The use of random variables and processes is one of the basic mathematical tools for assessing and analysis telecommunication networks performance since many factors are in their nature stochastic (e.g. thermal noise) or it is not possible to obtain their values (e.g. location of routes and user devices, session duration, the quality of the received signal). Very often these random variables have complex functional dependencies from other random variables, whose characteristics are known. This article describes a method of transformation probability density functions of random variables and analyzed finding the probability of directional antenna wavefront hitting a random receiver, for which it is an interference signal.