Microwave imaging with circular arrays: Comparative analysis between OAM and MIMO

The rotating phase of orbital angular momentum (OAM) waves can bring new degrees of freedom in the wireless communication and microwave imaging realms, and more generally for inverse problems. In this paper we consider a three-dimensional microwave imaging system under the Born approximation employing a uniform circular array (UCA) suitably designed to produce OAM beams with different pure azimuthal orders. The performance of the OAM-based imaging approach is compared to that achieved by exploiting the spatial diversity offered by the individual radiating elements in the UCA, i.e., implementing a multiple-input/multiple-output (MIMO) approach. To this aim, a fully vector formulation is employed and both point targets as well as extended 3-D PEC and dielectric objects are reconstructed implementing a truncated singular-value-decomposition scheme. Multiple azimuthal orders are considered, generated by circular arrays of dipoles arranged with different orientations. The results of such analysis indicate the feasibility of using the OAM degree of freedom in addition to spatial diversity for 3-D imaging reconstruction. In the circular array systems presented in this work - where the number of radiative elements and the array radius are determined by the geometry of the studied region and the desired azimuthal orders - the two approaches are shown to have comparable performance in most cases, the OAM system offering significant computational advantages. The proposed methods are of interest when scanning lines or curves are limited or cannot be implemented to probe the region of interest.