SYMBOL TIMING RECOVERY IN DIGITAL SUBSCRIBERS LOOPS IN THE PRESENCE OF RESIDUAL ECHO AND CROSSTALK

In This paper a general analysis is presented for the jitter performan ce of a common symbol timing recovery (STR) system employed in a digit al subscriber loop (DSL) transceiver employing adaptive echo cancellat ion for high-speed digital communications typical of evolving Integrat ed Service Digital Networks (ISDN's). The STR circuit consists of a pr efilter and a squarer followed by a narrow-band postfilter tuned to th e signalling rate. Exact analytical expressions for the mean and varia nce of the timing wave and for the rms phase jitter are derived as a f unction of the bandwidth of the postfilter for a given set of input pa rameters representing a particular digital subscriber loop and its noi se environment, including such effects as residual-echo and crosstalk. Numerical results, obtained for an experimental study of a 144 kb/s D SL timing recovery system, show that the presence of these disturbing signals can substantially degrade the STR performance. The effect of t he excess bandwidth factor of the prefilter on this degradations is al so investigated.