Analysis and Design of Transimpedance Amplifiers for Optical Receivers

Appendix E

Adaptive Equalizers

The adaptive equalizer has become an important building block, which is used in almost every communication system, be it for optical, wire-line, or wireless communication. In the following, we discuss the basics of the feedforward and decision-feedback equalizers. How do they cancel intersymbol interference? How do they adapt to the incoming signal? What are the implementation challenges?

Intersymbol Interference Canceler

The signal at the output of the receiver's linear channel invariably contains some intersymbol interference (ISI). This ISI is caused, among other things, by dispersion in the optical fiber (modal, chromatic, and polarization-mode dispersion) as well as the frequency response of the linear channel itself.

ISI can be mitigated with an ISI canceler. The effectiveness of the ISI canceler depends on whether the ISI is caused by a linear or a nonlinear distortion. In the linear case, the ISI canceler can apply the inverse transfer function and cancel the ISI almost perfectly. In the nonlinear case, cancellation is much harder. For this reason, a coherent transmission system, which is characterized by a mostly linear transfer function, can be compensated much better than a direct-detection system, which contains a square-law detector (cf. Chapter 2).

In many cases, the transfer function that causes the ISI is not known ahead of time and may even change over time (e.g., in the case of polarization-mode dispersion). Thus, an ISI canceler ...