Research On Physical-Layer Cooperative Multicast Beamforming

The future wireless communication systems take the Internet of Everything as the development vision,and is committed to dealing with the explosive growth of mobile data traffic,the massive device connection and the continuous emergence of all kinds of new services in the future.Being able to simultaneously deliver the same data stream to mul-tiple users at the same time-frequency resources,the physical layer wireless multicasting technique has the remarkable ability in enhancing the spectrum efficiency and has been considered an effective transmission approach to meet the demands of massive connectiv-ity and high spectrum-energy efficiency service in the future communication systems.By using spatial multiplexing gain and spatial diversity gain brought by multiple antennas,multicast beamforming can greatly improve the system spectrum-energy efficiency by forming narrow beams pointing to different multicast groups through precoding design.In the multi-group multicast scenarios,due to the large difference in user channel quality,the multicast group-rate is often limited by the user with the worst channel quality in one multicast group,which severely restricts the further improvement of the spectrum-energy efficiency of the multicast systems.To tackle this issue,taking advantage of users with better channel quality and enabling them to act as cooperators to assist in transmission,can break through the bottleneck of limited group-rate.However,different user channel characteristics result in complex and diverse interferences such as strong interference and weak interference among multicast grou ps.How to effectively manage these complex interferences through designing an efficient cooperative multicast beamforming scheme,remains to be investigated.Besides,the cooperators with better channel quality need to sacrifice their own energy to assist in transmission,which is unfair and unreasonable.Al-though the energy harvesting technology can provide continuous energy supply for these cooperators,it is difficult to control the balance between the energy supply and energy demand of each cooperative user.Finally,in the case of overload networks,the rank-one feasible solution of the multicast beamforming scheme is hard to obtain.Therefore,this thesis focuses on the core issue of "how to design an efficient co-operative multicast beamforming scheme to suppress the complex interference between multicast groups,break through the bottleneck of limited group-rate,achieve significant improvement of the frequency and energy efficiency of multicast systems,and provide fair and high quality service for users".Non-orthogonal multiple access(NOMA)technology(interference canceling)and rate-splitting multiple access(RSMA)technology(interfer-ence splitting)are the key techniques,assisted by simultaneous wireless information and power transfer(SWIPT)and reconfigurable intelligent surface(RIS).This thesis main-ly study the key technologies of physical layer cooperative multicast beamforming.The specific research content and innovation are briefly described as follows:1)For the scenarios where there exist strong interferences between multicast groups,we propose a SWIPT-assisted half-duplex cooperative non-orthogonal multicast scheme and design a cooperative non-orthogonal multicast beamforming and power splitting method,by adopting the successive interference cancellation(SIC)technique.The energy-constrained problem of the cooperators is solved,the strong interferences between groups are effec-tively suppressed,and the average system transmission power is significantly reduced.In specific,to minimize the transmission power,we formulate an optimization problem to jointly design the transmiter beamforming vector,the cooperative and distributed beam-forming vector and power-splitting ratio.Then we propose a low-complexity iterative algorithm based on successive convex approximation and a novel initial feasible point search algorithm.Furthermore,we construct a robust cooperative multicast beamforming and power splitting design scheme under the condition of imperfect channel state infor-mation.The infinite constraint caused by the uncertainty of channel state information is transformed into a finite one by using the S-Procedure method.Then an iterative algo-rithm combining the penalty function method and the successive convex approximation method is proposed to obtain the rank-one local optimal solution.The simulation results show that the proposed cooperative multicast scheme can greatly reduce the average sys-tem transmission power within a certain range,attributed to the multi-cooperation gain,the multiplexing gain of power-domain NOMA,as well as the flexible power-splitting ratio control of cooperators.The convergence and robustness of the proposed iterative optimization algorithm have also been verified.2)For the scenarios where there exist complex interferences such as strong inter-ference and weak interference among multicast groups,we propose a more flexible and generalized full-duplex cooperative multicast scheme based on RSMA and design a co-operative rate-splitting and multicast beamforming method,by adopting the idea of com-bining successive interference cancellation and interference neglecting,i.e.,interference splitting.The complex interferences among groups are effectively suppressed,the time resources are made full utilization,and a substantial reduction in system transmission power is achieved.In specific,to minimize the transmission power,we formulate an opti-mization problem to jointly design the transmitter beamforming vector,the rate-splitting vector,the cooperative distributed beamforming vector and the power-splitting ratio.Then we propose a low-complexity iterative algorithm and a novel initial feasible point search algorithm.Furthermore,we construct a robust full-duplex cooperative rate-splitting and multicast beamforming design scheme.In order to obtain a rank-one approximate so-lution,we develop a robust algorithm combining the positive semidefinite programming relaxation technique and penalty function method.The simulation results show that the proposed scheme has obvious advantages in reducing the average system transmission power.And it outperforms the other baseline schemes under various user deployment,network loads and target rates.That is attributed to the comprehensive utilization of full-duplex cooperation gain,spatial multiplexing gain as well as power multiplexing gain.3)To further enhance the channel power gain and then improve the system perfor-mance by actively controlling the channel environment,we propose a RIS-assisted co-operative multi-group multicast scheme and design a cooperative rate-splitting and phase shift controlling as well as multi-group multicast beamforming method,by utilizing the passive reflection signal characteristics of RIS and the idea of interference splitting.The complex interferences among groups are effectively suppressed,the maximization of the minimum group-rate is achieved,and the fair and high-quality multiuser service is provid-ed.In specific,under the constraints of given transmit power threshold and unit-modulus of the RIS phase shift,we formulate a minimum group-rate maximization problem to design the transmitter beamforming vector,the rate-splitting vector and the RIS phase shifts matrix.Then an alternate iterative optimization algorithm is proposed.The beam-forming,rate-splitting optimization sub-problem and the phase shift matrix optimization sub-problem are solved alternately and iteratively.Simulations demonstrate that the pro-posed RIS-assisted cooperative multi-group multicast scheme can obtain significant per-formance gain in terms of the minimum group-rate.In summary,in order to solve the problems of complex and diverse intergroup inter-ferences,limited energy of cooperators,as well as the difficulty to obtain rank-one solu-tion,this thesis focuses on three aspects of half-duplex cooperative non-orthogonal mul-ticast beamforming,full-duplex cooperative rate-splitting and multicast beamforming,as well as RIS-assisted cooperative rate-splitting and multicast beamforming.The complex interferences among groups in the system is effectively suppressed,and the frequency-energy efficiency of the system is significantly improved.