Transmission Power Optimization for Hybrid NOMA-URLLC System With STAR-RIS Assistance

The simultaneous transmitting and reflecting reconfigurable intelligent surface (STAR-RIS) has recently emerged as a cutting-edge technology for future wireless networks. In this paper, we explore an uplink STAR-RIS-enabled ultra-reliable and low-latency communications (URLLC) system utilizing hybrid non-orthogonal multiple access (NOMA). To enhance system performance, we pair one transmitting user with one reflecting user as a NOMA pair. Using a time division multiple access (TDMA) protocol among different pairs, our goal is to minimize the overall transmission power while meeting the individual reliability requirements of each user. This is achieved through joint optimization of block length and transmit power of users alongside beamforming design at the STAR-RIS. This non-convex optimization problem is then effectively approximated by estimating the channel dispersion in both the high signal-to-interference-plus-noise ratio (SINR) regime and across the entire practical SINR range. Then, we address this power minimization problem using an alternating optimization approach. Simulation results demonstrate the advantages of the proposed framework over conventional approaches, such as conventional RIS-based and OMA-based systems. Moreover, beamforming design, block-length optimization, and the rising of the element number of STAR-RIS significantly impact power reduction performance.