Analysis of the Range Ambiguity Function of Narrowband Near-field MIMO Sensing
Marcin Wachowiak, André Bourdoux, Sofie Pollin
TL;DR
The paper addresses the limitation of narrowband near-field sensing by systematically comparing SIMO/MISO and MIMO across diverse array geometries (ULA, UCA, URA, UPCA) and derives how MIMO processing shapes the near-field ambiguity function. By modeling the NF AF as a product of transmit and receive array factors and applying a quadratic Fresnel-based approximation, it shows a consistent $\sqrt{2}$ improvement in both beamdepth-based resolution and maximum NF range when moving from SIMO/MISO to MIMO, regardless of geometry. The analysis also reveals a twofold improvement in peak-to-sidelobe level due to squaring of the AF, with the best resolution achieved by UCA while planar arrays (URA, UPCA) offer superior PSL. The results provide design guidance for NF sensing systems, quantifying how geometry and MIMO processing jointly enhance localization performance in the near field.
Abstract
This paper compares the sensing performance of a narrowband near-field system across several practical antenna array geometries and SIMO/MISO and MIMO configurations. For identical transmit and receive apertures, MIMO processing is equivalent to squaring the near-field array factor, resulting in improved beamdepth and sidelobe level. Analytical derivations, supported by simulations, show that the MIMO processing improves the maximum near-field sensing range and resolution by approximately a factor of 1.4 compared to a single-aperture system. Using a quadratic approximation of the mainlobe of the array factor, an analytical improvement factor of $\sqrt{2}$ is derived, validating the numerical results. Finally, MIMO is shown to improve the poor sidelobe performance observed in the near-field by a factor of two, due to squaring of the array factor.