DISTRIBUTED HYBRID BEAMFORMING FOR MMWAVE CELL-FREE MASSIVE MIMO

The cell-free massive Multiple Input Multiple Output (MIMO) architecture is considered for millimeter Wave (mmWave) bands to overcome propagation challenges at higher frequencies such as significant pathloss, easy blockage, and shadowing. Instead of using a single and co-located extreme large antenna array, multiple large arrays with a smaller dimension in a hybrid architecture can be efficiently deployed over multiple Access Points (APs) in a distributed way. We propose a distributed hybrid beamforming concept for mmWave cell-free massive MIMO systems, which supports multi-antenna or even hybrid array based User Equipments (UEs) and multi-stream transmissions. Analog beamforming at both APs and UEs can be obtained in a joint and multi-phase manner to maximize the total effective array gain in the cell-free system. We also develop an interference aware digital precoding method to deal with multi-user/multi-stream transmissions, achieving a high multiplexing gain. Simulation results show a good potential of using the distributed hybrid beamforming in cell-free systems.