WiMAX Hybrid ARQ implementation in NS-2

1. 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 • BS Transmits Data • BS informs the location of ACK Channel • MS sends feedback (ACK/NACK) to the BS using the ACK Channel.

6. 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 BS Transmits Data

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. 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. BS informs the loc. of ACK Channel time (x1,y1) Burst#1 Frequency (x2,y2)

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 • BS grants Trx Chance. (HARQ UL_MAP) • MS transmits Data • 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

13. HARQ vs. MAC ARQ

14. Overview • 802.16 Hybrid ARQ • Modeling HARQ gain • HARQ impact on TCP • Future Work and Plan

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

16. Error Computation in Rel. 2.1 Packet size in slot • On receiving a packet Packet Max block size Last block size … Block1 Block2 Block3 For each block, indexk = f (modulation, block_size) k = f(mobility model, indexk) • i, 0itotal_subcarrier, calculate • Interferencepower[i] • signalpower[i] Bernoulli toss with pk = BLERk to determine Block error Then, PER = 1-(1- pk)

17. 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

18. Simplicity • Different Levels of Abstraction • Bit level • Subcarrier level • Block level

19. 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 2nd Transmission QPSK 1st Transmission 16QAM C1 C2 C3 C4 C1 C2 C3 C4 4 codes in one subcarrier block 2 codes in one subcarrier block

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

21. Block Level Abstraction Block1 Block 1’ Block1’’ SNR(1’’) BLER(1’’) SNR(1’) BLER(1’) SNR(1) BLER(1) SNR(1’’) = f(SNR(1), SNR(1’), BLER(1), BLER(1’’)) Given

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

23. Overview • 802.16 Hybrid ARQ • Modeling HARQ gain • HARQ impact on TCP • Future Work and Plan

24. 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

25. 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

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

27. 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

28. 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

29. 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