Degrees of Freedom of Holographic MIMO -- Fundamental Theory and Analytical Methods

Holographic multiple-input multiple-output (MIMO) is envisioned as one of the most promising technology enablers for future sixth-generation (6G) networks. The use of electrically large holographic surface (HoloS) antennas has the potential to significantly boost the spatial multiplexing gain by increasing the number of degrees of freedom (DoF), even in line-of-sight (LoS) channels. In this context, the research community has shown a growing interest in characterizing the fundamental limits of this technology. In this paper, we compare the two analytical methods commonly utilized in the literature for this purpose: the cut-set integral and the self-adjoint operator. We provide a detailed description of both methods and discuss their advantages and limitations.