On the power spectral density of digitally modulated signals and applications in code despreading

Two techniques for deriving the power spectral density (PSD) of digitally modulated signals are examined. Both concentrate on the autocorrelation function of modulated sequences, and one of them makes use of the discrete Fourier transform coefficients of this function. These methods provide general expression for PSDs which can be useful in a variety of situations; for example, in the case of the product pseudonoise (PN) codes with arbitrary modulating waveforms. Specific examples are given, where the dependence of the PSD on the relative shift between the two versions of a PN sequence is demonstrated. This could have implications on all systems which use the standard correlation operation (despreading) of PN codes and different pulse shapes.<>