On Landau’s Eigenvalue Theorem for Line-of-Sight MIMO Channels

An alternative derivation is provided for the degrees of freedom (DOF) formula on line-of-sight (LOS) channels via Landau’s eigenvalue theorem for bandlimited signals. Compared to other approaches, Landau’s theorem provides a general framework to compute the DOF in arbitrary environments, this framework is herein specialized to LOS propagation. The development shows how the spatially bandlimited nature of the channel relates to its geometry under the paraxial approximation that applies to most LOS settings of interest.