ratio 50%) 45

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Soft Frequency Reuse

Soft Frequency Reuse Scheme (Power Ratio 50%, Bandwidth Ratio 50%) 46

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Reuse Partitioning •Multiple partitions. •Two dedicated zones, one for CCUs, the other for CEUs. •Each sector can only consume CE resources from its own dedicated CE partition

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Comparison

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Questions 1. What does a coordination factor of 0 mean?

2. What does a coordination factor of 1 mean?

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Questions 3. What is the aim of ICIC?

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Questions 4. What is meant by: • Reuse Partitioning

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