Modern MIMO communication systems are almost exclusively designed under the assumption of locally plane wavefronts over antenna arrays. This is known as the far-field approximation and is soundly justified at sub-6-GHz frequencies at most relevant transmission ranges. However, when higher frequencies and shorter transmission ranges are used, the wave curvature over the array is no longer negligible, and arrays operate in the so-called radiative near-field region. This letter aims to show that the classical far-field approximation may significantly underestimate the achievable spectral efficiency of multi-user MIMO communications operating in the 30-GHz bands and above, even at ranges beyond the Fraunhofer distance. For planar arrays with typical sizes, we show that computing combining schemes based on the far-field model significantly reduces the channel gain and spatial multiplexing capability. When the radiative near-field model is used, interference rejection schemes, such as the optimal minimum mean-square-error combiner, appear to be very promising, when combined with electrically large arrays, to meet the stringent requirements of next-generation networks.

翻译：现代MIMO通信系统几乎完全基于天线阵列上局部平面波前的假设进行设计。这被称为远场近似，且在大多数相关传输距离下的亚6GHz频段中具有充分合理性。然而，当使用更高频率和更短传输距离时，阵列上的波曲率不再可忽略，阵列工作在所谓的辐射近场区域。本文旨在表明，经典的远场近似可能严重低估工作在30GHz及以上频段的多用户MIMO通信可实现的频谱效率，即使在超出夫琅禾费距离的范围内也是如此。对于典型尺寸的平面阵列，我们证明基于远场模型计算合并方案会显著降低信道增益和空间复用能力。当采用辐射近场模型时，干扰抑制方案（例如最佳最小均方误差合并器）与电大阵列相结合，似乎非常有希望满足下一代网络的严苛要求。