  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.
