Unifying Far-Field and Near-Field Wireless Communications in 6G MIMO

This letter proposes a unified channel representation across the far-field and near-field contexts in the wavenumber domain. Specifically, we commence by re-examining three approximations of the channel response, i.e., Fresnel, Fraunhofer, and Fourier plane-wave (FPW) series approximations, followed by a discussion of their limitations and circumstances under which these techniques may be employed, driving our motivation for this letter. Then, the FPW approximation is modified in accounting for the presence of evanescent waves, allowing it to exactly capture the channel characteristics at arbitrary elevation angles, constituting our proposed modified-FPW (M-FPW) for accommodating both far-field and near-field regimes. Based on the M-FPW, the wavenumber-domain channel representation is established, in which the channel matrix can be structured as a factorized form by employing our customized wavenumber-domain basis. Finally, numerical results are provided to reveal the efficiency and accuracy of the proposed M-FPW methodology in approximating channel responses, highlighting its pronounced robustness against fluctuations in the distance, carrier frequency, and array aperture size.