Circular Holographic MIMO Beamforming for Integrated Data and Energy Multicast Systems

Thanks to the application of metamaterials, holographic multiple-input multiple-output (H-MIMO) is expected to achieve a higher spatial diversity gain with lower hardware complexity. With the aid of a circular antenna arrangement of 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 derive the closed-form near-field resolution function in 3D space and show the asymptotic spatial orthogonality of near-field channel for circular antenna array. We then investigate the beamforming designs for IDEM systems, where the minimum rate of data users (DUs) are maximized while guaranteeing the energy harvesting requirements for energy users (EUs). Specifically, the asymptotically optimal fully-digital beamformer is first obtained based on the spatial orthogonality. Then, the alternating optimization is adopted for the H-MIMO beamforming, where the digital beamformer is obtained in closed form and the analog beamformers of three different control modes are then obtained, respectively. Scaling schemes are also investigated to further improve the IDEM performance. Numerical results verify the correctness of the resolution function and asymptotic orthogonality. Moreover, the proposed beamforming schemes with very low complexity outperform benchmark schemes.