On Rate Fairness Maximization for the Downlink NOMA with Improper Signaling and Imperfect SIC

Non-orthogonal multiple access (NOMA) is a key enabler for 6G networks due to its efficient spectrum utilization, which is garnering significant attention among the Internet of Things (IoT) community. This paper investigates the benefits of the improper Gaussian signaling (IGS) technique on the max–min fairness of the downlink NOMA system under imperfect successive interference cancellation (SIC), where both of the users have the potential to adopt IGS. We first investigate fairness optimization under perfect SIC. In this case, the max–min optimization is solved by the alternate optimization algorithm, where the impropriety degree and power level are iteratively optimized. The closed-form solution for conventional proper Gaussian signaling is also obtained. Then, a deep Q network-based solution is considered for the rate fairness maximization of the downlink NOMA system under IGS and imperfect SIC. The simulations presented for the IGS-aided NOMA system support the analysis, illustrating that IGS can efficiently improve the fairness achievable rate compared to the conventional proper one.