1. Aggressive HARQ Transmission IEEE 802.16 Presentation Submission Template (Rev. 9) Document Number: IEEE S802.16m-09/0047r1 Date Submitted: 2009-1-12 Source: Zheng Yan-Xiu, Yu-Chuan Fang, Chang-Lan Tsai, Chung-Lien Ho, Hsi-Min Hsiao, Ren-Jr Chen, Richard Li, E-mail: zhengyanxiu@itri.org.tw.zhengyanxiu@itri.org.tw ITRI Yih-Shen Chen, MTK Venue: IEEE Session #59, San Diego. Base Contribution: N/A Re: 802.16m-08/052, Call for Comments on 802.16m SDD (802.16m-08/003r6), Section 11.13.2 HARQ Purpose: To be discussed and approval by IEEE 802.16m TG Notice: This document does not represent the agreed views of the IEEE 802.16 Working Group or any of its subgroups. It represents only the views of the participants listed in the “Source(s)” field above. It is offered as a basis for discussion. It is not binding on the contributor(s), who reserve(s) the right to add, amend or withdraw material contained herein. Release: The contributor grants a free, irrevocable license to the IEEE to incorporate material contained in this contribution, and any modifications thereof, in the creation of an IEEE Standards publication; to copyright in the IEEE’s name any IEEE Standards publication even though it may include portions of this contribution; and at the IEEE’s sole discretion to permit others to reproduce in whole or in part the resulting IEEE Standards publication. The contributor also acknowledges and accepts that this contribution may be made public by IEEE 802.16. Patent Policy: The contributor is familiar with the IEEE-SA Patent Policy and Procedures: and.http://standards.ieee.org/guides/bylaws/sect6-7.html#6http://standards.ieee.org/guides/opman/sect6.html#6.3 Further information is located at and.http://standards.ieee.org/board/pat/pat-material.htmlhttp://standards.ieee.org/board/pat

2. Background In IEEE 802.16m-08/003r4, “The Draft IEEE 802.16m System Description Document” 10.2 HARQ Functions The HARQ operation relies on N-process (multi-channel) stop-and-wait protocol N-process requires N soft buffers or enough shared buffer to stored received samples and this has been specified in IEEE Std. 802.16e-2005 Basic scheduling rule: In order to avoid the risk of buffer overflow, maximum transmitted coded bits can not exceed the available soft buffer in an MS HARQ soft buffer might dominate MS baseband cost due to large soft buffer necessary for an MS to support high throughput.

3. Necessary Soft Buffer for N-process HARQ N-process HARQ throughput: Necessary buffer: –The necessary buffer increases with system throughput S Mbps. –The buffer also increases with the round trip delay T ms. –Higher code rate R c leads less storage due to coded bits stored in the soft buffers –Higher modulation order M order also reduces memory storage if symbol-level storage is considered –Bit-level buffer: –Symbol-level buffer: When HARQ throughput becomes large, the necessary buffer may be incredible large –IEEE 802.16e turbo decoder requires 2,880 soft bits for received samples and extrinsic information –HARQ soft buffer dominates baseband complexity S Mbps =100Mbps T ms =5ms S Mbps =1000Mbps T ms =5ms Rc=1/21000K [Soft Bits]10M [Soft Bits] Rc=4/5625K [Soft Bits]6.25M [Soft Bits] S Mbps =100Mbps T ms =5ms S Mbps =1000Mbps T ms =5ms Rc=1/2, M order =2 (QPSK, One retransmission) 500K [Symbols]5M [Symbols] Rc=1/2, M order =4 (16QAM, One retransmission) 250K [Symbols]2.5M [Symbols] Rc=1/2, M order =6 (64QAM, One retransmission) 167K [Symbols]1.67M [Symbols] Bit-Level Buffer Symbol-Level Buffer

4. Complexity Evaluation IEEE 802.16e 2x2 MIMO receiver necessitates gate count around 7M gates IEEE C802.16-07/015 (Intel) –Gate Count for 16e: 1M~1.5M gates –RAM 200~300KBytes –When HARQ transmission rate is 3Mbps, HARQ soft buffer requires 64KBytes=1024Kbits=1Mbits FACT: When transmission rate reaches 100Mbps @ code rate=1/2, HARQ soft buffer occupies at least 1M soft bits ≈ 6M ~ 12 M bits

5. FACT: Low Utilization of Soft Buffer H-ARQ operation scenario: –Packet error rate=0.1 –Buffer usage is geometrically distributed given N-processes HARQ –In most cases, half of buffers are vacant –For example: 6-channel HARQ buffer usage is shown in below table and, in 99.3%, at most three soft buffers are used. FACT: –At least half soft buffers can be reduced, as expected –Conventional buffer strategy is designed for worst-case scenario –Since at least half of HARQ soft buffer is unused in 99%, implementing less soft buffer is a viable way to significantly reduce baseband complexity Number of H-ARQ buffer in use 0123456 Occurrence0.530.350.0980.0150.00125.4×10 -5 10 -6 An example of the occurrence of number of HARQ buffers in use for 6-process HARQ

6. An Example for Aggressive HARQ Transmission Compared with Conventional HARQ Transmission Example: 4 memory buffers Compared with Conventional HARQ transmission, the results show that: –Aggressive HARQ transmission can support more HARQ processes 6-process H-ARQ provides extra 2 transmission opportunities –Extra 3 packets are transmitted  HARQ buffer is highly utilized A/X denotes ACK with X buffer used N/X denotes NACK with X buffer used

7. Example of Throughput Analysis 4 H-ARQ soft buffers Aggressive HARQ transmission operates 6, 8 and 12 HARQ processes Conventional HARQ scheduling can only operate 4 HARQ processes IEEE 802.16e –MPDU=2048 bytes=16,384 bits, T ms =20ms, R c =0.5, B bits =131,072 soft bits=4*MPDU/R c –Packet error rate=0.1 –Packet dropping effect is ignored IEEE 802.16e Conventional HARQ scheduling Throughput= 2.95Mbps Aggressive HARQ transmission with 6 H-ARQ processes Throughput= 4.46Mbps Throughput gain=50% Packet dropping rate=0.000933% Buffer saving under the same throughput=33.33% Aggressive HARQ transmission with 8 H-ARQ processes Throughput= 5.90Mbps Throughput gain=100% Packet dropping rate=0.005% Buffer saving under the same throughput=50% Aggressive HARQ transmission with 12 H-ARQ processes Throughput= 8,85Mbps Throughput gain=200% Packet dropping rate=0.041% Buffer saving under the same throughput=66.67%

8. Conclusion HARQ soft buffer costs too much memory –Large size of memory is redundant, which further contributes higher MS complexity Removing the memory restriction will highly reduces MS complexity BS can transmit more than MS HARQ soft buffer capability –Pros: higher transmission throughput lower memory constraint better user experience –Con: Potential packet dropping may degrade HARQ reliability –A flow control mechanism is required (C80216m-09_0048.ppt)

9. SDD TEXT Proposal 11.13.2.6 Aggressive HARQ Transmission In DL HARQ, 16m BS can transmit coded bits exceeding current available soft buffer capacity. The exceeding ratio is negotiated by BS and MS. 11.13.2.7 HARQ feedback A basic ACK/NAK channel to transmit 1-bit feedback is supported. An enhanced ACK/NAK control channel with some additional information is FFS.