Research And Simulation For Soft Decision Algorithm In QAM Based On OFDM System

Orthogonal Frequency Division Multiplexing (OFDM) has been successfully applied to a wide variety of digital communications applications over past several years. Although OFDM has been chosen as the physical layer standard for a diversity of important systems, the theory, algorithms, and implementation techniques remain subjects of current interest. The research purpose of this paper is to research and improve performance of the system before designing baseband chip of physical layer of wireless communication system.On the base of analysis and research of elementary theory and system infrastructure of OFDM, the paper propose a soft decision method applied in OFDM system and a simplified soft decision algorithm is also proposed. The theory of soft decision technique is analyzed firstly. Demodulation and channel decoding modules are concerned with soft decision algorithm. Demodulator only evaluates all kinds of symbols temporarily, which is called soft decision, and the useful information for decoder will not be missing. The performance of soft decision applied in quadrature amplitude modulation (QAM) method, especially 16-QAM and 64-QAM. The results, obtained by computer simulations on IEEE802.11a platform, are given in terms of Bit Error Rate (BER) versus E_b/N_o among hard decision, theoretical soft decision and simplified soft decision methods.Results of research and simulation in this paper show that the soft decision method greatly outperforms the hard method. Moreover, the simplified soft demapping method with low computational complexity has the same performance as the theoretical soft demapping method. Algorithm research and performance simulation in this paper objectively testify that the soft decision method applied in OFDM system can improve the performance. Theory and inference of simplified soft decision algorithm are research specially. This paper provides trustworthy basis for development of baseband chip of wireless communication system. Because of the similarity of the infrastructure of OFDM systems, the conclusions in this paper are broadly valid for other OFDM systems.