WiMAX Hybrid ARQimplementation in NS-2

SeungWoon Kim

Jeonghoon Mo

Information and Communications University{swkim,jhmo} AT icu.ac.kr

2

Overview

• 802.16 Hybrid ARQ

• Modeling HARQ gain

• HARQ impact on TCP

• Future Work and Plan

3

Overview

• 802.16 Hybrid ARQ

• Modeling HARQ gain

• HARQ impact on TCP

• Future Work and Plan

4

802.16 Hybrid ARQ Overview

• FEC + ARQ

– Chase Combining (CC) or Incremental Redundancy (IR)

– Stop and Wait ARQ

• Feedback– Dedicated Fast Feedback Channel

• Retransmission

5

Hybrid ARQ DL Operation

1. BS Transmits Data

2. BS informs the location of ACK Channel

3. MS sends feedback (ACK/NACK) to the BS using the ACK Channel.

6

BS Transmits Data

• HARQ DL-MAP IE– Defines 2D region for

HARQ in a frame.

• HARQ mode:– Chase Combining– IR with CTC, IR with CC– MIMO Chase, MIMO IR

HARQ, MIMO IR HARQ for CC, MIMO STC

7

BS Transmits Data

• HARQ DL-MAP Chase Sub-burst IE– Each subburst IE corresponds to

one subburst

– RCID: reduced CID

– ACID: HARQ Channel ID (4bit)

– AI_SN: ARQ Identifier Seq. Number (1bit)

8

BS informs the loc. of ACK Channel

• HARQ ACK region is informed to MS using HARQ ACKCH region allocation IE.

• One ACK occupy a half slot

– To transmit 16 ACKs, 8 slots are needed.

Burst#1

(x1,y1)

(x2,y2)

time

Frequency

9

MS sends the ACK/NACK

• Synchronous ACK Channel– HARQ_ACK_DELAY can be one, two or three (UCD)

– MS returns ACK/NACK after HARQ_ACK_DELAY frame.

• Predetermined location– The order of ack channels is the HARQ enabled bursts order

in DL MAP.

– MS needs to know the order of its burst in among HARQ enable DL_MAP_IEs in DL_MAP.

• ACK Channel Encoding– The 1 bit ACK/NACK information is encoded into length 3

code words over 8-ary alphabet.

10

Hybrid ARQ UL Operation

1. BS grants Trx Chance. (HARQ UL_MAP)

2. MS transmits Data

3. BS sends ACK/NACK

11

Ack Transmission for UL Data

• Two methods– Explicit Transmission by HARQ ACK-IE (bitmap)

– Implicit Transmission using AI_SN

• Explicit HARQ ACK-IE

• Implicit Transmission – IF AI_SN field in UL subburst-IE is toggled, ACK

– Otherwise, NACK (MS retransmits data)

• Fixed delay between : – HARQ_ACK_DELAY for UL_burst, 1, 2, or 3 frame time

12

HARQ Buffer Capability

• The maximal number of data bits the SS is able to store DL/UL HARQ

• Two parameters– Number of bits per channel

• Total number of bits that SS may buffer per channel

– Aggregation flag

• If flag == 1, buffer can be shared;

• otherwise, it cannot be shared

14

HARQ vs. MAC ARQ

HARQ MAC ARQHigh Complexity Receiver Simple Layer 2 Operation

Stop and Wait ARQ Selective Feedback

High Feedback Overhead Low Control Overhead

Packets out of Order Packets in Order

CC/IR gain No CC/IR Gain

Dedicated Ack Channel and fixed delay

No Dedicated Channel and Long Variable Delay

15

Overview

• 802.16 Hybrid ARQ

• Modeling HARQ gain

• HARQ impact on TCP

• Future Work and Plan

16

Modeling HARQ in NS-2

•How much is the HARQ gain?

