Beam-Domain Full-Duplex Massive MIMO: Realizing Co-time Co-frequency Uplink and Downlink Transmission in the Cellular System

Co-time co-frequency uplink and downlink (CCUD) transmission was considered challenging in the cellular system due to the strong self-interference (SI) between the transmitter and receiver of base station (BS). In this paper, by investigating the beam-domain representation of channels based on the basis expansion model, we propose a beam-domain full-duplex (BDFD) massive multiple-input multiple-output (MIMO) scheme to make the CCUD transmission possible. The key idea of the BDFD scheme lies in intelligently scheduling the uplink and downlink user equipment (UE) based on the beam-domain distributions of their associated channels to mitigate SI and enhance transmission efficiency. We show that the BDFD scheme achieves significant savings in uplink/downlink training resource and achieves uplink and downlink sum capacities simultaneously as the number of BS antennas approaches infinity. The superiority of the BDFD scheme over the traditional time-division duplex (TDD)/frequency-division duplex (FDD) massive MIMO is evaluated through simulation for the macrocell environment. The results show that the spectral efficiency gain can even exceed 2 $\times$ in the specific scenarios, since the BDFD scheme utilizes the time-frequency resource more efficiently in both the training and data transmission phases.