Circular Holographic MIMO Beamforming for Integrated Data and Energy Multicast Systems

Due to the innovative application of metamaterials, holographic multiple-input multiple-output (H-MIMO) is expected to achieve a higher spatial diversity gain with lower hardware complexity. Together with the aid of a circular antenna arrangement in H-MIMO, integrated data and energy multicast (IDEM) can fully exploit the near-field channel to realize wider range of energy focusing and higher achievable rate. In this paper, we focus on beamforming design and investigate the IDEM systems that maximize the minimum rate of data users (DUs) while meeting the energy harvesting requirements for energy users (EUs). Specifically, we first derive the closed-form near-field resolution function in 3D space and show the asymptotic spatial orthogonality of near-field channel for circular antenna arrays. Then, we design an asymptotically optimal fully-digital beamformer based on the spatial orthogonality. After that, we apply the alternating optimization to develop H-MIMO beamforming scheme, where the digital beamformer is given in closed form while the analog beamformers of three different control modes are obtained numerically, respectively. Scaling schemes are also investigated to further improve the IDEM performance. Numerical results verify the correctness of the resolution function and asymptotic orthogonality and demonstrate that the proposed beamforming schemes outperform benchmark schemes, with very low complexity.