Minimizing Energy Consumption in Cell-free Massive MIMO Networks

We consider a cell-free massive multiple-input multiple-output (MIMO) communication system that consists of distributed multi-antenna access points (APs) connected to a central processing unit (CPU), and APs coherently serve a set of single-antenna user equipments (UEs) in the same frequencytime resource in downlink (DL) communications. Serving all UEs by all APs leads to inefficient use of spectral resources as well as energy under low traffic load. This work focuses on minimizing the energy consumed to transmit a bit in the cellfree massive MIMO system for given spectral efficiency (SE) requirements. First, we propose an algorithm to achieve the max-min fairness in DL communications while minimizing the total power consumption of the cell-free system. Secondly, we present an algorithm to reduce the total power consumption for a given minimum target SE of the system by temporarily switching off a set of APs that is not contributing to overall system performance. Finally, we suggest a technique to reduce the average number of state transitions of APs in a system with dynamic UEs, over a period of time. It minimizes the average transition power consumption over a period where largescale fading values change multiple times. The numerical results show that the proposed algorithms reduce the overall energy consumption of the system compared to the situation where all APs are active.