Hybrid Active-Passive STAR-RIS-Based NOMA Systems: Energy/Rate-Reliability Trade-Offs and Rate Adaptation

This letter proposes efficient analytical frameworks to evaluate energy/rate-reliability trade-offs and rate adaptation of hybrid active-passive simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS)-based non-orthogonal multiple access systems. Under Nakagami-m fading channels and imperfect successive interference cancellation, closed-form approximations for the outage probability and energy efficiency are derived. Based on the asymptotic expressions at high signal-to-noise-ratio and low-rate regions, diversity orders and coding gain are provided, along with trade-offs between energy/rate and reliability. Besides, we also develop rate and energy-reliability expressions and closed-form solutions for allocating power resources to achieve user rate fairness and sum-rate maximization. Monte Carlo simulations corroborate the proposed frameworks, identify operating points for prioritizing passive, active, or hybrid STAR-RISs from energy perspectives, and clarify how to configure STAR-RIS elements to improve energy/rate-reliability and power allocation policy.