Codebook Design for Holographic MIMO: Near-Field Prospects and Road to Standardization

Holographic multiple-input multiple-output (HMIMO) is envisaged as a viable manner for manipulating electromagnetic field produced or perceived by antennas, in an effort to achieve an intelligent and endogenously holography-capable wireless propagation environment. This notion garners significant interest when engaging large antenna elements at high frequencies, such as millimeter-wave or terahertz. Under these conditions, operations often occur within the Fresnel region, i.e., near-field region, where assumptions of planar wavefront no longer apply. This article investigates the codebook solution for HMIMO, unmasking a number of challenges intrinsic in the near-field context and limitations of applying codebooks specified in current standards. Specifically, we proposed a two-phase codebook design empowered by artificial intelligence (AI)-based techniques, where angular and distance ingredients are resolved respectively at each phase, functioning in tandem for facilitating an efficient beam training at reduced pilot overhead. Then from the 3rd generation partnership project (3GPP) standardization perspective, we share potential design rationales influencing standardization, along with a novel signaling procedure for HMIMO beam sweeping.