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Future wireless communication systems are likely to adopt extremely largeaperture arrays and millimeter-wave/sub-THz frequency bands to achieve higherthroughput, lower latency, and higher energy efficiency. Conventional wirelesssystems predominantly operate in the far field (FF) of the radiation source ofsignals. As the array size increases and the carrier wavelength shrinks,however, the near field (NF) becomes non-negligible. Since the NF and FF differin many aspects, it is essential to distinguish their corresponding regions. Inthis article, we first provide a comprehensive overview of the existing NF-FFboundaries, then introduce a novel NF-FF demarcation method based on effectivedegrees of freedom (EDoF) of the channel. Since EDoF is intimately related tospectral efficiency, the EDoF-based border is able to characterize key channelperformance more accurately, as compared with the classic Rayleigh distance.Furthermore, we analyze the main features of the EDoF-based NF-FF boundary andprovide insights into wireless system design.

How to Differentiate between Near Field and Far Field: Revisiting the Rayleigh Distance