In this work, the polyharmonic distortion (PHD) model is used to analyze the behavior of high-speed nonlinear links. The model assumes the validity of the harmonic superposition principle for high-speed I/O links. From the PHD formalism, a frequency-domain X-parameter matrix formulation can be derived and shown to predict signal propagation in high-speed links. The formulation accommodates both port and harmonic dependence of the signals. The X-parameter formalism is reviewed as well as its application to circuit simulation. Advantages of the X-parameter representation are discussed and various types of simulations are performed and compared with standard simulation techniques.