Beamforming Design in Intelligent Reflecting Surface Aided Multicast System in Ultra-High Frequency Bands With Large-Scale Antenna Array

As the next generation mobile communication system goes to ultra-high frequency bands, such as terahertz (THz), we may achieve angular orthogonality by implementing a large-scale antenna-array. We maximize the achievable multicast rate among all users by jointly designing the hybrid transmit beamformers and the analog reflecting beamformer, subject to the power budget and norm on the phase-shifters. A novel asymptotically optimal structure of the reflecting beamformer at the IRS is found, based on which a closed-form asymptotically optimal solution of the hybrid transmit beamformer and the reflecting beamformer is also obtained. Numerical results demonstrates the feasibility of exploiting the angular orthogonality in the beamforming design of an IRS aided THz communication system.