Research On Hybrid Beamforming Algorithms For Multicast Transmission In Millimeter Wave Systems

Millimeter wave(mmWave)communication potentially gives a chance to address the challenge of bandwidth shortage.However,large-scale antenna arrays should be leveraged at the base station(BS)to provide significant beamforming gains.In fully digital beamforming schemes,each antenna requires one radio frequency(RF)chain.The prohibitive cost and power consumption make fully digital beamforming impractical in mmWave systems.To tackle this difficulty,Hybrid analog and digital beamforming has drawn significant attention in mmWave systems.Most of the existing hybrid beamforming structures can be categorized into fully-connected structure and patially-connected structure.In the fully-connected structure,each RF chain is connected to all the antennas,and hence full beamforming gain can be achieved.The latter can reduce the hardware complexity significantly,because each RF chain is connected to part of the antennas.Recently,the combination of multicasting and hybrid beamforming also attracts much attention.However,the existing works on hybrid beamforming for multicast transmission utilize the semi-definite relaxation(SDR)based method,which is complex;meanwhile,these works assume that the perfect channel state information(CSI)is available at the transmitter.However,in practical systems,perfect knowledge of CSI may not be available due to many factors.This paper studies low-complexity multi-group multicast hybrid beamforming and the robust design of multi-group multicast hybrid beamforming.In these two designs,we choose the low-complexity partially-connected structure.In the first part of our work,we formulate a quality-of-service(QoS)problem with the objective of minimizing the total transmit power at the BS,subject to an individual signal-to-interference-plus-noise ratio(SINR)constraint for each multicast group.We first adopt alternating minimization method to design the analog and digital beamformer alternatively.Then we solve each of the analog and digital subproblems through solving a sequence of convex problems via concave-convex procedure(CCP).Each convex CCP subproblem is solved by alternating direction method of multipliers(ADMM).In the second part of our work,we consider a multiplicative phase error model,which is more suitable for mmWave channel.We formulate a QoS problem,which aims to minimize the total transmit power while satisfying the outage constraint for each user's SINR.We also utilize the alternating minimization method to optimize these two different beamformers.Each of these two subproblems is then solved approximated by using the second-order Taylor expansion together with the Bernstein-type inequality.Simulations show that our low-complexity algorithm can achieve the same performance with very low complexity compared with the SDR-based algorithm,and the proposed robust design can satisfy the outage constraint well.