STAR-RIS Aided Multi-Antenna NOMA Downlink and Uplink Transmissions: A Low-Complexity Approach

The key idea of this paper is to leverage channel angle information to propose a low-complexity mode switching design in simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS) aided multi-antenna non-orthogonal multiple access (NOMA) systems, which is suitable to both downlink and uplink transmissions by applying the concept of angle reciprocity. Particularly, a location-based assignment algorithm is firstly provided to perform NOMA pairing, then the equal gain transmission at the base station and the cophase matching criterion at STAR-RIS are adopted to improve the performance of the reflected user and the effective channel gain of the paired users, respectively. According to the proposed design, considering the difference in the Rician factors of the channel from the base station to STAR-RIS caused by the change of line-of-sight and non-line-of-sight components, two different cases for channel statistics of the paired users are rigorously characterized. Subsequently, we utilize the above results to analyze the closed-form expressions for the outage probability and ergodic rate of all users under downlink and uplink transmissions. Further, the diversity orders and the high slopes corresponding to all expressions are also determined to pursue more insights. Finally, numerical results are presented to demonstrate our analysis and reveal that: 1) the various system factors have significant impact on the performances of the proposed design; and 2) the low-complexity design can reduce the overheads of channel estimation and signal processing at the expense of extremely-low or even no performance loss in contrast to the traditional maximum ratio transmission based design.