there is an important class of signals, namely, the aperiodic power signals, for which we have yet developed a frequency domain model the fourier series does not exist for aperiodic power signals that are not time limited. for such signals the fourier transform may or may not exist, and the concept of energy spectral density does not apply. however, since the average power is assumed to be finite we may use power spectral density functions to describe aperiodic power signals in the frequency domain