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In future B5G/6G broadband communication systems, non-linear signaldistortion caused by the impairment of transmit power amplifier (PA) canseverely degrade the communication performance, especially when uplink usersshare the wireless medium using non-orthogonal multiple access (NOMA) schemes.This is because the successive interference cancellation (SIC) decodingtechnique, used in NOMA, is incapable of eliminating the interference caused byPA distortion. Consequently, each user's decoding process suffers from thecumulative distortion noise of all uplink users. In this paper, we establish anew and tractable PA distortion signal model based on real-world measurements,where the distortion noise power is a polynomial function of PA transmit powerdiverging from the oversimplified linear function commonly employed in existingstudies. Applying the proposed signal model, we characterize the capacity rateregion of multi-user uplink NOMA by optimizing the user transmit power. Ourfindings reveal a significant contraction in the capacity region of NOMA,attributable to polynomial distortion noise power. For practical engineeringapplications, we formulate a general weighted sum rate maximization (WSRMax)problem under individual user rate constraints. We further propose an efficientpower control algorithm to attain the optimal performance. Numerical resultsshow that the optimal power control policy under the proposed non-linear PAmodel achieves on average 13\% higher throughput compared to the policiesassuming an ideal linear PA model. Overall, our findings demonstrate theimportance of accurate PA distortion modeling to the performance of NOMA andprovide efficient optimal power control method accordingly.

Capacity Analysis and Throughput Maximization of NOMA with Nonlinear Power Amplifier Distortion