Performance analysis of a QAM adaptive receiver for 1.6 Mbps digital subscriber line transmission

The asymmetric digital subscriber line service attempts to provide 1.6 Mb/s data transmission over a single twisted-pair copper loop for distances up to 18000 ft. The authors investigate the various system components necessary to realize such a system on a single integrated circuit. They discuss the modulation format, adaptive equalization and its convergence, and system sensitivity to sampling phase error. A comparison of conventional quadrature amplitude modulation (QAM) and pulse amplitude modulation (PAM) schemes is presented. It is concluded that a QAM-based system can meet the system objectives. The use of fractionally spaced equalization and error prediction as means of improving SNR and reducing the hardware complexity is investigated for such a system. It is shown that with 6 T/2-spaced feedforward equalizer taps, eight decision-feedback equalizer taps, and two error predictor taps, the proposed digital QAM system can achieve performance close to that of an ideal infinite-length equalizer