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Modern MIMO communication systems are almost exclusively designed under theassumption of locally plane wavefronts over antenna arrays. This is known asthe far-field approximation and is soundly justified at sub-6-GHz frequenciesat most relevant transmission ranges. However, when higher frequencies andshorter transmission ranges are used, the wave curvature over the array is nolonger negligible, and arrays operate in the so-called radiative near-fieldregion. This letter aims to show that the classical far-field approximation maysignificantly underestimate the achievable spectral efficiency of multi-userMIMO communications operating in the 30-GHz bands and above, even at rangesbeyond the Fraunhofer distance. For planar arrays with typical sizes, we showthat computing combining schemes based on the far-field model significantlyreduces the channel gain and spatial multiplexing capability. When theradiative near-field model is used, interference rejection schemes, such as theoptimal minimum mean-square-error combiner, appear to be very promising, whencombined with electrically large arrays, to meet the stringent requirements ofnext-generation networks.

Spherical Wavefronts Improve MU-MIMO Spectral Efficiency When Using Electrically Large Arrays