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WiMAX Hybrid ARQ Implementation in NS-2: Overview and Future Plans

Explore the modeling of HARQ gain, its impact on TCP, and future plans for 802.16 Hybrid ARQ. Dive into FEC, Chase Combining, and Stop-and-Wait ARQ methods in DL and UL operations. Understand the importance of ACK channels, HARQ ACK region, and Hybrid ARQ buffer capability. Compare HARQ versus MAC ARQ, and delve into modeling HARQ effects on TCP performance over wireless channels. Beneficial for those interested in wireless communication protocols and enhancing TCP efficiency.

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WiMAX Hybrid ARQ Implementation in NS-2: Overview and Future Plans

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  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

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