index. The measures of system performance are essentially criteria of optimality, and systems are designed taking them into account.
In some systems, certain parameters are either not constant or vary in an unknown manner. It may be desirable to design for the capability of continuously measuring them and changing the system so that the system performance criteria are always satisfied. Systems designed with these objectives are called adaptive systems.
Adaptive systems are an important issue in channel equalization, which is the recovery of a signal distorted in transmission through a communication channel with a nonflat magnitude or a nonlinear phase response. When the channel response is unknown, the process of signal recovery is called blind equalization.