  In this work, we propose 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. We consider
the downlink channel with single-antenna users and multiple-antenna APs. We
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 (CB) and zero-forcing (ZF) schemes and that performance remains
excellent with APS, in the presence of perfect and imperfect channel state
information (CSI).
