Influence of nonlinearity on partial-response signalling systems

The behaviour of data-transmission systems, in particular p.a.m. systems, using partial-response signalling techniques in the presence of Gaussian and impulsive noise is investigated. Such an investigation permits a meaningful study of practical systems, because even if nonlinearity has not been added to the system, all real systems have a finite dynamic range of linear operation and act as a soft limiter with a highsaturation level. A 3rd-order polynomial device expressed by Y = a1x + a2x3 with an optimum nonlinearity resemblance is employed to suppress the large excursions of the impulsive noise. The effects of the nonlinearity on the partial-response signalling-systems behaviour is searched. An equaliser-design approach in the frequency domain is presented that includes the constraints of elimination of performance degradation owing to the nonlinearity and minimisation of the receiver-output noise power. Several partial-response signalling systems which are of practical interest are considered, and for each system the receiving-filter (equaliser) characteristics are obtained. Finally, the behaviour of standard p.a.m. systems and partial-response signalling systems in the presence of nonlinearity are compared.