1. doc.: IEEE 802.22-07/0039r0 Submission January 2007 SooYoung Chang, Huawei TechnologiesSlide 1 IEEE P802.22 Wireless RANs Date: 2007-01-03 Authors: Soft Hybrid ARQ Design with Adaptive Frequency Interleaving for WRAN Systems Notice: This document has been prepared to assist IEEE 802.22. It is offered as a basis for discussion and is not binding on the contributing individual(s) or organization(s). The material in this document is subject to change in form and content after further study. The contributor(s) 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 802.22. Patent Policy and Procedures: The contributor is familiar with the IEEE 802 Patent Policy and Procedures http://standards.ieee.org/guides/bylaws/sb-bylaws.pdf including the statement "IEEE standards may include the known use of patent(s), including patent applications, provided the IEEE receives assurance from the patent holder or applicant with respect to patents essential for compliance with both mandatory and optional portions of the standard." Early disclosure to the Working Group of patent information that might be relevant to the standard is essential to reduce the possibility for delays in the development process and increase the likelihood that the draft publication will be approved for publication. Please notify the Chairhttp://standards.ieee.org/guides/bylaws/sb-bylaws.pdf Carl R. StevensonCarl R. Stevenson as early as possible, in written or electronic form, if patented technology (or technology under patent application) might be incorporated into a draft standard being developed within the IEEE 802.22 Working Group. If you have questions, contact the IEEE Patent Committee Administrator at patcom@iee.org.patcom@iee.org >

2. doc.: IEEE 802.22-07/0039r0 Submission January 2007 SooYoung Chang, Huawei TechnologiesSlide 2 Co-Authors:

3. doc.: IEEE 802.22-07/0039r0 Submission January 2007 SooYoung Chang, Huawei TechnologiesSlide 3 Introduction Currently, Hybrid ARQ is not included in the WRAN standard. Hybrid ARQ provides an efficient way to improve system error performance. It has been adopted in IEEE 802.16. In this proposal, we introduce an enhanced chase combining scheme for WRAN systems. –Low complexity & more robust This scheme prefers the situations where the frequency diversity can be explored by retransmissions (e.g., distributed subcarrier permutation).

4. doc.: IEEE 802.22-07/0039r0 Submission January 2007 SooYoung Chang, Huawei TechnologiesSlide 4 Distributed Subcarrier Permutation There are three types of subcarrier permutation: –Distributed subcarrier permutation; –Band-type adjacent subcarrier permutation; –Scatter-type adjacent subcarrier permutation. In the current draft, it claims that distributed subcarrier permutation “is suitable for the users with high frequency selectivity or far distance users” (P226, Draft v0.1). Distributed subcarrier permutation is important for real-time voice, multimedia and regular data application.

5. doc.: IEEE 802.22-07/0039r0 Submission January 2007 SooYoung Chang, Huawei TechnologiesSlide 5 Our Proposed Solution: [1] Frequency Interleaving Based on normal chase combining approach, when the distributed subcarrier permutation is used or when there are bursty interferences in the channel, a frequency interleaver can be added in the retransmissions.

6. doc.: IEEE 802.22-07/0039r0 Submission January 2007 SooYoung Chang, Huawei TechnologiesSlide 6 By frequency interleaving, frequency diversity can be achieved at the receiver while combining the retransmitted packets. The interleaving pattern is determined by the receiver and feed back to the transmitter using more than 1 bit ACK/NAK. –Soft HARQ A...B C D D C B A 1 st Transmission: 2 nd Transmission: Frequency … Our Proposed Solution: [1] Frequency Interleaving

7. doc.: IEEE 802.22-07/0039r0 Submission January 2007 SooYoung Chang, Huawei TechnologiesSlide 7 Our Proposed Solution – [2] Soft HARQ Feedback of soft HARQ –Conventional HARQ ACK/NAK feedback carries only 1 bit. –In practice, there are plenty of overheads associated with delivery of the 1 bit ACK/NAK (e.g. preamble…ECC, etc.) –It will be more efficient if more bits are contained in the ACK/NAK feedback  Soft HARQ

8. doc.: IEEE 802.22-07/0039r0 Submission January 2007 SooYoung Chang, Huawei TechnologiesSlide 8 Our Proposed Solution – [2] Soft HARQ The receiver has the knowledge of channel state information and the bursty interference  Utilizing the soft HARQ (with a constraint of N bits), the receiver shall inform the transmitter which frequency interleaving pattern is the best. The receiver should add a “feedback controller” to determine the interleaving pattern. The transmitter just uses the selected interleaving pattern indicated in the feedback to do interleaving, no increase in complexity. Transmitter Channel Estimation Feedback Controller Interference Detection Soft HARQ feedback Frequency Interleaving Pattern

