Adaptive Cancellation of Nonlinear Intersymbol Interference for Voiceband Data Transmission

Two related cancellation techniques based on "tentative" decisions are proposed for removing intersymbol interference, due to channel nonlinearities, for voiceband data transmission. Cancellation of intersymbol interference avoids the need for equalizing or "inverting" analog nonlinear operations as in the decision-feedback method previously proposed by Falconer. The nonlinear cancellation approach removes nonlinear interference terms without excessive noise enhancement and allows effective implementation with ROM\´s to generate the needed nonlinear signal variables that are to be weighted and summed. The first technique can be incorporated as an "add-on" unit to a present-day modem; the second technique requires a different receiver architecture but offers somewhat enhanced improvement. Simulation results for 9.6 kbit/s data transmission over nonlinear channel models show that both techniques can offer substantial performance improvements for 16-point QAM systems.