Research On Relaying Protocols And Best Relay Selection Strategy In Cooperative Communications

Recently,cooperative communication technology has been the research hot spot in the field of wireless communications.By the cooperative transmission of the relays,it can effectively resist shading and instable links in wireless channels,and the transmission performance could be improved.So this dissertation focuses on the relay technology in wireless cooperative communications for investigation,and the researches about the relaying protocols and best relay selection strategy are carried out in cooperative communications.And the main work and creative contents are described as follows:1.The performance of several typical relaying protocols are analyzed and compared.And for the problems of the noise amplification in amplify-and-forward?AF?protocol and the error propagation in decode-and-forward?DF?protocol,by introducing the incremental relay with ARQ mode,an incremental selection hybrid decode-amplify forward?ISHDAF?scheme is proposed.According to the feedback information from the destination node,the proposed scheme can properly utilize the methods of the direct transmission,the source node retransmission and the cooperative transmission,respectively.Theoretical analyses and simulation results show that the proposed scheme can improve the spectrum efficiency of the hybrid decode-amplify forward?HDAF?protocol.In addition,the bit-error-rate?BER?and the outage performance of the proposed scheme quite outperform that of the current incremental selection amplify forward?ISAF?scheme as follows.At a BER of 10-5,it obtains about 1dB performance gain than the ISAF scheme.And the outage performance is improved for about 0.7dB at outage probability of 10^-2,when compared with the ISAF scheme.2.This paper summarizes and compares the current relay selection strategy based on different criteria.Among them,the opportunistic relay selection policy can obtain the same diversity gain with the all relay cooperation by selecting the best relay,as well as the better adaptability.Therefore,combined with opportunistic relay,an optimal relay selection strategy of the HDAF protocol is proposed.Compared with relay selection strategies of the fixed relaying protocols,the BER and the outage performance of the proposed strategy are greatly improved,and more relays lead to much more significance in improving the performance.When the outage probability is 10^-2 and the number of the relays goes from 2 to 8,the proposed strategy can obtain about 5dB performance gain.Also at BER of 10^-3,the performance of the proposed strategy with 8 relays improves about 2dB than that with 2 relays.3.In order to improve the spectral efficiency of all-time cooperation,this paper investigates an optimal relay selection strategy with selective decode-and-forward?SDF?protocol based on threshold decision,and derives the approximate expression of the outage probability and the spectrum efficiency with high signal-to-noise?SNR?.The simulation and analysis show that,according to different SNR and the number of the relays,the proposed strategy can select an appropriate cooperation threshold,thereby improving the spectral efficiency and the outage performance.Under low-SNR and few relays,the outage probability under different thresholds is very close.If the cooperation threshold increases by 0.09,the spectral efficiency is improved by0.12.Under high-SNR and many relays,the spectral efficiency under different thresholds is similar.When the system outage probability is 10^-2,the cooperation threshold is decreased by 0.9,and1.1dB SNR gain can be obtained.With the rapid development in the theory and the applications of wireless communications,the cooperative communications,as one of the key technology,has been improved a lot too.Therefore,the researches on relaying protocols and best relay selection strategy in cooperative communications in this paper will have high theoretical and practical means in this field.