Electromagnetic Degree of Freedom of Holographic MIMO in Transformation Optics

Holographic multiple-input multiple-output (MIMO) communication is made up of an ultra-dense array of antennas, whose channel models and potential applications are increasingly drawing attention. The capacity of MIMO systems has long been constrained by the aperture's limitations on degree of freedom (DOF) which has become a significant impediment to the development of holographic MIMO. Inspired by transformation optics, this paper proposed that the electromagnetic DOF can be invariant when the aperture is reduced. Based on the electromagnetic information theory, our work is aimed to exploring the potential to overcome the aperture-size limitation in MIMO antennas, offering an alternative path to enhance the DOF in 6G communication systems.