rlc pdu size on hsupa

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Impact of Flexible RLC PDU Size on HSUPA Performance

Enrico Jugl, Michael Link, Jens Mueckenheim*

*Hochschule Merseburg, Germany

Outline

Motivation

Flexible RLC PDU Size Feature

Packet Data Performance

- Single User Performance- Multi-User Performance

VoIP PerformanceVoIP Performance

- Transmission Technique- Performance Criteria- Simulation Results

Conclusions

18-May-2011 Impact of Flexible RLC PDU Size on HSUPA PerformanceSlide 2

Motivation (1)

3GPP introduced an enhanced layer 2 (i.e. flexible RLC PDU size) for the downlink in Release 7 allowing for a more efficient transmission of higher data rates

- Required for evolution of HSDPA, e.g. 64 QAM, MIMO and dual-cell HSDPA

In Release 8 a similar layer 2 enhancement was added for the uplink by introduction of a MAC-i/is entity handling flexible RLC PDU sizes

- Allows for higher data rates given by advanced E-DCH features like 16 QAM and dual-cell HSUPA

Maximum achievable RLC throughput:

- RWIN: RLC window size in number of RLC PDUs- NRLC PDU: size of the RLC PDU (e.g. 336 bits, 656 bits)- NRLC header: size of the AM RLC PDU header (16 bits)- RTT: round trip time- TSP: timer status prohibit


,)(

max TSPRTT

NNRWINR headerRLCPDURLC

RLC +−⋅

=

Motivation (2)

With RWIN = 2047, RTT = 70 ms, TSP = 50 ms

- RRLC max = 5.4 Mbps for 336 bits PDU size- RRLC max =10.8 Mbps for 656 bits PDU size

By increasing RLC PDU size the maximum RLC data rate can be increased

- Problematic at cell edge if the UE is in power limitation, where a large PDU cannot be transmitted at all or with insufficient power only

Enhanced layer 2 can alleviate this tradeoffEnhanced layer 2 can alleviate this tradeoff

- Large PDUs can be used if allowed by radio conditions- Small PDUs can be used in power limited situations

If large PDUs are used

- RLC overhead is reduced, as well as the padding in the MAC-i PDUs- Transmission of less PDUs in a TTI allows for reduction of processing load in the terminals and the network equipment


Flexible RLC PDU Size Feature

RLC provides segmentation/concatenation of variable sized RLC SDUs (IP packets) into RLC/MAC-d PDUs

E.g. a RLC SDU contains an IP packet of 1500 bytes (MTU=1500)

The maximum RLC PDU size is 1505 octets RLC SDU

TCP/IP Payload TCP/IPheader

MTU: 576 or 1500

Example for a single logical channel:

The maximum RLC PDU size is 1505 octets (configurable)

The length of the data field is a multiple of 8 bits

RLC PDU size can vary according to the amount of data requested by current E-TFCI selection


Cf. 25.322 Rel-8

RLC PDU RLC PDUMAC-iheader

MAC-is PDU

RLC PDU RLC PDU

Max RLC PDU

Pad

.

Flexible size

H

H: MAC-is header

Simulation ScenarioParameter Value

# of NodeB (sites)/ sectors

Single user: 1 sectorMulti-user: 12 sites/ 3 sectors each (wrapped around)

Pathloss model COST 231 Okumura Hata urban

Cell radius 1000 m

Shadow fading Single cell: noMulti-cell: 7dB standard dev,50 m correlation length

UL receive diversity

2 way


diversity

Channel Model Single user: AWGNMulti-user: Mixture

Mobility Single user: noMulti-user: random movement with soft/softer handover

UL Target Load 85% (≡≡≡≡ 8dB noise rise)

Service 1 2 MByte FTP upload

Service 2 VoIP: 12.2 k AMR speech, 50% activity

Packet Data Performance – Single User (1)

Isolated radio cell with good radio conditions (AWGN) and a HARQ retransmission rate of 1%

For UE categories 5 & 6 about 5% throughput improvement compared to


improvement compared to fixed RLC PDU size of 336 bits due to the reduced RLC overhead

UE category 7: throughput significantly drops down to 6.5 Mbps due to RLC window size limitation

Maximum RLC PDU size: 12016 bits

Packet Data Performance – Single User (2)

RLC buffer occupancy limited to available RLC window size

Fixed RLC PDU size (336/ 656 bits): drops of available RLC PDUs in the


available RLC PDUs in the RLC window to zero disrupting the continuous data flow

Flexible RLC PDU size: there are always PDUs available for transmission

Packet Data Performance – Multi-User

Flexible RLC PDU size provides cell throughput increase of ~8%

- Reduced RLC overhead- Finer granularity of the RLC PDU size, allowing for a better exploitation of the uplink resourcesof the uplink resources

- Reduced probability of residual MAC-e block errors after HARQ (reduced TCP impact)

Only slight impact of the maximum RLC PDU size on throughput (should be chosen > 5000 bits)


VoIP – Transmission Technique/ Performance Criteria

MAC-d PDU size of 296 bits for the voice packet and 96 bits for the SID packet

Transmission over E-DCH using non-scheduled transmission mode with 2ms TTI

Non-scheduled grant of 318 bits (transport block size table 0)

244bit4

bytes12bit

AMR frameRoHC header

8bit

RLC UM

MAC-d PDUHdr./ Pad.

