LTE optimisation

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LTE Optimization

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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

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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

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eNodeB

MAC Scheduler DL

Physical Uplink Shared

Channel(PUSCH) Buffer Status Report

Physical Downlink Control Channel (PDCCH)

-Additional UL GRANT

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

Medium Access Control (MAC)

Transport Channels RLC (Radio Link Control) … … RLC (Radio Link Control) RLC (Radio Link Control) PDCP (Packet Data Convergence Protocol) RLC (Radio Link Control) PDCP (Packet Data Convergence Protocol) RLC (Radio Link Control) PDCP (Packet Data Convergence Protocol) Logical Channel (E-)RRC

(Radio Resource Control)

IP / TCP | UDP | … Application Layer NAS Protocol(s) (Attach/TA Update/…) Scheduling / Priority Handling HARQ

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

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64QAM 16 QAM QPSK 2 BITS 4 BITS 6 BITS

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.

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Sub-band CQI, can be created by

splitting the channel into several

sub-bands

The number of sub-bands depends on

the channel bandwidth

Wide

ba

nd

CQI

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ASSET – LTE

There are 4 schedulers • Max SINR

• Proportional Demand • Proportional Fair

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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.

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Are there any disadvantages of

MU-MIMO?

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MU-MIMO

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

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

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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.

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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

MU-MIMO for heavily loaded cells switches to Diversity

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

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

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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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•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 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..

TTI bundling

Number of TTIs bundled 1 4 Transmission bandwidth 360 kHz 360 kHz Required SNR (dB) -4 -8

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SINR

SINR ave = S

I + N

I = Iown + Iother

SNR = S

N

What is 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

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Cell-edge performance

LTE supporting Cell Not supporting LTE Reduced coverage may arise due to interference

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

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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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DLRS SINR= 10.32 dB DLRS SNR = 13.3 db SNR = S/N

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Cell Loads

Load (%) Interference Margin (dB) 35 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.

eNB eNB

X2

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

PDSCH PDSCH

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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).

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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

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inter-cell interference control (ICIC).

RSRPs of the serving and the strongest interfering cell

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Carriers

The following ICIC schemes are supported in ASSET:

• Reuse 1 (Prioritisation) • Soft Frequency Reuse • Reuse Partitioning

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REUSE 1(PRIORITISATION)

Carrier 1 Carrier 1 Carrier 1 Number of Partitions = 3 15 Mhz 5 Mh z A 1 A 2 A 3 A 1 A 2 A 3

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.

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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

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Soft Frequency Reuse

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

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Reuse Partitioning

•Multiple partitions.

•Two dedicated zones, one for CCUs, the other for CEUs.

•Each sector can only consume CE

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Questions

1. What does a coordination factor of 0 mean?

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Questions

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Questions

4. What is meant by: • Reuse Partitioning