Beamforming for NOMA Under Similar Channel Conditions Including Near-Field Formulation

Non-Orthogonal Multiple Access (NOMA) has emerged as a promising strategy for increasing spectral efficiency in wireless multiuser communications. The performance of NOMA highly depends on the users’ channel conditions, being enhanced when these are significantly different, i.e., one strong user and one weak user. On the contrary, rate fairness is compromised when users present similar channel conditions since the maximization of the sum capacity may result in the allocation of most resources to one user. In general, this problem can be avoided with correct user grouping. However, there are scenarios, e.g., picocells or femtocells, where finding users with distinctive channel conditions is not always possible. In this paper, we study the application of beamforming in a two-user Multiple-Input-Single-Output (MISO) downlink NOMA system to achieve rate fairness through channel shaping even when the conditions of both users are similar. The proposed formulation includes the effect of Near-Field (NF) radiation, which yields an accurate modeling of the problem at hand in a small-cell scenario. We use an optimization algorithm to jointly calculate both the power allocation and the complex weights to be applied to the elements of a given array. Numerical simulations across scenarios with distinctive or similar channel conditions show that the proposed approach outperforms alternative NOMA methods in ensuring rate fairness. Additionally, it yields improved overall rates compared to Far-Field (FF) formulation-based modelling and beamforming Time Division Multiple Access (TDMA) solutions.