Single-User Versus Multi-User Precoding for Millimeter Wave MIMO Systems

Given the cost and complexity associated with the deployment of a large number of radio frequency (RF) chains in millimeter wave (mmW) multi-input multi-output (MIMO) systems, this paper addresses the question of network efficiency considerations for hybrid precoding. We first establish the relevance of directional precoding structures as a low-complexity and robust solution to meet the data rate demands of single-user MIMO systems relative to the more complex and less robust eigen-mode-based precoding structures. Key to the relevance of the directional precoding structures is the sparsity of the mmW channel coupled with higher antenna dimensionality affordable due to smaller wavelengths. We then leverage the directional structure of the channel to propose a simple class of directional schedulers that offers a low-complexity and yet approximately fair separation plane in the user space. We then compare the performance of single-user precoding schemes with multi-user precoding schemes and show that from network efficiency considerations, it would be more worthwhile to expend the RF chain resource on multi-user transmissions.