Trends in Wireless Relay and Reflector Technology

As the world moves beyond 5G (B5G) and toward the realization of 6G networks, wireless communication systems must address unprecedented challenges in coverage expansion, spectral efficiency, energy optimization, and ultra-reliable low latency communication (URLLC). The increasing adoption of high-frequency millimeter-wave (mmWave) and sub-terahertz (THz) bands introduces signal propagation challenges due to path loss, atmospheric absorption, and blockage effects. To overcome these limitations, relay and reflector technologies using repeaters and intelligent metasurfaces are expected to enhance network connectivity, extend coverage, and optimize wireless transmission efficiency. Relay systems in distributed environments have recently become a focal point for improving the reliability and reach of mmWave communications. These systems, which include integrated access backhaul (IAB), reconfigurable intelligent surface (RIS) / intelligent reflecting surface (IRS), and network-repeater (NCR), enhance coverage and deployment flexibility without requiring fiber connections, thus reducing costs. Each system varies in its IAB employs a decode-and-forward mechanism, processing signals at higher layers; RIS/IRS uses a passive approach to reflect signals without amplification; and NCR amplifies and forwards signals using analog beamforming, enhancing direct communication between the base station and devices without decoding the information. These relay technologies are crucial for expanding the effective deployment and functionality of mmWave networks. This paper presents surveys of use cases and scenarios where the relaying technologies discussed in this paper are required, the latest technical trends of relay and reflector technologies, and international standardization activities.