On the impulsive nature of interchannel interference in digital communication systems

Impulsiveness, or a high degree of peakedness, of interchannel interference in digital communication systems typically results from the non-smooth nature of any physically realizable modulation scheme designed to transmit a discrete (discontinuous) message. Even modulation schemes painstakingly designed to be ‘smooth’ are not. The non-smoothness of the modulation can be caused by a variety of hardware non-idealities and, more fundamentally, by the very nature of any modulation scheme for digital communications. In order to transmit a discrete message, such a scheme must be causal and piecewise, and cannot be smooth, or infinitely differentiable. Recursive differentiation of a non-smooth transmitted signal eventually leads to discontinuities. When observed by an out-of-band receiver, the transmissions from these discontinuities may appear as strong transients with the peak power noticeably exceeding the average power, and the received signal will have a high degree of peakedness. This impulsive nature of the interference provides an opportunity to reduce its power.