I. Introduction
The strength of the received signal expressed either by the signal power, or by the signal swing depends on the transmission channel. Although the channel type is normally known in advance, the channel length may be unknown or may vary due to reconfiguration in the optical network. Consequently, the received signal amplitude is not known in advance, and it can vary over time, especially in dense wavelength division multiplexing (DWDM) optical systems. Due to variations in the signal level of digitally transmitted signals, an automatic gain control (AGC) circuit is customarily placed before the clock recovery (CR) and data recovery circuits.