Beamformer Design and Power Allocation for Two-Cluster Two-User NOMA System

Non-orthogonal multiple-access (NOMA) systems are envisioned as suitable solutions for both next generation cellular and local area networks, in order to support differentiated traffic requirements and a massive number of users. In this paper we consider a scenario with a base station (BS) equipped with multiple antennas serving four users, clustered into two groups of two users each, equipped with a single antenna. This simple but relevant scenario allows to capture the main features of NOMA systems, where in each cluster the user having the best channel applies successive interference cancellation (SIC) to remove the signal intended for the other user in the same cluster, while the other user treats interference as noise. Moreover interference among the two clusters also is treated as additional noise. In this scenario we address the problem of beamformer design and power allocation for the four users with the aim of minimizing the total transmit power while satisfying a minimum rate constraint for each user. Starting from the zero-forcing (ZF) and maximal ratio combining (MRC) beamformers we propose five solutions based on suitable combinations of fixed beamformers to simplify the beamformer design and power allocation. The performance of the proposed solutions is compared in terms of total consumed power and complexity of the various solutions.