Pulse or digital communications – Transceivers – Modems
Patent
1998-07-30
2000-10-31
Le, Amanda T.
Pulse or digital communications
Transceivers
Modems
375296, 379 9305, 379340, 379414, 455126, H04B 138
Patent
active
061413771
ABSTRACT:
An ADSL modem provides compensation for a nonlinear telephone impedance by predistorting transmit signals such that the voltage across the nonlinear impedance is correct independent of the impedance. Correcting the voltage across the nonlinear impedance reduces noise in the telephone due to intermodulation products generated in the voice band. The modem comprises a transmitter that includes (a) a signal source providing a source data signal and (b) a predistortion circuit for predistorting the source data signal to provide a modem transmit signal for coupling to a transmission loop having a nonlinear impedance coupled thereto. The modem device further includes a receiver for receiving a modem receive signal from the transmission loop. The source data signal comprises source digital words, each source digital word corresponding to a particular voltage level. The predistortion circuit includes a memory for mapping the source digital words to predistortion digital words, each predistortion digital word corresponding to a particular desired voltage level. The transmitter further includes a digital-to-analog converter for converting the predistortion digital words into the modem transmit signal. A feedback circuit coupled to the loop measures the distortion of the modem transmit signal to provide a feedback signal to the predistortion circuit that is indicative of the measured distortion. The predistortion circuit adjusts the amount of predistortion in response to the feedback signal.
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Hong Kyung-Yeop
Sharper Craig A.
Cisco Technology Inc.
Le Amanda T.
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