  Together with millimiter waves (mmWaves), massive multiple-input
multiple-output (MIMO) systems are key technological components of fifth
generation (5G) wireless communication systems. In such a context, geometric
considerations show that the largely adopted plane wave model (PWM) of the
channel potentially loses its validity. An alternative is to consider the more
accurate but more complex spherical wave model (SWM). This paper introduces an
intermediate parabolic wave model (ParWM), more accurate than the PWM while
less complex than the SWM. The validity domains of those three physical models
are assessed in a novel way. Finally, estimation algorithms for the SWM and
ParWM are proposed and compared with classical algorithms, showing a promising
performance complexity trade-off.
