A 28–GHz Bi-Directional Reflection-Amplifier-Based Phase Shifter for Active Reconfigurable Intelligent Surface (RIS)

This paper presents a low-power, bi-directional reflection-amplifier-based phase shifter (RAPS) for 28 GHz active reconfigurable intelligent surfaces (RIS). The architecture integrates a 90◦ hybrid coupler with an active reflection load consisting of a reflection amplifier (RA) and a 3-stage switched-filter phase shifter (SFPS). By leveraging the bi-directional nature of reflection-type topologies, the proposed RAPS provides a full 360◦ tuning range with <0.6 dB gain variation. The prototype consumes only 5.6 mW from a 0.35 V supply, representing a significant advancement in power efficiency for large-scale 6G active RIS deployments.