Wimax System Ctc Codec Algorithm Analysis And Design

As a crucial sub-system in wireless communication system, error-correcting codes impact markedly on the overall performance of communication system. Recognized as an optional channel coding technology in World wide Interoperability for Microwave Access (WiMAX) standard, the Convolutional Turbo Code (CTC), which concatenates several convolutional constituent encoders in parallel, has many advantages such as flexible codeword, large range for data block sizes, varying coding rates and so on. As a result of the characteristics approximating those of the pseudorandom code, CTC is able to provide performance very near to Shannon capacity. At the same time, CTC has a strong ability to fight the channel fading and various interfering effect, which makes it capable to perform satisfactorily even in very adverse communication environment. Such superb performance attracts enormous interest from researchers all over the world. Implementation of CTC codec in WiMAX system has been conducted and presented. Combined with classic turbo code, CTC encoder in IEEE 802.16e standard has been introduced, and the corresponding modification in structure to that of the classic turbo code has been analyzed.Principle of CTC decoding that adopts iterative decoding scheme between two constituent decoders has been analyzed. On the basis of Viterbi Algorithm and SOVA, procedure and mathematic principle of constituent decoding algorithm, such as MAP Algorithm, Enhanced Max-Log-MAP Algorithm and the like has been described and deduced in detail. Performance and complexity has been discussed and compared, so as to choose the most appropriate algorithm to be implemented in constituent decoders. Modification and improvement to CTC decoding procedure have been researched. Regarding the circularity of the CTC constituent encoder state, a new algorithm - Recursion Feedback Algorithm has been introduced, which helps to solve the problem come across in initializing forward and backward recursion between two successive decoding iterations. Consequently, at least a 0.5 dB performance gain has been achieved. CTC codec simulation system that conforms to the specifications in IEEE 802.16e OFDMA PHY has been designed and constructed based on MATLAB, according to the chosen constituent decoding algorithm and the modified iterative decoding procedure. Proposed solution to module implementation has been achieved through optimizing algorithm structure, improving algorithm procedure and analyzing configuration of certain key parameters. System simulation under AWGN channel has been inspected and the result shows that the designed CTC codec achieves satisfactory performance, as the CTC decoder with 8 iterations obtains a 10?7 Bit Error Rate (BER) when channel Signal Noise Ratio (SNR) comes to 3 dB.