A carrier and timing recovery technique for QAM transmission on digital subscriber loops

The asymmetric digital subscriber line (ADSL) service attempts to provide 1.6 Mb/s data transmission over a single twisted-pair copper loop for distances up to 18 kft. A simple timing and carrier recovery scheme for a quadrature amplitude modulation (QAM) approach is proposed. The algorithm uses the receiver decisions to eliminate the effects of intersymbol interference (ISI) as well as to provide estimates for samples of the pulse response at the demodulator output. These estimates are then used to generate a sampling phase error signal that can be used in a single phase locked loop configuration to correct carrier frequency, as well as timing offsets in the QAM based system. The algorithm exploits the characteristics of both the system as well as the environment, and is simple enough to allow integration with all other receiver blocks on a single integrated circuit. Preliminary results for an ADSL system with blind equalization starting from an unequalized state are presented. The new scheme is shown to be effective at acquiring and tracking frequency offsets as high as 112 ppm, without significantly degrading system performance