A Low-Complexity Design for STAR-RIS Aided Multi-Antenna NOMA Systems

In this paper, we propose a low-complexity mode switching design in a simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS) aided multi-antenna non-orthogonal multiple access (NOMA) system via exploiting channel angle information only. Particularly, a location-based assignment algorithm is firstly provided to perform NOMA pairing, then the equal gain transmission at base station and cophase matching criterion at STAR-RIS are adopted to improve the performance of reflected user and the effective channel gain of paired users, respectively. On the basis of to the above design, we study the exact channel statistics in two different cases to further derive the outage probability of reflected and transmitted users. Numerical results are presented to demonstrate our analyses and reveal that: 1) the various system factors have markedly impacts on our proposed design; and 2) the low-complexity design can reduce the overheads of channel estimation and signal processing at the expense of extremely low performance loss.