Transmission Design and Component Allocation for STAR-RIS Assisted NOMA Systems with Direct Link

This paper investigates a simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS) assisted downlink non-orthogonal multiple access (NOMA) system, consisting of a base station (BS), a STAR-RIS, a transmission user and a reflection user, where STAR-RIS uses mode switching protocols. Under the proposed transmission model, the base station serves the two users via the STAR-RIS and a direct link exists from the base station to the reflection user. By using the Beaulieu series, the statistical property of the Nakagami-m fading cascaded channel is characterized. Based on this, closed-from expressions of outage probability and diversity order for the reflection user and transmission user are obtained, respectively. In addition, to ensure the maximum gain in system throughput, a component allocation scheme is proposed, and the optimal allocation interval between the transmission and reflection components is obtained. Numerical results verify the theoretical analysis and demonstrate the superiority of the proposed scheme in terms of outage probability and system throughput compared to existing schemes.