Training and tracking modes of adaptive equalizer

An adaptive equalizer works in two modes.

Training mode

• The transmitter sends a fixed-length, known training sequence (e.g. a pseudo-random binary pattern or the 26-bit midamble in GSM) before or within each data burst.
• The receiver knows this sequence, so it computes the error e_k = x_k − d̂_k exactly and adjusts the tap weights (e.g. by LMS) towards their optimum.
• The training sequence is designed so that the equalizer can converge even in the worst channel; at the end of training the taps are near optimal for the current channel.

Tracking mode

• When user data follows, there is no known reference, so the equalizer uses its own decisions x̂_k (decision-directed adaptation): e_k = x̂_k − d̂_k.
• It continuously follows slow changes in the channel while data is being received.
• If the channel changes too much, decisions become wrong, so training is repeated periodically. In TDMA systems this happens in every time slot.

 |Training|  User data (tracking)  |Training| Data ...
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