Regularized phase alignment precoding for the MISO downlink

With the knowledge of both channel and data information at the base station prior to downlink transmission, we can increase the received signal-to-noise ratio (SNR) of each user without the need to increase the transmitted power. Achievability is based on the idea of phase alignment (PA) precoding where instead of removing the destructive interference, it judiciously rotates the phases of the transmitted symbols. In this way, for each user, the received interference from the other users add up coherently, and consequently we can glean higher received SNRs at all mobile terminals. In addition, it is well-known that the regularized channel inversion (RCI) improves the performance of channel inversion (CI). In line with this and similar to the RCI precoding, in this paper we propose the idea of regularized PA (RPA) which is shown to improve the performance of original PA precoding. To do so, we first rectify the original PA precoding by deriving a closed-form expression of its scaling factor. We then use this new analysis to select an appropriate regularization factor for the proposed RPA scheme. Finally, we drive an explicit formula regarding its received SNR. Since the focus of this work is on linear precoders, we show that the proposed RPA precoding outperforms CI, RCI, and PA precoders from both symbol-error rate (SER) and sum rate perspectives. We also consider the performance of RPA under imperfect channel state information at transmit side. We show that even in this case, RPA precoding is as sensitive as other linear precoders to channel imperfections.