  In this paper, we study the spatial bandwidth for line-of-sight (LOS)
channels with linear large-scale antenna arrays (LSAAs) in 3D space. We provide
approximations to the spatial bandwidth at the center of the receiving array,
of the form $C R^{-B}$, where $R$ is the radial distance, and $C$ and $B$ are
directional-dependent and piecewise constant in $R$. The approximations are
valid in the entire radiative region, that is, for $R$ greater than a few
wavelengths. When the length of the receiving array is small relative to $R$,
the product of the array length and the spatial bandwidth provides an estimate
of the available spatial degree-of-freedom (DOF) in the channel. In a case
study, we apply these approximations to the evaluation of spatial multiplexing
regions under random orientation conditions. The goodness-of-fit of the
approximations is demonstrated and some interesting findings about the DOF
performance of the channel under 3D and 2D orientation restrictions are
obtained, e.g., that, under some conditions, it is better to constrain the
receiving array orientation to be uniform over the unit circle in the 2D ground
plane rather than uniform over the 3D unit sphere.
