In this paper,the effects of the generalized exponent,the height and the zenith angle on the log-amplitude variance in the weak fluctuation are investigated.The theoretical results indicate that for the downlink,the log-amplitude variance of the Kolmogorov model is always smaller than that of the three-layer model,while for the uplink,there is a point of intersection in the log-amplitude variance curves of the two models.The different phenomena for the downlink and uplink are analyzed in detail.Further,we find a method to ascertain precise values of the boundary layer altitudes for the three-layer model under various atmospheric conditions through the analysis for the point of intersection.And at the point of intersection,the Kolmogorov model can be used to replace the three-layer model to simplify the analysis of the system performance.Moreover,the log-amplitude variance increases with the increase of the zenith angle.