Near-Field Beam Training for Holographic MIMO Communications:Typical Methods,Challenges and Future Directions

Holographic multiple-input multiple-output(HMIMO) has become an emerging technology for achieving ultra-high frequency spectral efficiency and spatial resolution in future wireless systems. The increasing antenna aperture leads to a more significant characterization of the spherical wavefront in near-field communications in HMIMO scenarios. Beam training as a key technique for wireless communication is worth exploring in this near-field scenario. Compared with the widely researched far-field beam training, the increased dimensionality of the search space for near-field beam training poses a challenge to the complexity and accuracy of the proposed algorithm. In this paper, we introduce several typical near-field beam training methods: exhaustive beam training, hierarchical beam training, and multi-beam training that includes equal interval multi-beam training and hash multi-beam training. The performances of these methods are compared through simulation analysis, and their effectiveness is verified on the hardware testbed as well. Additionally, we provide application scenarios, research challenges, and potential future research directions for near-field beam training.