Ber Analysis Of High Speed Links With Nonlinearity

Quest for higher data rates has led to more complex high speed serial links (SerDes) whose design is more challenging, time consuming, and expensive. As such the ability to accurately predict the performance of the link at the early stages of the design is essential. Bit Error Rate (BER) is the ultimate metric of performance. Due to the very low BER requirement (1e-12 or lower), it is not practical to predict the performance solely through simulation as it requires unacceptably long simulations time. Resorting to accurate analytical methods to augment the simulation is the only viable approach. Analytical methods use knowledge of channel models to calculate probability density function (PDF) of signal (Inter Symbol Interference (ISI), crosstalk, jitter etc.) at the decision point, and use tail probability to predict BER. Such methods based on Linear Time Invariant (LTI) system analysis have been studied extensively in the past. However, it is known that a typical high speed system suffers from significant nonlinearity in the signal path. We present a method to modify the PDF to account for nonlinearity in the path. Once the PDF is correctly modified, similar tail probability methods can be used to determine BER. Static, memory-less nonlinearities are modelled by polynomials in this paper. The methodology can also be adapted to deal with the time varying and frequency dependent nonlinearities.