Transmission Strategy Of MIMO Relay Communication Systems Based On SWIPT

Multiple-Input Multiple-Output(MIMO)communication systems which are equipped with multiple antennas can employ the spatial multiplexing gain to improve the spectrum efficiency of the system,and also adopt the diversity gain to improve the reliability of the system.Thun,MIMO communication system has attracted a lot of attention from scholars.However,the energy limitation of communication nodes restricts the lifespan of communication networks,becomes a bottleneck in the development of communication systems,and hinders the development of green communication.Simultaneous wireless information and power transfer(SWIPT)technology can extend the time of wireless communication system by utilizing the broadcasting characteristics of the radio frequency(RF)signal which energy and information can be transferred simultaneously,and become an important way to realize the green communication.In addition,the relay system has the advantages of expanding network coverage,reducing transmit power,improving system spectrum efficiency,and improving edge user service quality.Therefore,consider applying MIMO technology and SWIPT technology to the wireless relay communication systems.However,two technologies lead some severe challenges to the transmission methods and resource allocation design of the traditional wireless relay system.Therefore,this thesis employs multi-antenna precoding technology and mathematical optimization tools to achieve the efficient transmission of energy and information in MIMO SWIPT relay system.The specific content is as follows:Firstly,for the problems of the interference caused by multiple antennas and the design of SWIPT,a transmission method of SWIPT-based MIMO cooperative relay system and an optimization algorithm of the power splitting are proposed.When the direct link is affected by severe large-scale fading,a suitable transmission method for this scenario is proposed.Different from the traditional relay system,the relay node adopts the power splitting(PS)mode to harvest energy and decode information,and the harvested energy is used to provide the transmission power in the second phase.Furthermore,the transmission methods of MIMO SWIPT cooperative relay systems based on zero-forcing reception/zero-forcing transmission(ZFR/ZFT)and maximum ratio combining/maximum ratio transmission(MRC/MRT)procoings are proposed,and the achievable rates of relay systems corresponding to two precodings are derived.In addition,a MIMO SWIPT cooperative relay system based on SVD precoding is designed to diagonalize the MIMO channels on both sides of the relay node and eliminate the interference of multiple data streams.To maximize the achievable rate,the optimal PS closed expression for each antenna is obtainedSecondly,aiming at the problem of the impact of massive MIMO on the performance of SWIPT relay system,the transmission method of massive MIMO two-way relay system based on relay-SWIPT(R-SWIPT)technology are proposed.In the two-way relay communication scenario,the suitable transmission method which considers massive MIMO beamforming technique is proposed.Different from the traditional two-way relay,PS mode of SWIPT is applied to the large-scale antenna relay node.Two classical and linear relay precodings,i.e.,ZFR/ZFT and MRC/MRT,are adopted at the relay node.The de-terministic sum rates with two precodings are derived when the number of relay antennas increases to infinity.According to the deterministic rate,the asymptotic performance of four typical power scaling cases has been derived.Finally,the simulation results verify the correctness of the asymptotic performance,and the influence of the PS coefficient,position coefficient and the number of user antennas on the rate has been investigated.It is great significant to apply the theory to the practical application of the massive MIMO relay system.Thirdly,for the problem of the energy scarcity of user nodes or destination nodes in the wireless relay system,the transmission method and the optimization algorithms of MIMO two-way relay system based on user-SWIPT(U-SWIPT)technique are proposed in the unmannedaerial-vehicle(UAV)scenario.In the UAV-enabled two-way relay communication scenario,SWIPT technology is applied to the user nodes.The suitable transmission method is proposed and the precoding matrixes of the users and UAV relay and the PS factors of two users are jointly optimized to maximize residual energy of two users while satisfying the minimum signal-to-noise ratio(SNR)of users.To solve the optimization problem,two optimization algorithms are proposed,i.e.,iterative optimization algorithm and low-complexity generalized singular value decomposition(GSVD)algorithm.The two proposed algorithms have great performance and match the MIMO channel suitably to achieve the goal of maximizing the residual energy of users.It is worth noting that the low-complexity GSVD algorithm makes an excellent tradeoff between complexity and performance.The complexity of low-complexity GSVD algorithm is much smaller than the iterative optimization algorithm,but the performance is similar to that of the iterative optimization.Lastly,aiming at the problems of the transmission method of the multi-user relay system and the interference suppression,a transmission method of SWIPT-based multi-user MIMO relay system and an optimization algorithm are proposed.In the multi-user relay scenario,time switching(TS)mode of SWIPT is adopted at the relay node.The relay node uses the harvested energy in the first phase to forward the processed signal which is received in the second phase to multiple user nodes.To maximize the sum rate,the precodings of the source node and the relay node as well as TS factor are jointly optimized.Since the precoding of the energy transmission is not directly related to the objective function of the optimization problem,the optimal energy transmission precoding of source node is designed in advance and independently.Then,the precoding structure based on SVD-ZF technique,which converts the original complicated matrix optimization problem into a simple power allocation optimization problem,is proposed.Finally,the iterative optimization algorithm is employed to obtain the optimal information transmission precoding martixes of source node and relay node as well as TS factor.It is worth mentioning that although the iterative optimization algorithm is used to solve the power allocation of the source node and the relay node step by step,the power allocation optimization problem of the relay node is still a non-convex problem.The difference of convex(DC)algorithm is used to solve the above problem.The non-convex problem of relay node power allocation optimization is transformed into a solvable convex problem.In summary,this thesis applies the SWIPT technique and MIMO technique to the wireless relay communication system.By optimizing the resource allocation of the relay system nodes,the efficient transmissions of the energy and information in the relay system based on SWIPT and MIMO techniques are obtained.Thus,it leads to the improvement of spectrum efficiency and the growth of network service life and the green communication can be realized in a true sense.