A mmWave Polarization-Dependent Simultaneously Transmitting and Reflecting Reconfigurable Intelligent Surface Based on a Novel Independent 1-Bit Phase-Agile Unit Cell

To overcome the main limitation of conventional reconfigurable intelligent surfaces (RISs), simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS) can extend the coverage of signals to its backside to provide a more comprehensive service from all directions. This paper combines the design concepts of transmitarray and reflectarray and proposes a novel unit cell of STAR-RIS. The measured reflection loss and transmission loss are 0.43 dB and 1.26 dB, respectively. The ability to achieve independent 1-bit reconfig-urability for the reflection and transmission phases is validated. A Ka-band <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$20\times 20$</tex> STAR-RIS is realized. The sidelobe levels are both lower than -10 dB for transmitting and reflecting beams. These results demonstrate the feasibility of the STAR-RIS based on the proposed 1-bit phase-agile unit cell in mmWave applications.