On The Fixed Point Turbo Decoding And The Turbo Coded Cooperation Scheme

Turbo codes were put forward by French scholar C. Berrou etc in 1993. With excellent performance approaching the Shannon limit, Turbo codes are regarded as milestones in the history of channel coding theory. Nowdays, Turbo codes have been widely applied in many communication systems such as 3G. Because higher data rate and better performance are required in next-generation mobile communication, it is necessary to analyze the performance of different Turbo encoding and decoding schemes and study the implementation techniques for Turbo codes. Meanwhile, it is very important to investigate coded cooperation base on Turbo codes in the distributed cooperation communication system. The work of this thesis is to discuss the performance influence by different coding structure by analyzing the weight spectrum of Turbo codes and through performance simulation. Meanwhile, the fixed point Turbo decoding algorithms and the involved normalization scheme are investigate as well based on the input-output statistics analysis. Moreover, the Turbo coded cooperation scheme is analyzed and discussed for full-duplex relay communication system.Firstly, the Turbo encoding structure and its influence on the achieved performance are addressed in this thesis. Some commonly utilized Turbo encoding scheme, including punctured Turbo codes, asymmetric Turbo codes and nonsystematic Turbo codes, are reviewed at first. And the constrained subcode search algorithm for Turbo weight spectrum searching is employed to determine the weight spectrum for punctured Turbo codes and asymmetric Turbo codes. Based on the union performance model, the weight spectrum could be utilized to approximate the error floor performance of Turbo codes. And the analysis in this thesis shows that, symmetric Turbo codes with nonprimitive feedback polynomials may achieve a reasonable waterfall region performance, while the symmetric Turbo codes with primitive feedback polynomials may achieve a reasonable error floor regionperformance. On the other hand, asymmetric Turbo codes with one component code with nonprimitive feedback polynomial and another component code with primitive feedback polynomial may achieve a reasonable tradeoff between the achieved error floor and waterfall region performance.Secondly, several Turbo decoding algorithms are introduced and performance simulations are performed to determine the candidate decoding scheme for implementation. Only the fixed-point algorithm could be utilized in hardware implementation of Turbo decoder. Seemingly, the reasonable data width for all decoding variables in the fixed point processing, together with the normalization method in the recursive calculations of state variables, need to be carefully designed to guarantee the achieved performance. Considering a reasonable tradeoff between the achieved performance and the involved calculation complexity, Enhanced Max-Log-MAP algorithm is chosen as the fixed point implementation for Turbo decoding algorithm for its reduced complexity and nearly the same achieved performance. In this paper, based on the input-output statistics analysis, the uniform quantization pre-processing and the respective data width for the input data will be addressed at first. Then the emphasis will be laid on the normalization method for the recursive calculations of forward/backward state variables. It is shown that both the data width for quantization processing and the respective normalization parameters could be derived by using the input-output statistics analysis.Finally, the coded cooperation scheme based on Turbo codes is discussed for wireless relay network. After a survey of the cooperation communication system model and the cooperative communication schemes, the soft information combining based Turbo coded cooperation scheme is addressed for full-duplex relay model. The influences of the different locations of relay nodes on the achieved performance are investigated through simulations. The performance of the entire cooperative communication system was simulated, and the results validate that, significant performance gain could be achieved by using the coded cooperation over the relay communication scenarios.