Fractional Fourier Transformation Based XL-MIMO Near-Field Channel Analysis

Due to the impact of spherical wavefront effects, extremely large-scale multiple-input-multiple-output (XL-MIMO) near-field channels no longer exhibit sparsity in the angular domain. In this letter, we systematically delve into the characteristics of near-field XL-MIMO channels within the fractional Fourier transformation (FrFT) domain. Specifically, we commence by analyzing the sparsity exhibited by near-field channels in the fractional domain across various scenarios. Subsequently, we further investigate the orthogonality conditions governing near-field channels in the fractional domain, providing valuable insights for the design of near-field codebooks. Finally, we explore the symmetry properties of both near-field and far-field channels within the angular and fractional domains.