On the Role of the Number of Degrees of Freedom of the Field in MIMO Channels

The aim of this paper is to investigate the relationship between the effective number of degrees of freedom (ENDF) of a multiple input multiple output (MIMO) channel and the number of degrees of freedom (NDF) of the field. In particular, the NDF of the field represents an upperbound for the ENDF of a MIMO channel. Consequently, the evaluation of the maximum ENDF can be carried out by adopting the literature on the NDF based on the spatial bandlimitation properties of the field. This approach allows a simple and intuitive explanation of the maximum ENDF in a MIMO channel based on a geometrical interpretation of the spatial channel in terms of (spatial) Nyquist intervals.