Hardening the Channels by Precoder Design in Massive MIMO with Multiple-Antenna Users

In this paper, we investigate the potential of a novel precoder for a massive multiple-input multiple-output (MIMO) system with multiple-antenna users under the influence of both Rayleigh and keyhole channel models. The proposed precoder is based on maximum-ratio processing and aims to enhance the channel hardening effect at the users. This is achieved through dividing each channel vector by its norm square. Closed-form expressions for the achievable spectral efficiency of the conventional and proposed precoders are derived. If modeled with Rayleigh fading, due to the physical hardening property of this channel, a moderate gain in performance is produced by the proposed precoding method, where the highest spectral efficiency gain occurs for a small number of users and user antennas. Under Rayleigh fading, the results also demonstrate that similar performance is returned by increasing the number of users or user antennas. By comparison, the proposed precoder is shown to produce significant performance gains in the keyhole channels under the condition that the system is limited to single-antenna users. Corresponding to this, the results showcase how the performance of massive MIMO under keyhole channels is heavily dependent on increasing the number of served users over user antennas.