Partial Multiplexing Power-Frequency Multiple Access for Active STAR-RIS Systems: Outage and Ergodic Perspectives

This letter proposes a novel partial multiplexing power-frequency multiple access (PFMA) strategy to enhance the performance of downlink active simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS)-based systems. Unlike non-orthogonal multiple-access (NOMA), the proposed PFMA simultaneously serves two users without requiring successive interference cancelation (SIC) implementation. Closed-form expressions for the outage probability (OP) under Nakagami-m fading mediums are derived, along with the upper and lower boundaries at high signal-to-noise ratios. A joint optimization problem of power and bandwidth resources has been formulated and solved to maximize system throughput. This letter also provides insight into the energy-spectral efficiency trade-offs and multiplexing gain of the proposed PFMA by deriving an expression for the ergodic capacity (EC) and the upper and lower EC boundaries. The numerical results validate the advantages of the proposed PFMA over traditional orthogonal multiple-access, NOMA, and partial NOMA counterparts.