A baud-rate full-duplex transmission unit for subscriber loops

A baud-rate full-duplex digital transmission unit for 144-kb/s basic access over an existing subscriber loop plant is described. The adaptive operations performed include echo cancellation, decision feedback equalization, reference control, and timing recovery. The unit operates at the baud rate, allows a fully digital implementation, and requires no special training sequences. Baud-rate timing recovery is a crucial issue. A method which only estimates timing error when certain data sequences occur is described and applied to subscriber loops. The behavior of this method in a region of incorrect decisions due to intersymbol interference is analyzed as a one-dimensional random walk. This indicates that escape from this region will be rapid when the gain is moderately large. Timing issues at exchange and subscriber ends are discussed. Different start-up strategies involving staggered turn-on plus gear shifting for both ends are proposed to decouple the adaptive loops and yield relatively low convergence times. In particular, at the exchange end, timing recovery is initially decoupled by successively adapting the remaining loops at several instants within the baud period. The instant closest to the optimum is then that instant with a minimum timing estimate variance