Performance Comparison Between A Simple Full-Duplex Multi-Antenna Relay And A Passive Intelligent Reflecting Surface

In this paper, we propose a single RF-chain multi-antenna full-duplex (FD) relay built with $b$-bit analog phase shifters and passive self-interference cancellation. The hardware complexity of the proposed FD relay with 2-bit quantized analog phase shifters is on the same order as the hardware complexity of a passive intelligent reflecting surface (IRS). Next, assuming only passive self-interference cancellation at the FD relay, we derive the achievable data rate of a system comprised of a source, the proposed FD relay, and a destination. We then compare the achievable data rate of the proposed FD relaying system with the achievable data rate of the same system but with the FD relay replaced by an ideal passive IRS. Our results show that the proposed relaying system with 2-bit quantized analog phase shifters can significantly outperform the IRS-assisted system. Intuitively, the IRS system is in disadvantage since there the total transmit power $P_T$ is used entirely by the source, whereas in the FD relaying system the total power $P_T$ is shared by the source and the FD relay in addition to the noise-cleansing process performed by the decode-and-forwarding at the FD relay.