Full-Duplex Relaying in Machine-Type Communications With a Multi-Antenna Base Station

To address the problem of multiple access for a large number of devices in machine-type communication (MTC), we study the use of full-duplex relays. In-band-full-duplex (IBFD) allows devices to receive and transmit concurrently on the same frequency band, potentially doubling the spectral efficiency. On the downside, the use of IBFD raises mutual interference due to an increased density of concurrently transmitting nodes. This paper deals with the tradeoff between the increased spectral efficiency and increased interference in utilizing the IBFD uplink relays in a densely deployed MTC network. Using stochastic geometry, we develop a framework to evaluate the end-to-end outage probability and the uplink data transmission rate in a single-hop relay network for MTC. A fraction of MTC devices (MTCDs) can be scheduled to transmit in the same band as the relay nodes, with the rest transmitting on an orthogonal band. The relay nodes, therefore, operate either as IBFD or out-band-full-duplex (OBFD) depending on their associated MTCD and forward packets to base stations (BSs). The BSs employ linear zero-forcing filters to cancel the interference from their associated relays. We calculate the optimal fraction of in-band MTCDs and relative transmit powers of relay and MTCDs to maximize the area spectral efficiency. The appeal of in-band scheduling increases as more antennas are deployed at the BS.