The transition of Doppler reflectometry diagnostic to nonlinear regime of scattering is studied analytically with the use of the perturbation theory applied to Helmholtz equation. Density fluctuations with large radial correlation length, O-polarization of the probing beam, linear background density profile and slab geometry are considered. Formulae for onset of nonlinearity are derived for different parameter ranges. Obtained results are generalized for the arbitrary turbulence radial correlation length. The results of physical optics model are reproduced, while new formulae are obtained for different parameter ranges and their experimental relevance is demonstrated. Qualitative explanation for scattered signal nonlinear power dependence is proposed. Analytical results are validated with IPF-FD3D code full-wave numerical modelling for radially uniform and fully 2D turbulence..