The Markov parabolic equation for the second-order space-frequency coherence function is solved analytically. A technique allowing construction of an asymptotic solution for an arbitrary analytical model of the structure function of medium fluctuations is applied to solve a number of model problems. Propagation of plane (obliquely incident) and spherical waves in a random medium is analyzed. Fluctuations of the medium are characterized by a realistic model of a structure function that tends to a positive constant at large values of the transverse spatial difference variable.