  In this paper the reflection and transmission of waves by a three-dimensional
random medium are studied in a white-noise and paraxial regime. The limit
system derives from the acoustic wave equations and is described by a coupled
system of random Schr\"{o}dinger equations driven by a Brownian field whose
covariance is determined by the two-point statistics of the fluctuations of the
random medium. For the reflected and transmitted fields the associated Wigner
distributions and the autocorrelation functions are determined by a closed
system of transport equations. The Wigner distribution is then used to describe
the enhanced backscattering phenomenon for the reflected field.
