Mixed LoS/NLoS Near-Field Channel Estimation for Extremely Large-Scale MIMO Systems

Accurate channel estimation is essential to empower extremely large-scale MIMO (XL-MIMO) with ultra-high spectral efficiency in 6G networks. With the sharp increase in the antenna array aperture of the XL-MIMO system, the electromagnetic propagation field will change from far-field to near-field. Unfortunately, due to the near-field effect, the existing near-field XL-MIMO channel model mismatches the practical mixed line-of-sight (LoS) and non-line-of-sight (NLoS) channel feature. In this paper, a mixed LoS/NLoS near-field XL-MIMO channel model is proposed to accurately describe the LoS and NLoS path components simultaneously, where the LoS path component is modeled by the geometric free space propagation assumption while NLoS path components are modeled by the near-field array response vectors. Then, to define the range of near-field for XL-MIMO, the MIMO Rayleigh distance (MIMO-RD) is derived. Next, a two stage channel estimation algorithm is proposed, where the LoS path component and NLoS path components are estimated separately. Numerical simulation results demonstrate that, the proposed two stage scheme is able to outperform the existing methods.