Optimal Transmission Design For RIS-Assisted Uplink Multiple Access Systems

In recent years,NOMA technology and RSMA technology have received increasing attention.As the number of users increases and user requirements become higher,the communication environment of the system becomes more complex.RIS technology has been widely applied to various communication systems such as NOMA and RSMA to adapt to complex communication environments,as RIS can change the amplitude and phase of reflected electromagnetic waves to expand the coverage area.This paper proposes an alternate optimization scheme for power allocation factors,phase shifts at the RIS,and beamforming at the base station to maximize user achievable rates in RIS-assisted multiple access systems,effectively improving the achievable rates of users in the system.The main research work of this thesis is summarized as follows:Firstly,for the RIS-assisted NOMA uplink communication system,a scheme based on RIS assistance is proposed to maximize the user total rate,considering multi-user communication with the multi-antenna base station via the direct link and the reflection channel generated by RIS.In this scheme,by jointly optimizing the beamforming at the base station and phase shifts at the RIS,the total rate of the communication system is maximized in a multi-user scenario.To solve the non-convex problem of maximizing the total rate of multi-users,an alternating optimization approach is used based on CPS and MRC methods(CPS-MRC)under the assumption of perfect CSI.Then,an optimization scheme based on the LMA-SLN is proposed to address the joint beamforming and phase shift optimization problem using the powerful computational capabilities of neural networks when the channel state information is unknown.From the analysis of simulation results,it can be concluded that the proposed LMA-SLN optimization scheme can achieve convergence in fewer training iterations,approaching the results obtained under perfect CSI,and effectively solve the problem of maximizing the user’s total rate through beamforming and phase shift optimization.Secondly,an RIS-assisted RSMA system with two users communicating with a multi-antenna base station via direct link and RIS-assisted reflected link was considered,and a RIS-assisted RSMA system single-user rate maximization scheme was proposed.In this scheme,an alternating optimization scheme based on convex optimization was designed to optimize the base station’s receiving beamforming,RIS’s phase shift,and transmit power allocation.The achievable rate in the RIS-RSMA uplink system was maximized while satisfying the QoS constraints of one of the users.The non-convex rate maximization problem in the scheme was decoupled into three sub-problems.First,an alternating optimization algorithm based on CPS-MRC was used to alternate the optimization of the base station’s receiving beamforming and the RIS’s phase shift for initialization so that the subsequent optimization algorithm could further iterate.Then,the optimization expression of the power allocation factor was derived to solve one of the sub-problems,and the convex problem of beamforming with multiple-ratio concave-convex fractional programming was transformed into a convex problem using quadratic transform.The rank-1 constraint in the beamforming problem was handled by combining a penalty term method,and the other two sub-problems were solved using convex optimization methods.Finally,an alternating optimization scheme based on convex optimization was designed to solve the joint beamforming,phase shift optimization,and power allocation problem.Experimental results show that the proposed scheme can effectively improve the achievable rate of the considered RIS-assisted RSMA uplink communication system and is significantly better than the baseline scheme based on CPS-MRC.