Center-tap tracking algorithms for timing recovery

Fractionally spaced adaptive equalizers can readily compensate for timing-phase variations between the transmitter and receiver clocks that are on the order of several symbol intervals. When there is a frequency difference between these clocks, the equalizer compensates for the accumulated timing-phase difference by shifting the equalizer tap weights in the appropriate direction along the delay line. Performance, as measured by the mean-squared error, is not degraded until the center (largest) tap migrates toward one end of the equalizer. In this paper, we demonstrate how the spatial distribution of the equalizer coefficients can be used to lock the receiver sampling frequency to that of the transmitter. Replacing the standard envelope-derived timing recovery system with a closed-loop center-tap tracking algorithm offers significantly improved performance for narrow roll-off or severely-attenuated systems, reduced complexity, and a structure appropriate for all-digital receivers.