An Evanescent-Propagating Wave Conversion Method for Expanding the DoF in Holographic MIMO

Abstract: Holographic multiple-input multiple-output (HMIMO) systems deploy ultra-dense antennas in confined spaces, yet spatial degrees of freedom (DoF) fail to scale with element count. Only by harnessing evanescent waves can the potential gains of ultra-dense arrays be un‑ locked. However, in practical scenarios, antenna apertures are typically too small relative to communication distances to generate significant near-field effects for capturing these evanescent waves. This paper proposes a method to alter the dispersion relation via locally resonant meta‑ materials (LRM), thereby enabling the radiation of near-field evanescent wave components into the far field. This approach leverages the DoF gains  offered  by  ultra-dense  elements  within  a  confined  aperture.  Full-wave  simulations  validate  the  effectiveness  of  the  evanescent- propagating wave conversion method, demonstrating an increase in the spatial DoF radiated into the far field by the HMIMO system even with a limited aperture.

Keywords: spatial DoF; evanescent-propagating wave conversion; holographic MIMO