An adaptive equalizer is an equalizer that automatically adapts to time-varying properties of the communication channel.[1] It is frequently used with coherent modulations such as phase shift keying, mitigating the effects of multipath propagation and Doppler spreading.

Many adaptation strategies exist. They include:

• LMS Note that the receiver does not have access to the transmitted signal $x$ when it is not in training mode. If the probability that the equalizer makes a mistake is sufficiently small, the symbol decisions $d(n)$ made by the equalizer may be substituted for $x$.[2]
• RLS

A well-known example is the decision feedback equalizer,[3][4] a filter that uses feedback of detected symbols in addition to conventional equalization of future symbols.[5] Some systems use predefined training sequences to provide reference points for the adaptation process.