Codebook Design for Beamforming in Near-Field Cylindrical Antenna Array Systems

Extremely large antenna array (ELAA) is able to significantly improve the spectral efficiency, so it is regarded as one of the most important technologies for the next-generation networks. However, larger antenna aperture and higher frequency make the Rayleigh distances dramatically increased, so in sixth-generation (6G) networks, more and more communication scenarios will take place in the near-field region. Different from traditional far-field communication, electromagnetic waves transmitted in the near-field communication is widely considered to be spherical rather than planar, so techniques designed for far-field scenarios are no longer applicable. In this paper, we study the near-field communication scenario in which the base station is equipped with a cylindrical antenna array (CLA). Specifically, by exploiting the geometrical relationship between CLA elements and the near-field user, codebook design for beamforming in near-field CLA systems is studied for the first time. We first derive the beamforming gain in the elevation angle domain, the azimuth angle domain and the distance domain respectively. Then a 3-D near-field CLA codebook is proposed to make beamforming more effectively in near-field CLA systems. We obtain the sampling method in each domain by controlling the correlation between different codewords. Simulation results prove the effectiveness of the proposed codebook.