SOME STUDIES ON TRELLIS AND TURBO CODES (IMPLEMENTATION IN C)

Abstract

The main objective of this thesis is to study and implement two of the advanced and latest channel decoding algorithms and compare their performance. Although for implementing these decoding algorithm, the algorithms for channel encoder and AWGN channel (for adding noise) are developed as well. But our main objective is to study, analyze and compare the performance of decoding algorithms. The two decoding algorithms developed and implemented are (i) A Simplified Trellis-Based Decoder and (ii) Log-MAP-Based Iterative Turbo Decoder with reduced storage requirements. In “A Simplified Trellis-Based Decoder” a simplified branch metric and add-compare-select (ACS) unit is presented for use in trellis-based decoding architecture. This simplification is based on a complementary property of some feed forward encoders. As a result, one adder is saved in every other ACS unit. Hence only half the branch metrics have to be calculated. In “Log-MAP-Based Iterative Turbo Decoder” efforts are made to reduce the memory requirements for implementing the algorithm, although that is achieved at the cost of degraded speed performance. Also the odd-even symmetric interleaver structure used here for implementing turbo code is implemented with reduced storage memory requirements. For implementing the above algorithms an intense study of various decoding algorithms is done, and then algorithms are developed. Afterwards these algorithms are implemented in C language. Results are produced in form of text files, after executing the programs for encoder, AWGN channel and decoder in the sequence. After getting the results in form of text files, the decoded output is compared with the original file and bit error probability (PB) is calculated for different values of Eb/N0. Finally PB versus Eb/N0 is plotted for above algorithms and compared. At Eb/N0 of 2 dB PB for A simplified trellis based decoder is 2×10 and for Log-MAP-Based turbo decoder P-2B of 6×10 is obtained for a text file containing 3000 bit values. The error performance of Log-MAP-Based Turbo decoder is found to be better than the A simplified trellis based decoder but the speed of later is better than the former. -4