Research On The Channel Coding Technique Of OFDM Systems Based On Turbo Codes

Turbo code was applied to OFDM (Orthogonal Frequency Division Multiplexing) system for its excellent performance. Optimizing the design of decoding algorithms and interleaver were important methods to improve the error-correcting performance of Turbo code. By comparing the performance of anti-multi-path of OFDM system used in BPSK, QPSK, 16-QAM modulation, it was found that the Gray mapping QPSK has the best bandwidth efficiency. It was also found that the log-MAP algorithm was the most suitable algorithm for Turbo code among several decoding algorithms. Although increasing the iteration times, the length of interleaver and the code rate could get high coding performance, it has negative effect on the realtime and decoding complexity. The performance analyses of Turbo-OFDM systems with different parameters in low SNR regions were given in this paper. The Log-MAP decoding algorithm was optimized based on the comprehensive analyses of performance, complexity and realtime. The interleaver algorithm was refined to reduce the correlation between the information bits according the correlation criterion of the interleaver design, a new design method of helical interleaver is presented in this paper. The simulation result shows that the new designed interleaver can obtain 0.9dB coding gain when BER(Bit Error Ratio) is 10-4 by lowing thecorrelation. The application of the optimization Turbo-COFDM design scheme to the transmission of a static picture got perfect result.