Research On Space-time Coding And Precoding Technology In Cooperative Relay Systems

The use of cooperative relay communications in combination with multiple-input multiple-output(MIMO)technologies has been shown to be an effective means to extend the communication range and improve the system capacity.Cooperative communication can also achieve spatial diversity through virtual multiple antennas.Space-time coding technology in MIMO system has attracted much attention,because it can improve the performance through obtaining spatial diversity gain without channel information at the transmitter.Corresponding to this is the precoding technology which needs to acquire the channel state information(CSI).Furthermore,the space-time block codes(STBC)has been combined with precoding in order to obtain angular selectivity gain in addition to diversity gain.Hereafter,we call the combined technique “precoding-plus-STBC”.A number of precoding-plus-STBC methods have been proposed to improve the performance of MIMO system.These design schemes show that it can substantially improve the performance of MIMO system.Considering the similarity of relay and MIMO system,precoding-plus-STBC can be naturally applied to a relay system.In this paper,we focus on precoding-plus-STBC for correlated MIMO channels in employing amplify-and-forward(AF)relay system.The contributions of this paper are as follows:1.The research work of precoding,space-time coding and selection techniques in cooperative communication system is summarized.First,the work on precoding technology is divided into single relay system and multiple relay system according to the number of relays.Second,a model system of relay network is given with STBC.The received signal-to-noise ratio(SNR)is achieved respectively by employing AF protocol and decode-and-forward(DF)protocol.And finally the selection technology is divided into antenna selection,relay selection and joint relay and antenna selection.The three parts are the basis of the following chapters.2.We design the precoding matrices at both the source and relay nodes of MIMO relay networks to minimize the symbol error rate(SER)based on the statistical CSI.First compact SER expressions are derived,and then the optimization problem of designing the precoding matrices is formulated.This problem is highly nonlinear and nonconvex.We propose a two-step method by exploiting the correlations of the source-relay and relay-destination links.In the first step the two precoding matrices are proved to be in the eigenvector directions of the transmit correlation matrices.In the second step,the original joint problem is decomposed into two independent sub-problems which can be solved in lower complexity.Simulation results show that the precoding system combined with STBC outperforms the only STBC system in terms of SER,especially at low signal-to-noise ratios.3.To improve the SER performance,we apply the log-likelihood ratio(LLR)criterion to the relay selection system with precoding-plus-STBC.Meanwhile,we consider the joint design of precoding matrices at both the source and relay nodes in a multi-relay system based respectively on perfect and imperfect CSI.Moreover,the SER for relay selection system based on proposed scheme is analyzed.At first,we design the combined precoding matrices based on respectively the maximum SNR with perfect CSI and the minimum SER of the single-relay system with imperfect CSI.Then,different from traditional relay selection based on signal-to-noise ratio,the relay node is chosen with the maximum LLR.The superiority of the LLR is explored,and the criterion of relay selection based on LLR is derived.It is proved that the SER is minimized compared with the SNR scheme,which is consistent with the following simulation results.4.A multiple relay selection scheme based on LLR criterion is derived.In communications where multiple relays are used,the selection of multiple relays can obtain better SER performance than that of single relay.Using eigen-decomposition of matrices and monotonicity of functions,the joint design problem is transformed into two kinds of independent sub-problems,and a fast implementation algorithm is also proposed.Simulation results show that the scheme based on LLR is superior to that on SNR in the SER performance.Meanwhile the multi-relay system with the proposed scheme can improve the SER performance effectively compared that with only space-time coding or only precoding system.5.The performance of joint relay and antenna selection in precoding-plus-STBC system is studied.Two antenna selection schemes are proposed: In the first scheme,the optimal receive antenna on each relay node is selected based on maximizing the two-stage LLR/SNR,which is feedback from the destination.And in the second scheme,the optimal receive antenna is based on single-stage LLR/SNR,and the feedback is not required.For each joint selection scheme,the single best relay is selected at the destination based on the received LLR/SNR.Then the probability density function(PDF)and the cumulative distribution function(CDF)of received SNR are derived respectively,and both the outage probability and SER are analyzed.The simulation results show that the performance of single-stage antenna selection is similar to that of the two-stage scheme,when the number of receive antennas is small.Furthermore,the LLR criterion can also achieve better SER performance when there are fewer relays.