Downlink Power Control in Cell-free Massive MIMO with Partially Distributed Access Points

Abstract

Cell-free massive multiple-input multiple-output (MIMO) is a promising cellular technology in which a large number of distributed access points (APs) jointly serve a small number of user equipments (UEs). In this work, we investigate the impact of the spatial distribution of APs on the performance of a cell-free massive MIMO system, considering different downlink power control policies. Further, we analyze the performance for the case where only subsets of all APs serve the individual UEs: this scenario has lower backhaul requirements and associated CAPEX/OPEX costs. In this framework, we first propose a novel, tractable approximation for the average spectral efficiency (SE) of the transmission to a UE conditioned on the estimated channel gains. This approximation is then used to develop different downlink power control policies. Further, we extend the policies to the scenario when power control is coordinated only among subsets of APs and not across the subsets. Through extensive system-level simulations, we evaluate the improvement in SE by spreading out the APs and the SE loss when a subset of all APs serve a UE and coordination across subsets is absent.