As a result of the rapid evolution of mobile communications in the last decade many propagation models have been developed for radio planning purposes. To verify the various propagation models, accurate measurements of the received signals have to be performed especially at the edges of mobile communication cells, where the received power is at very low level and signal to noise ratio is degraded. In these cases narrowband receivers such as spectrum analyzers have to be employed. The settings of the receivers have to be examined carefully, since they are of crucial importance, in order to discriminate the measured signals from the noise. On the other hand, noise has to be removed from the measurements and uncertainty has to be estimated correctly; thus the “clean” levels will be suitable for verifying the propagation models and optimize the radio planning. In this paper a method of noise reduction for such signals is presented.