Research On Optimization For MIMO Multiuser Cooperative Ambient Backscatter Communication Systems

Internet of Things(IoT)is one of the most important application scenarios of 5G and future wireless communication systems.However,the existing IoT communication technologies adopt active radio-frequency(RF)units,which induce high transmission power consumption and circuit power consumption.Hence,low-power and high efficiency transmissions cannot be realized.In addition to the existing IoT technologies,ambient backscatter communication(AmBC)technology directly uses existing RF signals in the environment as the communication carriers.In AmBC,backscatter devices transmit the information passively by modulating the bits into the amplitude and/or phase of the reflected signal,which can be realized by switching the antenna impedance.By doing so,the backscatter devices do not require the high-power and high-cost active RF units,and they need neither dedicated RF signal sources nor additional spectrum.Therefore,AmBC is with extremely high energy efficiency and high spectral efficiency.However,AmBC still faces a few problems,including unstable RF signal sources and the difficult scheduling of devices.Cooperative ambient backscatter communication(CABC)solve the problems by combing AmBC with traditional wireless communication systems in a cooperative and full-duplex manner,achieving stable,reliable,high-speed,and energy-efficient transmissions.In this thesis,we investigate a multi-input multiple-output(MIMO)multi-user CABC system and propose several optimization algorithms for different types of CABC systems,which significantly improve the performance of backscatter transmission.Firstly,this thesis studies a space division multiplexing(SDM)based MIMO multiuser cooperatively-receiving ambient backscatter communication(CRABC)system,that is,the cooperative receiver can receive signal of the RF signal source and all of the backscatter devices simultaneously.The sum-rate maximization problem of the backscatter links is formulated,in which the beamforming(i.e.,precodin)matrix is optimized under the minimum rate requirement of the direct link and the minimum energy requirement of the backscatter devices.Since the formulated problem is non-convex,we apply sequential parametric convex approximation(SPCA)to the problem and design an efficient iterative algorithm.The simulation results show that the beamforming design can significantly improve the sum-rate of the backscatter link.Secondly,we study a time division multiplexing(TDM)based MIMO multi-user CRABC system.In each time slot,the cooperative receiver simultaneously receives signals from both the conventional RF source and a single backscatter device.To maximize the sum-rate of the backscatter links,it is necessary to jointly optimize the beamforming matrix,time allocation,and backscatter devices' power reflection coefficients under the constraints of the minimum rate requirement of the direct link and the minimum energy requirements of all backscattering devices.The problem has multiple coupled optimziation variables and is non-convex.This thesis designs a block coordinated decent(BCD)based iterative algorithm to solve the problem.This thesis also compares the performance of SDM and TDM based CRABC under the same conditions.Simulation results show that the system has better rate performance in TDM mode when the numbers of transmitting/receiving antennas and that of backscattering devices are small.When the antenna array of the MIMO is large,the SDM based CRABC system is more advantageous.Finally,a MIMO multi-user full-duplex cooperative ambient backscatter communication(FCABC)system is investigated in this thesis.A full-duplex access point receives the signal reflected from the backscatter devices while transmitting the RF signal to a legacy user and the backscatter devices.Then,under the constraints of residual selfinterference strength,rate requirement of the legacy user,and harvested energy of the backscatter devices,the precoding matrix and the backscatter device reflection coefficient matrix are jointly optimized to maximize the sum-rate of the full-duplex backscatter link.After that,an efficient iterative algorithm is designed to solve the problem.Simulation results show that the joint optimization scheme can significantly improve the performance of backscatter transmissions.We also evaluate the effects of residual self-interference on the rate performance in the simulation results.