Near-Far Field Beamforming for Holographic Multiple-Input Multiple-Output

Recently, holographic multiple-input multiple-output (HMIMO) has motivated its potential use to support high-capacity data transmission with spatially quasi-continuous apertures. As a practical instance of HMIMO, reconfigurable refractive surfaces (RRSs) equipped with numerous metamaterial elements are utilized as antennas by refracting incident signals from signal sources. In this paper, we investigate a multi-user communication system with an RRS deployed as the base station (BS)'s transmit antenna. To mitigate the high overhead of accurate channel state information (CSI) acquisition, the codebook design and beam training are employed to perform beamforming. Given the large scale of RRS, users are likely to be randomly distributed in both the near and far fields around the BS, which is unknown in advance. By considering radiation characteristics in both fields, a near-far field codebook is designed to be applicable to all users, regardless of their locations. To reduce overhead, a multi-user beam training is proposed to serve all users simultaneously by enhancing each codeword capable of covering multiple areas. Considering a general case that includes users in both fields, simulation results indicate that, without prior knowledge of user distribution, the proposed scheme outperforms state-of-the-art ones in terms of sum rate and overhead.