To combat the perturbations introduced during the transmission via time-varying communication channels, modern digital communication systems employ a technique called equalization. Preset equalization transmits a training sequence which is compared to a similar sequence at the receiving end. The difference between the received and local sequences is used to adjust a special filter. Adaptive equalization continually measures the characteristics of the channel from the data received and dynamically adjusts the filter. To take advantage of the powerful features of equalization, good channel error performance is necessary.

The equalizer sits between the demodulator and the detector.