Stability-Oriented STAR-RIS Aided MISO-NOMA Communication Systems

Abstract

Simultaneously transmitting and reflecting reconfigurable intelligent surfaces (STAR-RISs) have received great attention due to the capability of achieving full-space coverage. In this paper, the queue-stable STAR-RIS assisted non-orthogonal multiple access (NOMA) communication system is investigated to ensure the stability of queueing systems. To tackle the challenge of infinite time slots required for stability, the long-term stability-oriented problem is reformulated to maximize the per-slot queue-weighted sum rate (QWSR) of users. In particular, the rate weight allocated for each user is determined by the length of a data queue, which is maintained at the base station (BS) and pending to be delivered to each user. Then, the QWSR is maximized by jointly optimizing the NOMA decoding order, the active beamforming coefficients (ABCs) at the BS, and the passive transmission and reflection coefficients (PTRCs) at the STAR-RIS. To handle the highly-coupled and non-convex problem, the blocked coordinate descent and the successive convex approximation methods are invoked to iteratively and alternatively optimize the problem. Simulation results revel that: i) our proposed STAR-RIS assisted NOMA communication achieves better performance than the conventional schemes; ii) the reformulated per-slot QWSR maximization problem is proven to ensure the system stability.