Wireless communication systems have almost exclusively operated in the far-field of antennas and antenna arrays, which is conventionally characterized by having propagation distances beyond the Fraunhofer distance. This is natural since the Fraunhofer distance is normally only a few wavelengths. With the advent of active arrays and passive reconfigurable intelligent surfaces (RIS) that are physically large, it is plausible that the transmitter or receiver is located in between the Fraunhofer distance of the individual array/surface elements and the Fraunhofer distance of the entire array. An RIS then can be configured to reflect the incident waveform towards a point in the radiative near-field of the surface, resulting in a beam with finite depth, or as a conventional angular beam with infinity focus, which only results in amplification in the far-field. To understand when these different options are viable, an accurate characterization of the near-field behaviors is necessary. In this paper, we revisit the motivation and approximations behind the Fraunhofer distance and show that it is not the right metric for determining when near-field focusing is possible. We obtain the distance range where finite-depth beamforming is possible and the distance where the beamforming gain tapers off.
翻译:无线通信系统几乎完全在天线和天线阵列的远处运作,其传统特征是,天线和天线阵列的传播距离超过佛朗霍费尔距离,这是自然的,因为Fraunhofer距离通常只有几波长。随着主动阵列和被动的可调相智能表面(RIS)的出现,似乎发报机或接收机位于不同阵列/地表元素的距离和整个阵列的Fraunhofer距离之间。然后,可配置RIS,将事件波形反映到离地表辐射近处的一个点,导致波束与有限深度相隔,或作为具有无限焦点的常规角束,这只能导致远处的振动。要了解这些不同的选择何时可行,就必须对近地行为进行准确的描述。在本文中,我们重新审视Fraunhofer距离背后的动机和近点,并表明它不是确定近地距离距离的正确度指标,而远处的距离是有限的深处,我们获得伸缩的距离。