AoD-based Statistical Beamforming for Cell-Free Massive MIMO Systems

Abstract

Cell-free massive MIMO system is one of a promising technology of 5G wireless communications that can provide high throughput from the basestation cooperation. To capitalize on the gain obtained by the basestation cooperation, the downlink channel state information (CSI) should be available at the basestations. In the popularly used frequency division duplexing (FDD) system, the downlink CSI must be fed back from the users. However, due to a large number of antennas and basestations, the feedback overhead is a serious concern in the cell-free systems. Recent studies have shown that the uplink and downlink channels have similar angle-of-departures (AoDs), so-called angle reciprocity. In this paper, we present an AoD-based statistical beamforming scheme for the cell-free massive MIMO systems that does not rely on the CSI feedback. Also, we provide an efficient solution for the power allocation problem that minimizes the total power consumption of the basestations. Simulation results demonstrate that the proposed scheme saves approximately 12% transmit power and has a 22% higher coverage probability compare to the conventional cellular systems.