Distributed Beamforming Design for Combined Downlink and Uplink Transmissions in Cell-Free Massive MIMO Systems

The authors propose distributed beamforming designs that jointly optimize the downlink precoders and combiners, as well as the uplink combiners and precoders, in a cell-free massive MIMO system with multi-antenna access points and user equipments. The designs aim to reduce the iterative bi-directional training overhead by reusing the downlink beamformers for uplink transmissions.