Ring-type Codebook Design for Reconfigurable Intelligent Surface Near-field Beamforming

Reconfigurable intelligent surface (RIS) is a research hotspot for the fifth generation evolution (5GE) and the sixth generation (6G) wireless communication systems to extend high-frequency coverage. Due to the increase of the carrier frequency of the wireless communication system and the large scale of the RIS array, the near-field region is significantly expanded. Using the plane wave assumption-based discrete Fourier transform (DFT) codebook for the RIS near-field beamforming may result in a large signal-to-noise ratio (SNR) loss. Furthermore, for practical RIS with a large number of elements, it is more cost-effective to implement only discrete phase shifts with a small number of control bits for each element, for example, 1-bit for two-level (0 or $\pi$ ) phase shifts, which generates the mirror grating lobe with the same power as the main lobe and results in power leakage. In this paper, a ring-type codebook is proposed based on the Fresnel principle, which can achieve SNR enhancement as well as mirror grating lobe suppression for 1-bit beamforming in near-field region. Compared with DFT codebook, the proposed codebook improves the average throughput by 14% and achieves a main grating lobe suppression ratio of about 17.9dB.