Research On Joint Channel And Frequency Offset Estimation For OFDM System Based On Soft Decision Feedback

By virtue of the high frequency spectrum utilization efficiency and the good anti-multipath interference ability, orthogonal frequency division multiplexing (OFDM) has been widely used in all kinds of wireless communication system, and become the research focus of the field of wireless communication.In wireless communication OFDM system, in order to restore the transmitted signal, the receiver must know the channel response; In addition, OFDM system is very sensitive to the Carrier Frequency offset (CFO), a very small CFO can cause severe performance degradation in OFDM, so the CFO estimation is necessary too; And channel estimation performance and CFO estimation performance often influences each other; so, joint carrier frequency offset and channel estimation in OFDM system will help to improve the system performance. This paper focuses on the channel estimation and carrier synchronization technologies of the OFDM systems, studies the channel estimation algorithm and the joint carrier frequency offset and channel estimation algorithm based on the soft decision feedback. The main content includes:1. The shortcomings of the traditional channel estimation methods and the influence of the CFO to OFDM system have been analyzed;2. The advantages and disadvantages of the traditional decision feedback which doesn’t use the decoding information and the hard decoding decision feedback channel estimation algorithm have been analyzed; a soft decision feedback channel estimation and symbol detection algorithm based on the BCJR decoding is put forward; then the performance comparison of the proposed algorithm and the existing algorithms is given for the different number of the pilots and the different doppler frequency shift. The results show that, when the channel is slowly time-varying or faster time-varying, soft decision feedback performance is superior to the hard decision feedback, and the hard decision feedback is superior to the traditional decision feedback; And in the case of faster time-varying channel, the advantages of soft decision feedback are more obvious than the case of slowly time-varying channel, so we can say that the soft decision feedback methods resistant the effects of time-variation more effectively;3. On the basis of the disadvantage of the joint CFO and channel estimation algorithm based on the Expectation-Maximization in the OFDM systems, an improvement algorithm that apply the soft symbols estimation method based on the BCJR decoding in the joint CFO and channel estimation algorithm based on the EM algorithm is proposed. The simulation results of the performance of the two algorithms show that, in the sparse and the non-sparse channel environment, the performance of soft decision feedback methods is superior to the hard decoding and the soft decoding which are both non-feedback, this is because that soft decision feedback estimation can restrain the mistake spreading that hard decision can’t correct.