Sum-Rate Maximization of RIS-Aided Digital and Holographic Beamformers in MU-MISO Systems

TL;DR

This paper proposes a novel RHS-RIS-based hybrid beamforming approach for MU-MISO systems, jointly optimizing digital, holographic, and passive beamformers to maximize sum-rate, demonstrating significant performance gains over traditional methods.

Contribution

It introduces a new joint beamforming design framework using an alternating maximization approach for RHS-RIS systems in multi-user MISO scenarios.

Findings

Achieves 8-13 bps/Hz sum-rate improvement over non-RIS systems.

Employs fractional programming and Riemannian conjugate gradient algorithms for optimization.

Demonstrates effectiveness through simulation results in multi-user scenarios.

Abstract

Reconfigurable holographic surfaces (RHS) are intrinsically amalgamated with reconfigurable intelligent surfaces (RIS), for beneficially ameliorating the signal propagation environment. This potent architecture significantly improves the system performance in non-line-of-sight scenarios at a low power consumption. Briefly, the RHS technology integrates ultra-thin, lightweight antennas onto the transceiver, for creating sharp, high-gain directional beams. We formulate a user sum-rate maximization problem for our RHS-RIS-based hybrid beamformer. Explicitly, we jointly design the digital, holographic, and passive beamformers for maximizing the sum-rate of all user equipment (UE). To tackle the resultant nonconvex optimization problem, we propose an alternating maximization (AM) framework for decoupling and iteratively solving the subproblems involved. Specifically, we employ the zero-forcing criterion for the digital beamformer, leverage fractional programming to determine the radiation amplitudes of the RHS and utilize the Riemannian conjugate gradient algorithm for optimizing the RIS phase shift matrix of the passive beamformer. Our simulation results demonstrate that the proposed RHS-RIS-based hybrid beamformer outperforms its conventional counterpart operating without an RIS in multi-UE scenarios. The sum-rate improvement attained ranges from 8 bps/Hz to 13 bps/Hz for various transmit powers at the base station (BS) and at the UEs, which is significant.