Demonstration of Millimeter-Wave Reconfigurable Intelligent Surface (RIS) With Built-In Sensors for Automatic Tracking of Direction-of-Arrival (DOA)

This letter proposes a reconfigurable intelligent surface (RIS) with built-in sensors concept for millimeter-wave (mmWave) applications. The proposed method utilizes a unit cell structure, transmitting a portion of the received electromagnetic wave through the via connecting the top metal patch and the bottom ground. The Rotman lens and power detector then convert the radio frequency (RF) phase information from the unit cells into dc data, eliminating the need for expensive RF chains. Afterward, based on a feedback loop, a microcontroller unit (MCU) reads the dc data to estimate the transmitter (TX) position and applies predetermined bias voltages from a code book to the varactor diodes mounted on the top layer of the 10 × 10 unit cells. This process enables the RIS to steer the reflected beam toward the user equipment (UE) according to the TX position. The RIS beamforming functionality is validated through error vector magnitude (EVM) measurements with a commercial mmWave cellphone. An algorithm integrated into the MCU automates these processes, providing a cost-effective and simple RIS structure.