Nonlinear Propagation and Information Theory

We identify three general categories of nonlinear capacity impairments. Absent exact solutions, this can guide us as to what to look for in numerical simulations; it can also assist our thinking about system design. The channel capacity (or the equivalent dimensionless quantity, the maximum possible spectral efficiency) is not typically employed as a system metric: bit error rates (the fractional number of bits that are in error after reception), or the more ad hoc motions of "power penalties" (the equivalent increase in signal power required to overcome the deletrious effects of a particular noise source), are used instead, because they correspond to directly measurable quantities. Theoretically, however, the capacity is a more principled measure, and insight about the other metrics can be extracted, given an analytical formula for the capacity. Finally, the problem has intrinsic interest from an information theoretic standpoint. For these reasons, we believe it to be important to understand the effect of nonlinear propagation in terms of capacity or spectral efficiency.