TECHNIQUES FOR HIGH-SPEED IMPLEMENTATION OF NONLINEAR CANCELLATION

Nonlinear cancellation (NLC) can significantly reduce intersymbol interference (ISI) in long-haul direct-detection fiber-optic systems, and can thereby more than double the dispersion-limited data rate and/or distance. The ISI reduction by NLC is achieved by subtracting the interference caused by previously detected symbols; this subtraction, however, requires previous decisions to be fed back to the decision element, and delays in the feedback loop can severely limit the maximum data rate of a detector operating with NLC. In this paper, we present techniques for breaking the bottleneck caused by the feedback loop. First, we show how to simplify the loop to avoid high-speed switching of analog signals by using multiple decision elements, each with a different threshold level. We then show how to use lookahead computation to increase the delay permissible in the feedback loop. These techniques permit NLC to be implemented in integrated-circuit form at rates limited only by the detector switching speeds. These techniques are also useful in other applications requiring speedup of feedback loops with decision elements in the loop (e.g., decision feedback equalizers).