Impact of Flexible RLC PDU Size on HSUPA Performance

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Impact of Flexible RLC PDU Size on HSUPA Performance

Enrico Jugl, Michael Link, Jens Mueckenheim* *Hochschule Merseburg, Germany

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Outline

Motivation

Flexible RLC PDU Size Feature Packet Data Performance

-Single User Performance -Multi-User Performance VoIP Performance VoIP Performance -Transmission Technique -Performance Criteria -Simulation Results Conclusions

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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

-N_{RLC PDU}: size of the RLC PDU (e.g. 336 bits, 656 bits)

-N_{RLC header}: size of the AM RLC PDU header (16 bits)

-RTT: round trip time

-TSP: timer status prohibit

, ) ( max TSP RTT N N RWIN R_RLC RLCPDU RLCheader + − ⋅ =

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Motivation (2)

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

-R_{RLC max} = 5.4 Mbps for 336 bits PDU size

-R_{RLC 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 tradeoff

Enhanced 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

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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/IP header

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 PDU MAC-i header MAC-is PDU RLC PDU RLC PDU Max RLC PDU P a d . Flexible size H H: MAC-is header

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Simulation Scenario

Parameter Value

# of NodeB (sites)/ sectors

Single user: 1 sector

Multi-user: 12 sites/ 3 sectors each (wrapped around)

Pathloss model COST 231 Okumura Hata

urban

Cell radius 1000 m

Shadow fading Single cell: no

Multi-cell: 7dB standard dev, 50 m correlation length UL receive

diversity

2 way diversity

Channel Model Single user: AWGN

Multi-user: Mixture

Mobility Single user: no

Multi-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,

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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

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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

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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 resources of 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)

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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) 244bit 4 bytes 12 bit AMR frame RoHC header 8 bit RLC UM MAC-d PDU Hdr./ 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%

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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 loss

to 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

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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

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Thank you!

Nash Technologies GmbH

Thurn-und-Taxis-Str. 10

D-90411 Nuremberg

www.nashtech.com

[email protected]

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Backup

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RLC Rate Limit – WS Optimum for Peak Rate

Tx window state variable VT(...)

Exactly when Tx window is full, the

One SR arrives per TSP. SR acknowledges PDUs up to the situation one RTT earlier. 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 r

Result: Mean RLC rate R:

R = WS / (TSP + RTT) = r Note: TSP > RTT ⇒step size > WS/2 TSP = RTT ⇒step size = WS/2 TSP < 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)

RTT

MS

(upper edge)

S

(actually submitted)

A

(lower edge)

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UTRAN Architecture

MAC-d RLC RRC PDCP Logical Channels BCCH DCCH DTCH

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 DSCH FACH MAC-hs/ MAC-ehs HS-DSCH w /o M A C -c/ sh w /o M A 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

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Data Flow through Layer 2 – UTRAN Side

Mac-is PDU: Reordering queue distribution Reordering queue distribution DCCH DTCH DTCH DATA Header MAC-d DATA DATA DATA RLC PDU: RLC

Reordering Reordering Reordering

Disassembly & Reassembly MAC-d PDU: Disassembly & Reassembly Disassembly & Reassembly 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 distribution distribution MAC-d Flows HARQ Demultiplexing MAC-i DATA MAC-i PDU: L1 MAC-is

Mapping info signaled to Node B

MAC-i header

LCH-ID Padding

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L DATA DATA

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