Channel Codec for MB-OFDM UWB

1. Channel Codec for MB-OFDM UWB 2006/10/27 Speaker: 蔡佩玲

2. Outline • Specification for UWB • Conventional Viterbi decoder • Modified Viterbi decoder for UWB • Simulation • Conclusion • Future Work • References

3. Outline • Specification for UWB - System Parameter - Convolutional Encoder - Puncture Pattern • Conventional Viterbi decoder • Modified Viterbi decoder for UWB • Simulation • Conclusion • Future Work • References

4. System parameter

5. Output Data A D D D D D D Output Data B Output Data C Convolutional Encoder • Assume a mother convolutional code of R = 1/3, K = 7. Having a single mother code simplifies the implementation. • Generator polynomial :g0=[1338], g1=[1658], g2=[1718] • Higher rate codes are achieved by puncturing the mother code. Puncturing patterns are specified in latest revision of 09/2004.

6. Puncture pattern • Additional coding rates are derived from the “mother” rate R = 1/3 convolutional code by employing “puncturing”. Puncturing is a procedure for omitting some of the encoded bits at the transmitter and inserting a dummy “zero” metric into the decoder at the receiver in place of the omitted bits.

7. Outline • Specification for UWB • Conventional Viterbi decoder - Review of Viterbi decoding - Decoder architecture • Modified Viterbi decoder for UWB • Simulation • Conclusion • Future Work • References

8. Review of Viterbi decoding • Viterbi decoding is one decoding algorithms used with convolutional encoding. It is well suited to hardware decoder implementation. But its computational requirements grow exponentially as a function of the constraint length, so it is usually limited in practice to constraint lengths of K = 9 or less. • Convolutional codes are usually described using two parameters: the code rate , k/n, and the constraint length, K. -k : the number of bits into the convolutional encoder. -n: the number of channel symbols output by the convolutional encoder. -K: the constraint length parameter, denotes the "length" of the convolutional encoder.

9. Review of Viterbi decoding (cont.) • Given the unique mapping between a trellis path and an input sequence, the most likely input sequence (shortest path) through the trellis corresponds to the most likely input sequence. - Viterbi algorithm is an efficient method for finding the shortest path through a trellis. • Viterbi decoding algorithm : -The first step : Recursively compute the shortest path. -The second step :Tracing back and decoding the shortest path.

10. Decoder Architecture • The Viterbi decoder contains three main units : transition metric unit (TMU), add–compare–select unit (ACSU), and survivor memory unit (SMU). -TMU calculates the transition metrics from the input data. -ACSU recursively accumulates transition metrics (TM) as path metrics (PM), and makes decisions to select the most likely state transition sequence. -SMU traces the decisions to extract this sequence.

11. Outline • Specification for UWB • Conventional Viterbi decoder • Modified Viterbi decoder for UWB -Introduction -Combined techniques -Decoder architecture • Simulation • Future Work • Conclusion

12. Introduction • Some techniques have been developed to increase the decoding speed. These techniques include lookahead, sliding block, and parallel processing. • These techniques are combined to achieve the highest desired data rate. For lower data rates, it is possible to disable some parts of the decoder for power saving by proper analysis of the effects of puncturing on word length and trace back length.

13. acquisition truncation + K-D K+D maximum Best state Modified Decoder Conventional Decoder truncation maximum K-2D Best state Combined techniques • Lookahead Conventional Viterbi decoder determines the best state with one way truncation. But with the lookahead technique, it uses two way truncations. When forward and backward ACS iterations meet, the overall beststate is determined. Starting from this state, the best path is traced back and the corresponding source bits are computed as a parallel output.

14. Combined techniques (cont.) • The acquisition and truncation procedure can be performed independently and hence in parallel. It is most efficient to use nonoverlapping contiguous blocks of length 2D for this operation. The result is a number of uniquely decoded states with distance 2D transitions. -The Same Decoding Performance as a Conventional Viterbi Decoder.

15. Combined techniques (cont.) • Sliding block The Sliding Block Viterbi Decoder has the computation efficiency of the Viterbi algorithm. The Sliding Block Viterbi Decoding approach reduces decode of a continuous input stream to decode of independent overlapping blocks, without constraining the encoding process.

16. Decoder architecture • The proposed decoder architecture, which combines lookahead, 2-level parallelism, and serialized sliding block techniques. The decoder is divided into 3 blocks.

17. Outline • Specification for UWB • Conventional Viterbi decoder • Modified Viterbi decoder for UWB • Simulation • Conclusion • Future Work • References

18. Simulation • Simulation Parameter • Information Data Rate 200 Mb/s • Modulation : QPSK • Quantization : 3 bits • Coding rate = 5/8 • Required Eb/N0=4.3dB • Modified Viterbi decoder • Estimation length = 5L • Trace back length=3L

19. Simulation (cont.) • Simulation Parameter • Information Data Rate 480 Mb/s • Modulation : DCM • Quantization : 3 bits • Coding rate = 3/4 • Required Eb/N0=4.6dB • Modified Viterbi decoder • Estimation length = 5L • Trace back length=3L

20. Simulation (cont.) • Compare Modified Viterbi Decoder (L=6) with conventional Viterbi decoder( Length = 5L=30) • Simulation parameter • Information Data Rate 200Mb/s • Modulation : QPSK • Quantization : 3 bits • Coding rate = 5/8 • Required Eb/N0=4.3dB • Coding Loss = 1.8dB

21. Simulation (cont.) • Compare the performances of different data rates with the same length L = 6 • Simulation parameter • Quantization : 3 bits • Modified Viterbi decoder • Estimation length = 5L • Trace back length=3L

22. Outline • Specification for UWB • Conventional Viterbi decoder • Modified Viterbi decoder for UWB • Simulation • Conclusion • Future Work • References

23. Conclusion • The modified Viterbi decoder has higher speed than the conventional Viterbi decoder. On the other way, the performance of the modified Viterbi decoding is not so ideal as the conventional Viterbi decoder. But adjusting the block length L would get the performance close to the desired spec. • Puncturing has large influences to decoding performances. Decoding would probably not be working with losing too much information bits. Relations of length L and the code rates

24. Outline • Specification for UWB • Conventional Viterbi decoder • Modified Viterbi decoder for UWB • Simulation • Conclusion • Future Work • References

25. Future Work • The ratio of the trace back length and estimation length is 3/5. This ratio could be any other rational number. Since different ratios could bring different performances, it would be possible to pick a ratio to meet the optimal simulation performance. Furthermore, it is also researchable to find the length, L, to both get the better performance and the high throughput.

26. Outline • Specification for UWB • Conventional Viterbi decoder • Modified Viterbi decoder for UWB • Simulation • Conclusion • Future Work • References

27. References • [1]“Viterbi Decoder for high-speed Ultra-Wideband communication systems ”, Jun Tang and Keshab K. Parhi. • [2]“A CMOS IC for Gb/s Viterbi Decoding: System Design and VLSI Implementation”, Herbert Dawid, Gerhard Fettweis, and Heinrich Meyr, IEEE Transactions on very large scale integration (VLSI) systems. • [3]“A 1-Gb/s, Four-State, Sliding Block Viterbi Decoder”, Peter J. Black, and Teresa H.-Y. Meng, IEEE Journal of solid-state circus. • [4]“A Technique for Demapping Dual Carrier Modulated UWB OFDM Signals with Improved Performance”, Zhongjun Wang Wenzhen Li Lee Guek Yeo Yanxin Yan Yujing Ting Masayuki Tomisawa. • [5]”Multiband OFDM physical layer specification”

28. Thank you for your attention