Performance Analysis of Cooperative NOMA Networks over Compound Fading Channels

The emergence of non-orthogonal multiple access (NOMA) technology has redefined traditional resource allocation in wireless networks with its capability to superimpose multiple user signals over the same transmission resources. In this respect, this paper studies the performance of cooperative relaying NOMA systems over a compound fading channel, namely, the α−Lomax fading model. Specifically, we consider a network with one base-station communicating with one far-user and one near-user with the latter acting also as a relay based on the decode-and-forward protocol. The channels associated with the two users are assumed to be statistically independent. Novel closed-form analytical expressions for the probability density function and cumulative distribution function of the minimum of two α-Lomax random variables were derived. These expressions were then used to derive an accurate ergodic sum rate (ESR) analytical expression for the cooperative NOMA system and its corresponding upper bound expression. To highlight the achievable performance gains, results for the cooperative orthogonal multiple access (OMA) system were also included. The derived expressions enabled us to examine the impact of various system parameters, including the power allocation factors, fading parameters and transmit power, on the system performance. Monte Carlo simulations were included to verify the correctness and accuracy of our analysis. The findings revealed that to achieve maximum ESR, the power allocation factors should be carefully optimized. It was also shown that the optimized cooperative NOMA system always outperforms the cooperative OMA approach in all considered scenarios. Furthermore, the results clearly indicated that increasing α and/or λ will enhance the system performance for both the cooperative NOMA and OMA schemes.