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WiMAX Hybrid ARQ implementation in NS-2 SeungWoon Kim Jeonghoon Mo Information and Communications University {swkim,jhmo} AT icu.ac.kr.

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Presentation on theme: "WiMAX Hybrid ARQ implementation in NS-2 SeungWoon Kim Jeonghoon Mo Information and Communications University {swkim,jhmo} AT icu.ac.kr."— Presentation transcript:

1 WiMAX Hybrid ARQ implementation in NS-2 SeungWoon Kim Jeonghoon Mo Information and Communications University {swkim,jhmo} AT icu.ac.kr

2 2 Overview Hybrid ARQ Modeling HARQ gain HARQ impact on TCP Future Work and Plan

3 3 Overview Hybrid ARQ Modeling HARQ gain HARQ impact on TCP Future Work and Plan

4 Hybrid ARQ Overview FEC + ARQ –Chase Combining (CC) or Incremental Redundancy (IR) –Stop and Wait ARQ Feedback –Dedicated Fast Feedback Channel Retransmission

5 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 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 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 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 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 10 Hybrid ARQ UL Operation 1.BS grants Trx Chance. (HARQ UL_MAP) 2.MS transmits Data 3.BS sends ACK/NACK

11 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 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

13 13 HARQ parameters

14 14 HARQ vs. MAC ARQ HARQMAC ARQ High Complexity ReceiverSimple Layer 2 Operation Stop and Wait ARQSelective Feedback High Feedback OverheadLow Control Overhead Packets out of OrderPackets in Order CC/IR gain No CC/IR Gain Dedicated Ack Channel and fixed delay No Dedicated Channel and Long Variable Delay

15 15 Overview Hybrid ARQ Modeling HARQ gain HARQ impact on TCP Future Work and Plan

16 16 Modeling HARQ in NS-2 How much is the HARQ gain? Oirginal Transmission Retransmission Block1 Error Block 1’’ Error??? Block 1’’ = Block1 + Block 1’ Block 1’ Receiver may or may not recover the error

17 17 Error Computation in Rel. 2.1 On receiving a packet  i, 0  i  total_subcarrier, calculate 1)Interferencepower[i] 2)signalpower[i] 3) Block1Block2 Block3 Packet Max block size Packet size in slot For each block, index k = f (modulation, block_size)  k = f(mobility model, index k ) Bernoulli toss with p k = BLER k to determine Block error Then, PER = 1-  (1- p k ) Last block size …

18 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

19 19 Modeling Objectives Simple –Not too complicated Close to Real World –Error is small

20 20 Simplicity Different Levels of Abstraction –Bit level –Subcarrier level –Block level

21 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 1 st Transmission 16QAM 2 nd Transmission QPSK C3 C4 4 codes in one subcarrier block 2 codes in one subcarrier block

22 22 Subcarrier Level Abstraction Keep Track of SNR per subcarrier Use EESM or some other methods to have representative values High Complexity

23 23 Block Level Abstraction Block1Block 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 24 Block Level Abstraction Would like to find combined SINR  c (n), combined block error rate BLERc(n) Compute BLER from  c (n) # Retransmission BLER Combined SINR

25 25 Overview Hybrid ARQ Modeling HARQ gain HARQ impact on TCP Future Work and Plan

26 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 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 28 Retrx. # vs. TCP Thput [1] Persistent Retransmission is the best

29 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 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 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


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