Equalizer-based symbol-rate timing recovery for digital subscriber line systems

This paper derives a class of equalizer-based symbolrate timing recovery algorithms, which are to be employed in digital subscriber line (DSL) systems. These algorithms are intended to break the coupling of the adaptive decision-directed timing recovery and the baud-spaced equalizer in tracking mode, once acquisition has been reached. As the derivation is based on a generalized complex-valued system model, the results can be applied for all common DSL systems employing baseband and passband modulation schemes, like PAM, QAM, and CAP. Exemplarily, an algorithm based on the ratio of the largest precursor tap weight and the cursor tap weight is derived for very high data-rate DSL (VDSL) based on QAM transmission. It is shown by means of simulations, that destructive coupling is prevented. A simple modification of the complex-valued equalizer structure with cursor-normalized tap weights allows an easy implementation of the derived timing recovery algorithm.