9. doc.: IEEE 802.22-07/0039r0 Submission January 2007 SooYoung Chang, Huawei TechnologiesSlide 9 System Flowchart: Transmitter Transmit a new packet Is the feedback ACK or NAK? Can retransmit? Get the interleaving pattern in the feedback Interleave the modulated symbols and retransmit No Yes NAK ACK

10. doc.: IEEE 802.22-07/0039r0 Submission January 2007 SooYoung Chang, Huawei TechnologiesSlide 10 System Flowchart --- Receiver Receive a packet Error? Can retransmit? Determine the interleaving pattern in the next retransmission Feedback to the transmitter No Yes No NAK feedback New packet? Symbol level chase combining Demodulate & Decode Yes No ACK feedback

11. doc.: IEEE 802.22-07/0039r0 Submission January 2007 SooYoung Chang, Huawei TechnologiesSlide 11 Message Flow: An Example TransmitterReceiver Packet 1 ACK No error Packet 2 Error occurs NAK + interleaving pattern x Interleaved Packet 2 NAK + interleaving pattern y Combining Error occurs TransmitterReceiver Interleaved Packet 2 ACK Combining No error …

12. doc.: IEEE 802.22-07/0039r0 Submission January 2007 SooYoung Chang, Huawei TechnologiesSlide 12 The Pattern Selection Algorithm: An Example A simple metric for adaptive interleaving –We choose an interleaving pattern to maximize the mutual information. –Assume is the channel gains encountering by the i-th transmitted symbol in the first n-1 times transmissions; are the channel gain of the i-th transmitted symbol when the m-th interleaving pattern is used; there are totally K transmitted symbols. Note the receiver is able to detect the interference, let the corresponding channel gain be zero if a symbol is put on a subcarrier with interference. Hence, in the n-th transmission, the controller will choose an interleaving pattern which maximizes the summation of the mutual information of each symbol, i.e.: –Where is the transmission power and is the power of noise.

13. doc.: IEEE 802.22-07/0039r0 Submission January 2007 SooYoung Chang, Huawei TechnologiesSlide 13 Simulation Configurations [1] Distributed subcarrier permutation –One subchannel contains 48 subcarriers, those subcarriers are discretely allocated. –Hence, the channel gains of these subcarriers are assumed to be i.i.d. Each subcarrier has an equal probability to encounter a larger interference. Three cases are considered: –No frequency interleaving No frequency interleaving –Fixed frequency interleaving The frequency interleaving patterns of each transmission are predetermined. –Adaptive frequency interleaving The frequency interleaving patterns are determined by the receiver online, and feed back to the transmitter.

14. doc.: IEEE 802.22-07/0039r0 Submission January 2007 SooYoung Chang, Huawei TechnologiesSlide 14 Simulation Configurations [2] Information bits per coded block: N_DBPB = 48; Length of coded block: N_CBPB = 96; QPSK, data rate ½, convolutional code specified in the standard; Randomly generate 32 interleaving patterns –The ARQ feedback contains 6 bits, namely 1 bit for ACK/NAK and 5 bits for pattern selection. Total number of transmissions = 4;

15. doc.: IEEE 802.22-07/0039r0 Submission January 2007 SooYoung Chang, Huawei TechnologiesSlide 15 Simulation: with interference Probability of interference per subcarrier = 0.2

16. doc.: IEEE 802.22-07/0039r0 Submission January 2007 SooYoung Chang, Huawei TechnologiesSlide 16 Simulation: with interference Probability of interference per subcarrier = 0.05

17. doc.: IEEE 802.22-07/0039r0 Submission January 2007 SooYoung Chang, Huawei TechnologiesSlide 17 Simulation: without interference

18. doc.: IEEE 802.22-07/0039r0 Submission January 2007 SooYoung Chang, Huawei TechnologiesSlide 18 Conclusions Frequency interleaving can greatly reduce the FER. –When the probability of interference per subcarrier is 0.05, adaptive frequency interleaving is about 7 dB better than no frequency interleaving. –Even there is no interference, adaptive frequency interleaving is still about 5 dB better than no frequency interleaving. Soft-HARQ with Adaptive frequency interleaving performs better than fixed frequency interleaving –Around 1dB gain compared with fixed frequency interleaving in the simulations. –The extra feedback cost is very mild. For example, in a channel with average FER of 1%, the average additional # of bits to support “soft HARQ” is about 8 bits per frame (with the probability of interference per subcarrier 0.2).