Spectral efficiency enhancement in multiuser NOMA over Nakagami-m fading channels through power allocation and pairing strategies under hardware and SIC imperfections

Non-orthogonal multiple access (NOMA) is a key enabler for future sixth-generation (6G) and massive machine-type communication networks, offering superior sum spectral efficiency (SSE) and user fairness compared to traditional orthogonal multiple access (OMA) schemes. However, practical deployments face challenges due to hardware impairments (HI) and imperfect successive interference cancellation (iSIC), which cause signal distortion and residual interference, degrading system reliability. Most existing works overlook these impairments or assume ideal fading scenarios, thereby limiting their real-world applicability. This work addresses these issues by developing a comprehensive outage probability analysis for both downlink (DL) and uplink (UL)-NOMA over Nakagami-m fading channels, a versatile model for realistic fading conditions. Closed-form expressions are derived using Gamma-function-based modeling, and a low-complexity optimization of power allocation (PA) and user pairing is proposed to enhance system performance under impairments. The results demonstrate that employing a strong-weak (SW) user pairing strategy, along with optimally tuned PA factors, yields a notable improvement in SSE [Formula: see text] in DL and [Formula: see text] in UL. The accuracy of the proposed analytical models are substantiated through a strong agreement with simulation outcomes. The integration of user pairing with optimized PA not only enhances spectral efficiency (SE) but also facilitates efficient resource utilization while maintaining low computational complexity. This comprehensive approach positions the proposed system as a viable solution for meeting the performance and scalability requirements of next-generation wireless networks.