Research On Joint Hybrid And Passive Beamforming Technology For Millimeter-wave Multi-antenna Backscatter Communication Systems

With the rapid development of global 5G communications,massive machine-type communications(m MTC)is widely used in fields such as industry and agriculture,smart medical care,and smart cities.However,the launch of the millimeter wave(mm Wave)frequency band has greatly increased laying cost and energy consumption of the m MTC network.Backscatter communication,which is an emerging technology for the green Internet of Things(Io Ts),can effectively solve the above problems,for its characteristics of low cost and low power consumption.At the same time,the ultra-large spectrum bandwidth of mm Wave can significantly increase the communication rate of backscatter communication.This thesis proposes a multi-antenna backscatter and mm Wave fusion communication system.The backscatter device(BD)can harvest energy from the incident signal and transmit the information to the receiver by backscatter modulation.This thesis proposes a joint beamforming algorithm for hybrid beamforming(HBF)at the RF source(RS)and passive beamforming(PBF)at the BD to maximize the communication rate of the backscatter link.For the monostatic backscatter mm Wave communication,an optimization problem of joint beamforming constrained by the energy harvesting of BD is investigated.This thesis assumes that the HBF design can ignore the energy harvesting constraint,since the process is operated at the BD.Then,this thesis proposes a joint beamforming scheme: the classic HBF algorithm is adopted at RS,and the PBF design under the energy harvesting constraint at BD is solved by utilizing the semi-positive definite relaxation(SDR)algorithm.For the symbiotic radio(SR)mm Wave communication,this thesis establishes an optimization problem based on the detection algorithm of successive interference cancellation(SIC)to maximize the communication rate of the backscatter link,while ensuring the normal communication between the primary receiver(PR)and the primary transmitter(PT).A joint beamforming algorithm based on alternate optimization is proposed in this thesis.With given PBF matrix,a modified orthogonal-matching-pursuit(MOMP)algorithm is used for HBF; when HBF matrix is fixed,the optimization problem of PBF can be decomposed into multiple sub-optimization questions and the corresponding algorithm is proposed.Subsequently,based on the alternate optimization algorithm,this thesis proposes two low-complexity algorithm,which reduce the algorithm complexity within an acceptable performance loss.Finally,numerical simulations verified the performance advantages of the joint beamforming design.The communication rate of the BD can be significantly improved for different signal-to-noise ratio(SNR)and antenna number.The more the number of antennas,the more obvious the performance improvement.When the number of quantization bits of the reflection coefficient is 2,it can almost achieve the performance of continuous value.In addition,the two low-complexity algorithms proposed in this thesis have a small performance loss compared to the joint optimization algorithm based on the MOMP algorithm at the condition of small antennas number and low SNR,showing its potential to practical situation.