Overview of Near-Field Rayleigh Distance for 6G with Extremely Large-Scale Antenna Array

Extremely large-scale antenna array (ELAA) is a feature common to several key candidate technologies for sixth generation (6G) mobile networks. Due to the very large number of antennas in ELAA, the electromagnetic radiation field needs to be modeled with near-field spherical waves, which is different from the traditional plane wave-based radiation model of 5G massive MIMO. Therefore, near-field communications will become critical in 6G wireless networks, where the most critical is to delineate the boundary between the far-field and near-field regions, i.e., Rayleigh distance, also known as Fraunhofer distance. This paper summarizes the detailed derivation process for calculating the Rayleigh distance based on different criteria in existing near-field communications. Based on the phase difference criterion, the classical Rayleigh distance in MISO scenario and the MIMO Rayleigh distance (MIMO-RD) and MIMO advanced Rayleigh distance (MIMO-ARD) in MIMO scenario are introduced. The uniform-power distance (UPD) is introduced for the MISO scenario based on the amplitude/power ratio criterion of the signal between the array elements. Finally, based on the array gain criterion affecting the propagation rate, the effective Rayleigh distance (ERD) is introduced for the MISO scenario considering the angle factor. This paper provides a general understanding of the Rayleigh distance calculation for 6G near-field communications. The expected research directions and the work already done in the lab are summarized at the end of this paper.