Research On The Application Of Turbo Codes In COFDM

Turbo codes have been arresting great attentions from the academia of communications and informatics because of their excellent performance in error correction. To improve the efficiency of transmitting encoded data in wireless channel, the application of Turbo codes in coded orthogonal frequency division multiplexing(COFDM) is researched in this paper. A novel COFDM scheme is proposed. It employs Turbo codes, OFDM modulation and differential coding technologies and is termed as Differential Turbo coded OFDM(DT-OFDM).Firstly, the principal of Turbo codes and i nterleaver design are discussed. A modified MAX-LOG-MAP decoding algorithm is proposed after a depiction of modified BCJR algorithm. It employs simple and explicit butterfly-type and tree-type operations, which cause the new algorithm much easier for implementation. B ased on the analysis o f performance limit of Turbo codes, it is made clear that the distance spectrum in optimizing codes is the objective of designing optimal interleaver. A new concept, called spread-extent, is also introduced in this paper. After an overview of some generally used approaches of interleaving, a new interleaver is brought forward, named the golden-spread interleaver. It possesses high spread-extent and randomness and can make the distance spectrum of output codes better. To verify their effectiveness, simulation studies are conducted to test the performance of the proposed Turbo codes in AWGN channels and flat fading channels. In these simulations, the modified MAX-LOG-MAP decoding algorithm and the golden-spread interleaver are applied. On the application of Turbo code in COFDM, the DT-OFDM scheme is presented, together with a differential soft-detection method of DT-OFDM. By comparing the amplitudes and phase differences of neighboring subcarriers of DT-OFDM signals, the credibility information of the transmitted data can be obtained, which in turn is used as the soft input of the iterative decoding algorithm of Turbo codes. On the realization of this new scheme, the influence of the fixed-point quantification upon the modified MAX-LOG-MAP algorithm is evaluated. Simulations show that: if the input data is 5 bits at length, the unfavorable effect of quantification error is negligible. Finally, an implementation of the DT-OFDM demodulator is given. The key point of this demodulator is a Turbo decoder based on fix-point digital signal processor (DSP). Compared with other DSP-based Turbo decoder, the proposed decoder is of much high decoding speed.