WWiSE Group Partial Proposal on Turbo Codes

1. WWiSE Group Partial Proposal on Turbo Codes September 15, 2004 Airgo Networks, Bermai, Broadcom, Conexant, Realtek, STMicroelectronics, Texas Instruments S. Coffey, et al., WWiSE group

2. WWiSE contributors and contact information • Airgo Networks: VK Jones, vkjones@airgonetworks.com • Bermai: Neil Hamady, nhamady@bermai.com • Broadcom: Jason Trachewsky, jat@broadcom.com • Conexant: Michael Seals, michael.seals@conexant.com • Realtek: Stephan ten Brink, stenbrink@realtek-us.com • STMicroelectronics: George Vlantis, George.Vlantis@st.com • Texas Instruments: Sean Coffey, coffey@ti.com S. Coffey, et al., WWiSE group

3. Contents • Overview of partial proposal • Motivation for advanced coding • Specification of turbo code • Performance results • Summary S. Coffey, et al., WWiSE group

4. Transmitter block diagram Add pilots Turbo encoder, puncturer Interpol., filtering, limiter MIMO interleaver Upconverter, amplifier Symbol mapper IFFT D/A Add cyclic extension (guard) S. Coffey, et al., WWiSE group

5. Overview of partial proposal • WWiSE complete proposal contains optional LDPC code • FEC coding is a distinct module • In principle any high-performance code could be used instead of the LDPC code • This partial proposal discusses an alternative choice for optional advanced code • System proposed is identical to the WWiSE complete proposal in all respects except that the optional LDPC code is replaced by the turbo code described here S. Coffey, et al., WWiSE group

6. Motivation for advanced coding • Higher achievable throughput at the same robustness • Capacity difference between rate ¾ and rate 5/6 with MMSE, 64-QAM is 3 dB • So any code that achieves 3 dB gain enables a rate increase from ¾ to 5/6: an 11% increase without robustness penalty • Compare rates 5/6 and 7/8: 2 dB difference for a 5% increase Viability of 2x2 MIMO, BCC, rate ¾ in any mode implies similar robustness of 2x2 MIMO, new code, rate 5/6 S. Coffey, et al., WWiSE group

7. Motivation for advanced coding • In addition: • Better coding enhances coverage and robustness • Provides partial alternative to increased number of receive chains or high performance MIMO detection • Advantages carry over to all MIMO configurations and channel bandwidths S. Coffey, et al., WWiSE group

8. Specification of turbo code S. Coffey, et al., WWiSE group

9. Systematic bit 1+D +D3 g(D) = Parity bit 0 1+D2+D3 1+D +D3 Turbo interleaver Parity bit 1 g(D) = 1+D2+D3 Turbo encoder Uses the constituent codes used in the 3GPP/UMTS standard encoder: S. Coffey, et al., WWiSE group

10. Turbo code frame format • Data payload is padded to a multiple of 512 bits • Result is divided into blocks of 2048 bits and 512 bits • Number of 512 bit blocks is in the range 1-4 • All 512 bit blocks are placed at end of frame • Each block is encoded as a separate turbo codeword S. Coffey, et al., WWiSE group

11. Turbo code frame format, contd. • E.g., 4000 byte packet has approximately 15 blocks of length 2048, 1-4 of length 512 • At 135 Mbps, 2048 bit blocks span 5 OFDM symbols; 512 bit blocks span 2 OFDM symbols S. Coffey, et al., WWiSE group

12. Turbo interleaver design • Two interleavers are proposed, one for each supported block size: 2048 and 512 bits • Though algorithmically generated, implementations will hardwire interleavers, so number of block sizes should be minimized • Each interleaver is a contention-free inter-window shuffle interleaver • Designed to minimize memory contention in parallelized decoding • Equivalent performance to 3GPP/UMTS interleavers S. Coffey, et al., WWiSE group

13. Code rate Puncture interval 2/3 4 3/4 6 5/6 10 Puncturer • Parity bits are punctured at regular intervals: • Systematic & tail bits not punctured; pad bits are • All code rates are easily derivable from mother code S. Coffey, et al., WWiSE group

14. Code rate Puncture interval 2/3 2 3/4 3 5/6 3 Puncturing tail codewords • Tailing codewords, i.e., codewords of length 512 information bits, are punctured to a lower code rate • This facilitates low latency decoding: tail codeword blocks are lower rate and can be decoded with fewer iterations, without affecting operating point • Puncture intervals for tail blocks: S. Coffey, et al., WWiSE group

