CELL-FREE MASSIVE MIMO SYSTEMS WITH MULTI-ANTENNA USERS

In this paper, we investigate the impact of multiple-antenna deployment at access points (APs) and users on the performance of cell-free massive multiple-input multiple-output (MIMO). The transmission is done via time-division duplex (TDD) protocol. With this protocol, the channels are first estimated at each AP based on the received pilot signals in the training phase. Then these channel information will be used to decode the symbols before sending to all users. The simple and distributed conjugate beamforming technique is deployed. We derive a closed-form expression for the downlink spectral efficiency taking into account the imperfect channel state information (CSI), non-orthogonal pilots, and power control. This spectral efficiency can be achieved without the knowledge of instantaneous CSI at the users. In addition, the effects of the number antennas per APs and per users are analyzed in the case of using mutual orthogonal pilot sequences and data power control.