Research On Relay Selection And Channel Estimation For Wireless Cooperative Communication Systems

The emerging spatial diversity technology,Cooperative Communication,can be used to expand the coverage of existing cellular communication system,improving the link quality and spectral efficiency.Cooperative Communication will continue to play an irreplaceable rolein the field of wireless communications,though more attention should be paid in the theory research and implementation of it.In wireless relay cooperative systems,the number of relay nodes,which can be used for information forwarding,is usually more than one.If all the relay nodes in the idle state participate incommunication,it will cause a lot of waste.How to select one or more from these nodes to participate in the communication tomaximize the cooperative diversity gain,is the task of relay selection.Relay selection method determines which relay node participate in cooperative,andselecting "good" relay node can guarantee communication reliability and validity,so relay selection strategy is an important aspect of cooperative relay system.In current related studies,researchers usually assume that the channel state information(CSI)is known and determined for the relay node and the destination.But in the actual communication environment,instantaneous CSI value is usually changed with time,and many environmental factors may affect CSI,such as the node mobility,thereby CSI cannot be expressed bya fixed value.Therefore,from the point of view of communication reliability,only when the corresponding nodes in the communication system can master the real-time CSI,we can recover the information from the source.This requires the estimation of the channel parameters,which is an important research direction of the wireless relay cooperative system.In this paper,the key technologies of wireless relay system are researched,and we propose relay selection strategiesfor two-way relay networks with Nakagami-m channel,as well as two individual channel estimatorsfor distributed OWRN.The main achievements of this paper are as follows:1.Selecting the optimal relay is one of the most efficient schemes with space diversity for two-way opportunistic relay networks.However,in actual systems,the optimal relay maynot beobtained due to a variety of reasons,such as load balancing and scheduling issues,therefore,it is necessary to carry out the second,third or even the more universal Nth relay selection to ensure the operation of the network.In addition,when the optimal relay is selected in the cooperative diversity network,if the source node or the destination node generates error,the performance of the N-th relay should be used to evaluate the resulting performance loss of the system.In this paper,we propose a N-th relay cooperative communication method in AF relay system with independent flat Nakagami-m fading channel.The closed form formula of the probability density function of SNR is given,and the closed form expressions of outage probability and SER are derived based on it.Simulation results prove the correctness of the theory,and illustrate the difference between optimal relay selection strategy andthe Nth relay selection strategyused to complete cooperative communicationwhen the system cannot choose the optimal relay selection schemein outage performance and SER.Simulation results also show the influences of relay number and relay order on system performance,such as SER and diversity performance.2.A bidirectional relay selection mechanism is proposed for the three stage two-way AF relay system,and the performance of the proposed system in the independent flat Nakagami-m is studied.Firstly,the signal transmission process of the three stage two-way AF relay system is described,and then,according to the characteristics of the system model,a relay selection method which is different from the “best”single node selection criterion is presented.Then,in order to verify the validity of the method,we describe the outage probability of the system according to the definition,and then derive the lower bound closed formby zooming SNRs,as well as the approximate analytic expression when SNR tends to infinity.In order to further understand the relationship between the parameters of the system,the diversity multiplexing tradeoff is derived.And then SER is analysized.Finally,Monte-Carlo simulation is carried out to prove the accuracy of the results of theoretical analysis and the achieved asymptotic diversity order.Simulation results show that the performance of the proposed method is better than the best single node selection criteria.3.A channel estimation method of a classic distributed OWRN is proposed,which is composed of N+2 nodes in the network using a distributed space-time coding(DSTC) technology to transmit data.With superimposed pilot structure,Maximum Likelihood(ML)estimator and Maximum a Posterior(MAP)estimator are designed for individual channels,and ML estimator is usedin closed orthogonal training.Due to the complexity of the MAP structure,a low complexity iterative gradient descent estimator is designed in the individual channel.Then,the accuracy of the theoretical analysis is proved by Monte-Carlo numerical simulation.Finally,the estimation accuracy of the estimator is verified by analyzing the Normalized Mean Squared Error(NMSE)performance of the two-hop channel parameters estimation.