Aerial STAR-RIS-Based Symbiotic Systems With Semi-NOMA Transmission: Performance Analysis and Optimization

This work proposes a novel semi-nonorthogonal multiple access (NOMA) and data transmission technique, called Semi-NOMA, to enhance the spectrum utilization of aerial simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS)-aided symbiotic networks without using successive interference cancellation approaches as classical NOMA. In particular, the proposed scheme is investigated with active and passive STAR-RIS models combined with infinite blocklength (IBL) and finite blocklength (FBL) regimes under discrete phase-shift alignments. For IBL scenarios, the ergodic capacity and outage probability are derived under both approximation and asymptotic frameworks. Besides, a joint optimization problem of the power allocation factor and energy splitting coefficient is also formulated to maximize the ergodic sum capacity (ESC), where closed-form solutions are derived for both active and passive STAR-RIS models. For FBL scenarios, not only the approximation and asymptotic frameworks are derived for the average achievable rate and block-error rate but also an approximated convex form is derived for a nonconvexity optimization problem of min–max blocklength. Numerical results corroborate the efficacy of the proposed Semi-NOMA over the baseline schemes, the developed mathematical frameworks, and the solutions of the ESC maximization and min–max blocklength.