Intelligent Reflecting Surface vs. Conventional Full-Duplex Relay in MmWave MIMO Networks: A Comprehensive Performance Comparison

This paper aims to provide a comprehensive performance comparison between intelligent reflecting surfaces (IRSs) and conventional full-duplex (FD) relays, making it clear under which conditions and with what protocols, the performance boundaries are achieved by the two methods. We target millimeter-wave (mmWave) multiple-input-multiple-output networks that are key players for the next generation mobile communication and consider the scenarios where an IRS or a FD relay is adopted to establish transmission links between two terminals that may suffer from blocked direct channels. In the comparison, both amplify-and-forward and decode-and-forward relaying schemes are considered. Furthermore, practical power consumption models for IRS and FD relay are built. Aimed at maximizing the energy efficiency, we optimize the element/antenna sizes, active/passive beamforming and power allocation for IRS-aided and FD relay-aided systems, respectively. Our numerical results show that the superiority of each relaying scheme highly depends on the carrier frequency. Though IRSs exhibit lower spectral efficiency than active relays, this gap could be narrowed by decreasing the carrier frequency. Moreover, IRSs' energy efficiency is higher than that of active relays at sub-6 GHz. Specially, when the transmission/receiving gain of transceivers is very high, IRSs can further achieve higher energy efficiency at mmWave band. Compared to amplify-and-forward relays, decode-and-forward relays exhibit better performance in general, however, increasing transmit power can substantially enhance the performance of amplify-and-forward relays. Finally, we apply our conclusion to the relevant works to reveal the reasons for their seemingly conflicted findings.