Effective Rate of Downlink NOMA over k-μ Shadowed Fading with Integer Fading Parameters

Non-orthogonal multiple access (NOMA) is being considered as a strong candidate for the multiple access technique for beyond fifth-generation (B5G) communication standards. Most of the existing research related to the performance analysis of NOMA is based on the Shannon's capacity theory which, although very useful, fails to determine the delay-limited performance of the system. In this paper, we analyze the performance of a two-user downlink NOMA system over generalized κ-μ shadowed fading in terms of effective rate (ER), which evaluates the system performance under a delay-limited quality-of-service (QoS) constraint. We derive exact closed-form expressions and also the corresponding low-SNR and high-SNR approximations for the sum ER of the downlink NOMA system. Furthermore, using Jensen's inequality, we derive an upper-bound on the sum ER which represents the average achievable sum-rate for the downlink NOMA system. A corresponding time-division-multiplexed orthogonal multiple access (OMA) system is also analyzed for the purpose of comparison. Our results show that while NOMA consistently outperforms OMA over the practical SNR range, the relative gain becomes smaller in more severe fading/shadowing conditions, and is also smaller in the presence of a more strict delay QoS constraint.