Oirginal Transmission

Retransmission

Block1 Error

Error

Block 1’’ Error???Block 1’’ = Block1 + Block 1’

Block 1’

Receiver may or may not recover the error

17

Error Computation in Rel. 2.1

• On receiving a packet

i, 0itotal_subcarrier, calculate

1) Interferencepower[i]

2) signalpower[i]

3)

Block1 Block2 Block3

PacketMax block size

Packet size in slot

For each block,

indexk = f (modulation, block_size)

k = f(mobility model, indexk)

Bernoulli toss with pk = BLERk to determine Block error

Then, PER = 1-(1- pk)

Last block size

)cepower[i]interferen

r[i]signalpowe(log10

SINR[i]

10

k

k

blocksubcarrier

i

iSINR

k eEESM__#

0

][

)_#log(k

kkk blocksubcarrier

EESMSIR

),(_ kkk SIRindexlookuptableBLER

…

18

Important Factors

• HARQ Region

• Modulation and Coding Rate– Each subburst can have different modulation and

coding.

• # of subchannels that data is using

• Subchannelization Method: PUSC, FUSC, AMC

20

Simplicity

• Different Levels of Abstraction

– Bit level

– Subcarrier level

– Block level

21

Bit Level Abstraction

• Code block: [c1, c2, …, cN]

• SNR computation per each code symbol

• Due to Adaptive modulation and coding, a group of code in the same subcarrier may be separated into different subcarriers in the retransmission.

• High Complexity

C1 C2 C3 C4 C1 C2

1st Transmission16QAM

2nd TransmissionQPSK

C3 C4

4 codes in one subcarrier block 2 codes in one subcarrier block

22

Subcarrier Level Abstraction

• Keep Track of SNR per subcarrier

• Use EESM or some other methods to have representative values

• High Complexity

23

Block Level Abstraction

Block1 Block 1’ Block1’’

SNR(1)BLER(1)

SNR(1’)BLER(1’)

Given

SNR(1’’)BLER(1’’)

SNR(1’’) = f(SNR(1), SNR(1’), BLER(1), BLER(1’’))

24

Block Level Abstraction

• Would like to find combined SINR c(n), combined

block error rate BLERc(n)

• Compute BLER from c(n)

n

i

nc in

1

1 )()(

# Retransmission

BLER

Combined SINR 10,

25

Overview

• 802.16 Hybrid ARQ

• Modeling HARQ gain

• HARQ impact on TCP

• Future Work and Plan

26

TCP over Wireless Channel

• TCP is sensitive to losses.

• Factors affecting TCP performance– FEC factors

• Recovering Power

– ARQ factors• Fragmentation Size

• Maximum try of Retransmission

• Coding rate when with FEC

• Treatment of crucial fragment, especially the last fragment of a TCP packet

– TCP factors • AWND size

• RTO, RTT and CWND size

27

A Few Related Work

• Retransmission helps TCP performance. What is the best retransmission strategy? [1]

– Retrx. with Fixed Limit

– Persistent Retransmission

– Adaptive Retransmission

• Video and HARQ [2]

– Modeled the channel to Multistate Markov Chain (MSMC)

• more accurate than two-state Gilbert-Elliott model

28

Retrx. # vs. TCP Thput [1]

• Persistent Retransmission is the best

29

Related Work [2]

• Showed HARQ improves Residual Packet Error Rate (RPER) and TCP throughput– Error correction codes are beneficial in some

range of SNR, and with high mobility

30

Future Work

• HARQ Implementation to Release 2.1

– By early November, alpha version

– By late November, beta tested version

• Study the implication of HARQ on TCP performance– By December, 2007

– TCP timeout, packet error rate, throughput, delay

31

References

[1] F. Vacirca et al, “Optimal Design of Hybrid FEC/ARQ Schemes for TCP over Wireless Links with Rayleigh Fading”, IEEE TRANSACTIONS ON MOBILE COMPUTING, VOL. 5, NO. 4, APRIL 2006

[2] Hang Liu and Magda El Zarki, “Performance of H.263 Video Transmission over Wireless Channels Using Hybrid ARQ,” IEEE JSAC, Dec. 1997