Power Minimization for Cell-Free Massive MIMO

In this paper, we investigate the total power minimization problem for the downlink cell-free massive multiple-input multiple-output (mMIMO) system, subjecting to the users' quality-of-service (QoS) constraint. Specifically, the total power consists of circuit power, data transmission power, and back-haul power. Two precoding schemes are considered in this paper, namely conjugate beamforming (CB) and zero-forcing (ZF) method. It is proved that the power minimization problems for both precoders can be reformulated as the second-order-cone programming (SOCP) and hence, are easily determined. Simulation results indicate that, in the low QoS SE regime, the total power required by ZF precoder is larger than that of CB case. However, the superiority of CB precoder in power saving vanishes in both medium and high QoS SE regimes.