Study And Implementation Of Turbo Encoding And Decoding In OFDM System

Turbo code,as soon as its appearance has caused the stir in information and code theorists due to its outstanding performance of closing with Shannon limit, and it obtained widespread research and application. It is determined as one of the third generation mobile communication system's channel coding plans.The Coded OFDM (COFDM) system which is combined channel coding technology with OFDM( Orthogonal Frequency Division Multiplexing) technology, unifies both's merit. It has become the research hot spot.This paper studies the Turbo Coded OFDM system which unifies the Turbo code and OFDM technology, and implements the Turbo codec system based on OFDM down data link project.At first, the Turbo encoding and decoding principles are thoroughly studied theoretically in this paper, and several kind of commonly used decoding algorithms are deduced in detail. Based on the Log-MAP algorithm, several kind of simplified algorithms are also deduced, such as Max-Log-MAP algorithm, Constant-Log-MAP algorithm and Linear-Log-MAP algorithm.Through the simulation and comparison of each algorithm's performance, it is known that the Constant-Log-MAP algorithm has better performance and simpler structure. So it becomes the following selected scheme of hardware realization.There are so many parameters that affect the Turbo code's performance. The following parameters are simulated and analyzed in oder to know their influences on Turbo code, such as component code, iterative times, interleaver length and so on .Under the premise of guaranteeing the performance and giving proper attention to both complexity of realization and real-time of communication, the parameter selection scheme of optimization design is obtained. After the introduction of wireless channel's characteristic, this paper analyzes the performance of Turbo in Rayleigh channel. It makes big improvement in performance by using of channel interleaver. It can play an guiding role for design of actual communication system.As the OFDM interleaves signal in frequency domain, and Turbo interleaves in time domain, so the combined system can anti burst noise and frequency selectively decline well. This paper introduces the structure of Turbo Coded OFDM system, and analyzes its performance.The simulation results indicates that using of Turbo as channel coding in OFDM system can obtain obvious coding gain. According to the 3GPP protocol, the Turbo encoding and decoding system is designed and realized on FPGA. Aiming at the problem that the iterative process is complex, and it demands too many memories, this paper proposes a new memeory scheduling method, and designs a new decoding structure. It saves half of memories to traditional decoding structure.Because of the big iterative delay of Turbo code, the data speed is low. Based on the high data speed requirement of OFDM down data link project, this paper uses several parellel decoder. In order to saving the memory of FPGA further and reducing cost, this paper uses external memory SRAM as its storage medium.At last, the encoding and decoding system is implemented on FPGA.The test results indicate that it satisfies the system request.