Full-Duplex Cell-Free Massive MIMO

Abstract

This work studies a novel full-duplex (FD) cell-free massive multiple-input multiple-output (MIMO) network, where a very large number of multiple-antenna access points (APs) simultaneously serve many single-antenna uplink and downlink users in the same frequency band. The APs operate in the FD mode while the users in the half-duplex (HD) mode. The APs apply a simple conjugate beamforming/matched filtering scheme with the channel state information acquired via the uplink training with orthogonal pilots transmitted from the users. By an analysis with a large number of APs, residual self-interference (RI) is proved to be the main limitation of the cell-free massive MIMO systems. A simple power control method to mitigate this limitation is also proposed. The closed-form expressions of uplink and downlink achievable rates are derived with a finite number of APs and the channel estimation error taken into account. Under considered parameter settings, numerical results show that when the RI is sufficiently low, the FD mode can achieve a spectral efficiency gain of 140% over the HD mode in the cell-free massive MIMO system. They also confirm that the FD cell-free massive MIMO systems outperform the FD collocated massive MIMO systems in terms of spectral efficiency.