Abstract

Cell-Free massive MIMO system performance can be improved with cooperative beamforming strategies. Typically, local beamforming strategies are assumed at base stations (BSs) to avoid extensive channel state information exchange via backhaul links. A fully distributed framework relying on a novel over-the-air (OTA) signaling mechanism has been recently proposed to design cooperative beamformers for cell-free massive MIMO systems. However, the existing distributed exact solution has a suboptimal global convergence behaviour because of outdated information used at each BS. This paper proposes a Newton method with an adaptive regularization to design uplink receivers for the fully distributed framework. Numerical results show a significantly faster convergence of the proposed method compared to the distributed exact solution and gradient methods.