Near‐Field 3D Stochastic Channel Modelling and Performance Analysis for Cooperative STAR‐RIS Systems With Cylindrical Transmit Arrays

ABSTRACT This paper develops a three‐dimensional geometry‐based stochastic model for cooperative STAR‐RIS‐assisted multiple‐input single‐output systems operating in the near‐field regime. A cylindrical antenna array (CA) is employed at the base station, and all propagation links are formulated using exact Euclidean distances to accurately capture spherical wavefront effects without relying on planar‐wave approximations. Both line‐of‐sight and clustered non‐line‐of‐sight components are modelled under a Rician framework, ensuring geometry‐consistent phase evolution across BS–STAR‐RIS, STAR‐RIS–user and inter‐STAR‐RIS links. Closed‐form expressions for spatial correlation and temporal autocorrelation are derived to characterize near‐field channel behaviour. Numerical results show that the proposed CA‐based configuration achieves faster spatial decorrelation and improved spectral efficiency compared with conventional uniform rectangular array and uniform linear array baselines under identical element counts and spacing. In particular, cooperative double STAR‐RIS deployment yields approximately a 4‐bit/s/Hz spectral‐efficiency improvement at 30 dB SNR compared to single‐surface operation. These findings confirm that spherical‐wave modeling and inter‐surface cooperation significantly enhance near‐field wireless performance.