Active reconfigurable intelligent surface for the millimeter-wave frequency band: Design and measurement results

Reconfigurable intelligent surfaces (RISs) will play
a key role to establish reliable low-latency millimeter wave
(mmWave) communication links for indoor automation and
control applications. In case of a blocked line-of-sight between
the base station (BS) and the user equipment (UE), a RIS
mounted on a wall or on a ceiling enables a bypass for the
radio communication link. In this work, we present an active
RIS for the mmWave frequency band. Each RIS element uses a
field effect transistor (FET) to amplify the reflected signal and an
orthogonal polarization transformation to increase the isolation
between impinging and reflected radio wave. By switching the
bias voltage at gate and drain of the FET we can establish four
states for each RIS element: two reflection states with different
phase shifts, an active amplification and an off state. We present
results of the active RIS with 37 patch antenna elements arranged
in a hexagonal grid for a center frequency of 25.8 GHz. The RIS
field patterns obtained by numerical simulations and by empirical
measurements in an anechoic chamber are compared. They show
a good match and the received power is improved by 12 dB in
the active mode of the RIS compared to the reflective mode.