A Study On Turbo Code And Turbo Equalization Of Communication System

Equalization and channel coding are the main techniques that improves the performance of wireless communication system. ISI caused by multipath is the major obstacle in shortwave channel. In this dissertation the equalizers widely used, such as DFE,MLSE, are introduced.Turbo codes are a class of forward error correction codes that offer energy efficiencies to the limits predicted by information theory. Iterative decoding techniques are applied in conjunction with special encoding strategies which combine powerful codes in such a way that allows each code to be decoded independently. The way the information is passed from one decoder to the next one, and the type of soft output decoding algorithm which is used, are keys to the improvement in performance from one iteration to the next. The dissertation introduces the fundamentals of turbo codes. Encoding and decoding structures are explained.In turbo decoder, two main algorithms, MAP and SOVA, are compared with both outstanding simulating results. An optimized maximum a posteriori decoding algorithm is described and compared with a soft output Viterbi algorithm. This optimized algorithm is used in all simulations throughout this thesis. Small interleaver sizes and low SNR are emphasized during the investigation. Both the additive white Gaussian noise channel model and Rayleigh fading channel model are used in computer simulations. Additionally, under low SNR, the classic equalizer failed to acquire a satisfying outputs. The received data without equalization were tried to be dealt with using Iterative decoding techniques directly. The simulation and the experience show that Turbo codes can achieve a better result without equalization.Iterative equalization and decoding(turbo equalization) offers another means under ISI channel, low-complexity, sub-optimal forms of equalization were used. When error-correcting codes ere used on the ISI channel, the optimal decoder/detector works on the combined trellis of the channel and the code. The simulation of Turbo equalization is similar with that of Turbo code.Another part is the implementation of several algorithms, Viterbi and Max-Log -MAP, used in turbo decoder based on ADSP21060.