Efficient Codebook Design and User Scheduling for Large STAR-RIS-Aided Downlink URLLC Transmission

To ensure ultra-reliable, low-latency communications (URLLC) despite blockages, we propose using simultaneously transmitting and reflecting (STAR) reconfigurable intelligent surfaces (RISs). These surfaces can transmit and reflect signals, expanding coverage on both sides of the RIS. However, phase shifter adjustments in STAR-RIS are limited by coupling constraints. We introduce a codebook design for phase-shift adjustment that accounts for these constraints. Leveraging this design, we jointly optimize base station beamforming and STAR-RIS phase-shifting to maximize the number of admitted URLLC users in a downlink multi-user communication system. Numerical results show that the proposed STAR-RIS codebook surpasses existing designs and effectively admits more URLLC users.