Iterative AP selection, MMSE precoding and power allocation in cell‐free massive MIMO systems

In this work, the authors proposed the iterative access point (AP) selection (APS), linear minimum mean‐square error (MMSE) precoding and power allocation techniques for cell‐free massive multiple‐input multiple‐output (MIMO) systems. They considered the downlink channel with single‐antenna users and multiple‐antenna APs. They derive sum‐rate expressions for the proposed iterative APS techniques followed by MMSE precoding and optimal, adaptive, and uniform power allocation schemes. Simulations show that the proposed approach outperforms existing conjugate beamforming and zero‐forcing schemes and that performance remains excellent with APS, in the presence of perfect and imperfect channel state information.