Abstract: A method to maximize energy efficiency is proposed for uplink non-orthogonal multiple access (NOMA) systems assisted by simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS). Firstly, the optimization problem of maximizing energy efficiency is constructed, with base station beamforming, STAR-RIS phase shift, users’ power and time allocation as variables. Then, the problem is simplified by deducing the relationship between beamforming and STAR-RIS phase shift as well as the minimum power required by the users. Finally, an iterative method is proposed to optimize STAR-RIS phase shift, power allocation and time allocation alternately. In each iteration, semi-definite programming method is used to optimize STAR-RIS phase shift, Dinkelbach algorithm to optimize power allocation, and function extremal method to optimize time allocation. The simulation results show that in the time switching protocol, time allocation has little effect on energy efficiency, while STAR-RIS phase shift has great effect on energy efficiency.

Key words: non-orthogonal multiple access ,  simultaneously transmitting and reflecting reconfigurable intelligent surface ,  energy efficiency ,  power ,   phase shift