Compact Inhomogeneous MIMO Antenna Arrays Inspired by Transformation Optics

The capacity of multiple-input-multiple-output (MIMO) systems has long been constrained by the aperture’s limitations on degree of freedom (DOF). Inspired by transformation optics, this article proposes inhomogeneous antenna arrays to reduce aperture size while maintaining DOF. Calculation on dyadic Green’s function confirms that equivalent inhomogeneous compact arrays calculated by transformation optics can effectively harness evanescent waves to convey information into the far field. A system transformation matrix is introduced to ensure the invariance of DOF across systems with different apertures. In addition, the exploration of 3-D antenna arrays highlights their benefits in balancing high DOF with the complexities of engineering implementation. It can be anticipated that with the further advancement and development of metamaterials, holographic MIMO, through the increased DOF, will lead to an enhancement in channel capacity with miniaturized devices.