Near-Field Channel Estimation for Uniform Circular Arrays Based on an End-to-End Spherical Wavefront Channel Model

Extremely large-scale multiple-input multipleoutput (XL-MIMO) systems emerge as a critical enabler for $\mathbf{6 G}$ networks demanding enhanced spectral efficiency. The adopted extremely large aperture array (ELAA) expands its near-field region by orders of magnitude compared to a usual array. Accurate channel estimation plays a pivotal role in enabling effective beamforming and supporting near-field propagation characteristics. However, prevailing channel estimation methods suffer from physical-model mismatch, uniform linear array (ULA)-centric algorithmic constraints, and narrowband assumptions. To address these challenges, this paper proposes novel solutions for channel modeling and estimation in near-field XL-MIMO systems utilizing uniform circular arrays (UCAs). First, we present an end-to-end spherical wavefront (E2ESW) reflection channel model that reconstructs non-line-of-sight (NLoS) paths through virtual source transmission equivalence, rigorously preserving spherical wavefront phase variations across antenna elements. Second, based on the simultaneous iterative gridless weighted (SIGW) framework, we develop a polar-domain SIGW (P-SIGW-UCA) channel estimation algorithm. Simulation results demonstrate that the designed algorithm significantly reduce channel estimation errors in near-field scenarios compared to conventional approaches.