Holographic Beamforming for Ultra Massive MIMO With Limited Radiation Amplitudes: How Many Quantized Bits Do We Need?

Reconfigurable holographic surfaces (RHSs) are novel metamaterial antennas with small-size and lightweight structures. They can achieve amplitude-controlled beamforming by leveraging the holographic technique, which shows great potential to meet the increasing data demands in next generation networks. In this letter, we investigate a downlink RHS-aided multi-user communication system, where the BS equipped with an RHS transmits to the users via holographic beamforming. Considering that the RHS only has limited amplitude values rather than continuous ones in practice, we study the discrete amplitude-controlled holographic beamforming and analyze the effect of radiation amplitude discretization on the sum rate. We present the lower bound of the sum rate with discrete amplitude values, based on which the number of the minimum radiation amplitude quantized bits is derived. Simulation results verify our theoretical analysis. The relations among the size of the RHS, the number of users, data rates, and quantized bits are revealed.