  In this paper we show that the eigenfunctions can be found exactly for
systems whose delay-Doppler spread function is concentrated along a straight
line and they can be found in approximate sense for systems having a spread
function maximally concentrated in regions of the Doppler-delay plane whose
area is smaller than one. The interesting results are that: i) the
instantaneous frequency of the eigenfunctions is dictated by the contour level
of the time-varying transfer function; ii) the eigenvalues are restricted
between the minimum and maximum value of the system time-varying transfer
function, but not all values are possible, as the system exhibits an inherent
quantization.
