Performance Analysis And Optimized Design Of Relay Selection Techniques For Cooperative Communication Networks

The rapid development of wireless communication networks brings great challenges for current quality of service(QoS)requirements,such as higher throughput,more connection density and higher mobility.In cooperative communication system,multiple users collaborate with each other or dedicated relays are used to form a virtual distributed antenna system by obtaining the space diversity.Cooperative diversity can combat fading,extend service coverage,improve system capacity and achieve spatial degrees of freedom,therefore,cooperative relay is an important topic.Applying relay selection(RS)technique to cooperative communication systems is a straightforward and effective approach for taking advantages of space diversity and improving spectral efficiency.This thesis proposed several different relay selection schemes for cooperative networks and studied the impacts of perfect channel state information(CSI)and imperfect CSI,and some important analytical closed-form expressions and approximated expressions are obtained,which will provide important insights for the design of transmission scheme and optimize the system parameters.Firstly,for a two-user multi-relay cooperative non-orthogonal multiple access(NOMA)system with distributed space time coding(DSTC),two dual relay selection strategies are proposed,namely,two-stage dual relay selection with fixed power allocation(DRS-FPA)and twostage dual relay selection with dynamic power allocation(DRS-DPA).Furthermore,lower and upper bounds on the outage probability for the DRS-FPA scheme and the exact outage probability for the DRS-DPA scheme are obtained in closed-form,respectively.Numerical results show that the proposed two-stage DRS schemes not only yield better outage performance than the existing single relay selection(SRS)schemes without sacrificing spectral efficiency,but can also achieve full diversity gain.Secondly,a novel full-duplex(FD)RS with dynamic power allocation(FD-DPA-RS)is presented,which can adaptively adjust the value of power allocation factor according to the current CSI.Compared with the existing FD-RS with fixed power allocation(FD-FPA-RS),the proposed FD-DPA-RS scheme has a significantly better outage performance,while the diversity gain is zero due to the residual self-interference of FD relay.To solve this problem,a new adaptive-duplex(AD)relay selection scheme with fixed power allocation(AD-FPA-RS)is proposed,which can switch opportunistically between FD and half-duplex(HD)relaying modes.The simulation and numerical results show that the proposed AD relay selection schemes not only yield better outage performance than both the existing FD and HD relay selection schemes,but also can achieve full diversity gain.Thirdly,the impact of imperfect CSI on the two-stage relay selection scheme of cooperative NOMA network is analyzed,and then an optimization scheme is proposed.Applying the minimum mean square error(MMSE)channel estimation error model,the closed-form expres-sion of outage probability is obtained,the results demonstrate that imperfect CSI degraded the system performance.To further improve the performance,a dynamic power allocation strategy based the impact of channel estimation error is proposed,and then the closed-form expression of the system outage probability is given.The results show that the dynamic power allocation strategy can achieve superior performance.Finally,the performance of antenna selection for high-speed railway(HSR)mobile relay(MR)distributed antenna systems with imperfect channel estimation is studied.In HSR systems,the fading channel varies fast due to the large Doppler spread.It is difficult to accurately estimate,track,and predict the fast time-varying fading coefficients,thus,channel estimation error is usually inevitable and it may seriously degrade system performance.The channel estimation error is quantified through the estimation mean squared error(MSE),which is expressed as a closed-form function of the maximum Doppler spread and pathloss.Several performance metrics,such as outage probability,symbol error rate and spectral efficiency lower bound are developed.The analytical and simulation results quantify the significant impacts of channel estimation error on antenna selection for HSR systems in practical applications.And then,an optimum transceiver is designed to minimize the sum MSE of all the users for multi-user multiple-input multiple-output(MU-MIMO)systems with distributed antennas.The optimum designs are performed under two different power constraints: a constraint on the total power of all distributed antenna units(DAUs),and constraints on the power consumption of each DAU.