Using the modulo inverse prefilter as a data scrambling device

Abstract

The modulo-inverse prefilter is a nonlinear digital filter that contains a standard linear infinite impulse response part and a nonlinear modulo number operation in the feedback that constrains output. This filter has been used in data transmission systems as a transmit-end equalizer to combat intersymbol interference on dispersive channels because it can act as the pseudo-inverse of any finite infinite impulse response channel function F(z). The author shows that since its output appears very white across the Nyquist band, independent of input, it has applications as a data scrambling device or whitener. This application is possible because it is very difficult to find the correct 'key' or filter coefficients by simply observing the incoming data stream, and there are no synchronization problems between the transmitter and receiver because the receive end is self-synchronizing.<>