RTP

Maximum number of HARQ transmissions is 4, target average value 2.05

Minimum set E-TFCI: 318 for fixed and120 for flexible PDU size

Performance criteria:


296 bit

MAC-e transport block: 318 bits

- Packet delay <= 90 ms- 95%tile of the VoIP frame loss rate <= 2%- Probability of exceeding 80% uplink cell load <= 2%

VoIP Performance – Simulation Results

The VoIP packet delay increases with higher path loss caused by

- Higher number of HARQ transmissions in case of fixed RLC PDU size

- Allocation of several HARQ processes for transmission of the whole MAC-d PDU in case of flexible RLC PDU size

A delay higher than 90 ms is considered to be a packet lossto be a packet loss

About 2 dB coverage gain for flexible RLC PDU size

In multi-UE scenario, the VoIP capacity is slightly improved by 6% for flexible RLC PDU size compared to fixed PDU size

- SID frames can now be transmitted with a smaller RLC PDU size

- In case of power limitation the RLC PDU can be segmented by MAC-is at the UE


Conclusions

Flexible RLC PDU size feature in uplink was investigated by dynamic system simulations for packet data services in single- and multi-user scenarios, and for VoIP over E-DCH

For UE categories 5 and 6 the single user throughput improves by about 5% compared to fixed RLC PDU size of 336 bits due to the reduced RLC overhead

In case of multi-users, a maximum gain of about 8% was detected for UE category 6

- Reduced RLC overhead- Finer granularity of the RLC PDU size allowing for better exploitation of the available uplink load- Reduction of call drops caused by TCP timeouts by improvements of the behavior at cell edge- Reduction of call drops caused by TCP timeouts by improvements of the behavior at cell edge

No significant impact of the maximum RLC PDU size on the performance, as long as this parameter is chosen larger than 5000 bits

RLC window size limitations are resolved enabling for about 11.3 Mbps RLC throughput with UE category 7 (16 QAM) compared to 6.5 Mbps for fixed RLC PDU size of 336 bits

Performance in power limitation at cell edge for VoIP over E-DCH users can be improved too

- Using smaller packet sizes in power limitation packet loss can be prevented at cost of an increased transmission delay � Improved coverage, about 2 dB gain

- Capacity gain of about 6% in multi-user scenarios


Thank you!

Nash Technologies GmbHThurn-und-Taxis-Str. 10

D-90411 Nurembergwww.nashtech.com

[email protected]

BackupBackup

RLC Rate Limit – WS Optimum for Peak Rate

Tx windowstate variableVT(...)

Exactly when Tx window is full, the

One SR arrives per TSP.SR acknowledges PDUs up to the situation one RTTearlier. RLC window jumps by a fraction of WS.

Parameters:• RLC RTT• TimerStatusProhibit TSP > RTT• Available MAC-is peak rate r• RLC window size WS is optimum for

RTT, TSP and rResult: Mean RLC rate R:R = WS / (TSP + RTT) = r

Note: TSP > RTT ⇒ step size > WS/2TSP = RTT ⇒ step size = WS/2TSP < RTT ⇒ step size < WS/2

SR – status report


TSP

WS

Time

window is full, the next SR arrives.

RTT With TSP > RTT:R < WS / (2 * RTT)

RTTMS (upper edge)

S (actually submitted)

A (lower edge)

UTRAN Architecture

MAC-d

RLC

RRC PDCP

Logical Channels BCCHDCCHDTCH

SRNC

CRNC

DCH

Upper phy

c/sh

MAC-d flows

Evolution from Rel-7

• Enhanced layer 2 which is already available for HSPDA is also supported for E-DCH

c/sh

MAC-es/MAC-is

MAC-d flows

E-DCH in Rel-8

• Additions in RRC to choose between MAC-

MAC-c/sh

Transport Channels

MAC-b

BCH

CRNC

NodeB

Upper phy

DSCHFACH

MAC-hs/MAC-ehs

HS-DSCHw

/o M

AC

-c/s

h

Slide 16 Impact of Flexible RLC PDU Size on HSUPA Performance18-May-2011

w/o

MA

C-c

/sh

MAC-e/MAC-i

EDCH

choose between MAC-e/es and MAC-i/is

• RLC now supports flexible PDU size (UM & AM)

• New MAC-is entity with link to MAC-d and MAC-c

• New MAC-i entity located in the Node B

• MAC-i entities from multiple NodeB may serve one UE (soft HO)

MAC-c

Data Flow through Layer 2 – UTRAN Side

Mac-is PDU:

Reordering queue distribution

Reordering queue distribution

DCCH DTCH DTCH

DATAHeader

MAC-d

DATA

DATA DATA

RLC PDU:RLC

Reordering Reordering Reordering

Disassembly & Reassembly

MAC-d PDU:



TSN SS

TSN: Transmission Sequence Number (6 bits)

SS: Segmentation Status (2 bits)

LCH-ID: Logical Channel Identifier (4 bits)

- Maps to MAC-d flow ID

L: Length of MAC-is SDU in

Mac-is SDU


distributiondistribution

MAC-d Flows

HARQ

Demultiplexing

MAC-i

DATA

MAC-iPDU:

L1

MAC-is

Mapping info signaled to Node B

MAC-i header

LCH-ID Padding (Opt)L DATADATA

Transport block:

LCH-ID => MAC-d flow ID

Read UE id(FDD only)

Cf. 25.319 Rel-8

L: Length of MAC-is SDU in octets (11 bits)

F: Flag indicating if more fields are present in MAC-i header or not (1 bit)

- 0: Flag is followed by additional set of LCH-ID, L, F field

- 1: Flag is followed by MAC-is PDU

F

rlc pdu size on hsupa

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