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Large-Scale Integration of Hybrid-Method Digital Subscriber Loops

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3 Author(s)
O. Agazzi ; Univ. of California, Berkeley, CA, USA ; D. Hodges ; D. Messerschmitt

This paper reports on studies concerning the feasibility of large-scale integrated realization of the circuits needed to provide hybrid-mode full-duplex digital transmission at 80 kbits/s or higher rates over standard local telephone loops. Alternative means of achieving the required 60 dB or so of echo cancellation have been studied in detail. The conclusion is that a combination of analog and digital circuit techniques permit practical MOSLSI realization of the complete modem, including filters, echo canceller, timing recovery, and A/D and D/A converters, without need for external circuit elements, trimming, or adjustments. The preferred system configuration has been evaluated by means of analysis, simulation, and laboratory and field measurements. A complete full duplex system, including an experimental NMOS integrated circuit echo canceller, was built and tested. Measurements showed a bit error rate lower than l0^{-8} with line attenuation up to 40 dB, operating at 80 kbits/s. We conclude that a fully integrated MOSLSI ciruit to implement all functions for a hybrid-mode digital local loop is entirely feasible.

Published in:

IEEE Transactions on Communications  (Volume:30 ,  Issue: 9 )