15. Parallelization of turbo decoders • Parallelization: • Divide trellis into a number of (possibly overlapping) segments and decode each in parallel • Any reasonable number of iterations can be achieved without affecting latency • End-of-packet latency: • Tailing codewords can be decoded with fewer iterations . . . Block 3 Block 1 Block 2 S. Coffey, et al., WWiSE group

16. Complexity • Compare to state complexity of 64-state BCC decoding equivalent throughput • System assumptions: M-state constituent codes, I iterations, soft-in soft-out algorithm extra cost factor of a, BCC duty factor of b • Decoder must process 2 x 2 x I x b trellis transitions (I iterations, 2 constituent codes, forward-backward for each, less duty factor), each of which costs aM/64 as much • Overall complexity is 4I abM/64 times as much as 64-state code • E.g., with M = 8, I = 7, a = 1.5, b = 0.7, we have 3.675 times the state complexity • This does not account for other differences such as memory requirements and interleaver complexity S. Coffey, et al., WWiSE group

17. Performance results S. Coffey, et al., WWiSE group

18. Simulation setup • All combinations of: • Channels B, D, AWGN • 20 MHz and 40 MHz • Rate ¾ and rate 5/6 • BCC and turbo code • All simulations under ideal conditions S. Coffey, et al., WWiSE group

19. AWGN, 20 MHz S. Coffey, et al., WWiSE group

20. AWGN, 40 MHz S. Coffey, et al., WWiSE group

21. Channel model B NLOS, 20 MHz S. Coffey, et al., WWiSE group

22. Channel model B NLOS, 40 MHz S. Coffey, et al., WWiSE group

23. Channel model D NLOS, 20 MHz S. Coffey, et al., WWiSE group

24. Channel model D NLOS, 40 MHz S. Coffey, et al., WWiSE group

25. Turbo and LDPC codes • Turbo codes have a more extensive and stable literature, build on Viterbi decoding, have straightforward encoders • LDPC codes have more flexibility, which may in principle be used to allow code design matched to decoder structure; more amenable to analysis • To first order, both give the same performance tradeoffs S. Coffey, et al., WWiSE group

26. Conclusions • Turbo code compensates for code rate increase to 5/6 from ¾ • Very well studied, extensive literature on performance and implementation • Latency at end of packet may be handled by tailing; applicable to LDPC codes also • Generally similar issues and tradeoffs as LDPC codes S. Coffey, et al., WWiSE group

27. References IEEE 802.11 documents: • IEEE 802.11/04-0886-00-000n, “WWiSE group PHY and MAC specification,” M. Singh, B. Edwards et al. • IEEE 802.11/04-0877-00-000n, “WWiSE proposal response to functional requirements and comparison criteria,” C. Hansen et al. • IEEE 802.11/04-0952-00-000n, “WWiSE partial proposal on turbo codes: specification,” S. Pope et al. Parallelization: 4. K. Blankenship, B. Classon, and V. Desai, “High-throughput turbo decoding techniques for 4G,” Int. Conf. on 3G Wireless & Beyond, 2002. 5. E. Yeo, B. Nikolic, and V. Anantharam, “Iterative decoder architectures,” IEEE Communications Magazine, August 2003, pp.132-140 S. Coffey, et al., WWiSE group

28. References, contd. 6. Z. Wang, Z. Chi, and K. K. Parhi, “Area-efficient high-speed decoding schemes for turbo decoders,” IEEE Trans. VLSI Systems, vol. 10, no. 6, pp. 902-912, Dec. 2002 • S. Yoon and Y. Bar-Ness, “A parallel MAP algorithm for low latency turbo decoding,” IEEE Communications Letters, vol. 6, no. 7, pp. 288-290, July 2002 Interleavers: 8. A. Nimbalker, K. Blankenship, B. Classon, T. Fuja, and D. Costello, “Inter-window shuffle interleavers for high-throughput turbo decoding,” Proc. Int. Symp. on Turbo Codes, 2003. 9. A. Nimbalker, K. Blankenship, B. Classon , T. Fuja, and D. Costello, “Contention-free interleavers,” Proc. Int. Symp. on Info. Theory, 2004. S. Coffey, et al., WWiSE group