Optical true time delay based hybrid beamforming for RIS-aided mmWave multi-user MISO systems

Reconfigurable intelligent surfaces (RISs) have emerged as a promising wireless technology for the 6th generation (6G) wireless systems. Herein, we investigate the joint passive- and hybrid- beamforming (JPHBF) optimization problem in RIS-aided millimeter-wave (mmWave) multi-user MISO (MU-MISO) systems. Furthermore, we introduced codebooks into analog- and passive- beamforming design for low training and feedback overheads. We proposed optical true time delay (OTTD)-based sub-connected hybrid beamforming (SC-HBF) with a low hardware cost and low channel estimation overheads. Compared to the traditional SC-HBF implementation, only about half of the optical true time delay pool based phase control units (OTTDP-PCUs) are required. We design the codebook-based passive- and analog- beamforming (PABF) under the constraints improved by the use of about half of the OTTDP-PCUs and the passive nature of RISs. The results show that the use of multiple RISs significantly improves the sum rate, specifically an improvement of approximately 284.5% at 25 dBm total transmit power. Although only about half of the OTTDP-PCUs are used, the proposed OTTD-based SC-HBF achieves near-optimal sum rates. Moreover, we presented a design example of the optical wavelength matrix used to map the OTTDP-PCUs.