Millimeter-waves above 100 GHz are supposed to be used in the sixth generation (6G) wireless communication to realize high-speed and large-capacity communication with performances far exceeding those of 5G. Since the millimeter waves are vulnerable to blockages by physical obstacles such as trees and buildings, narrowing of the service coverage becomes a more severe problem than in conventional microwave communications. As such, one of the challenges for realizing 6G is to expand the coverage area even in the presence of obstacles.

One of the promising solutions for mitigating the coverage holes is using metasurface-based high efficiency reflectors with the anomalous reflection property in which a reflector reflects incident waves in a desired arbitrary direction different from the specular direction. Metasurface reflectors provide non-line-of-sight communications by bypassing obstacles. This leads to easy coverage designs with flexible reflector deployments on fixed walls or windows of buildings. Furthermore, metasurface reflectors can achieve high-efficiency operation by optimizing the design, which enables communication coverage expansion with lower power consumption than densely deployed base stations.

However, there has been no demonstration of metasurface reflectors in the millimeter-wave bands above 100 GHz for the beyond-5G/6G applications. The biggest reason for this is that it was a challenge to accurately measure the complex permittivity of the dielectric substrate and the conductivity of the metal layer in the millimeter-wave bands above 100 GHz. Reliable material measurements are essential for the optimized design of high-efficiency metasurface reflectors for the beyond-5G/6G applications.

In collaboration with Osaka University, researchers in AIST developed the world's first metasurface reflector that reflects incident waves in a desired arbitrary direction at 140 GHz.

The reflection efficiency excluding the internal material losses was calculated to exceed 99 %. Furthermore, high efficiency operations were experimentally demonstrated with measured efficiency, including practically unavoidable material losses, of up to 88%. This is the world's first demonstration of a highly efficient metasurface reflector in the D-band. The developed technology is expected to contribute to flexible coverage designs of the beyond-5G/6G communication by bypassing obstacles and providing non-line-of-sight communications.