RFGlass: Optically Transparent Transmissive Metasurface for Coverage Expansion

Sub-6 GHz has been widely deployed in a variety of mobile services. Yet, the radio wave in a high-frequency band bears significant path loss, and thus frequently introduces coverage hole problems. The sporadic coverage outage becomes more severe for outdoor-to-indoor, indoor-to-outdoor, and cross-room communications. When penetrating through a window, the radio signal experiences considerable attenuation due to the intrinsic impedance mismatch between glass and free space. In this article, we propose RFGlass, an optically transparent metasurface, to enhance the coverage in spots without a line-of-sight path or at cell edges. RFGlass comprises sub-wavelength units, each capable of generating phase response to incident signals. Encoding specific phase shifts, RFGlass can steer the incident beam to the direction that needs coverage enhancement. Applying a glass substrate and metal-mesh-based patterns, RFGlass attains an optical transparency of 82 percent. We have fabricated a 472 mm x 472 mm prototype, consisting of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$24 \times 24$</tex> unit cells, and evaluated it through the over-the-air test. Experimental results show that RFGlass improves the signal strength over the non-line-of-sight path up to 11.55 dB, and reduces the symbol error rate 85 percent compared to ordinary glass.