Codec supporting PCM modem communications over a universal digital loop carrier

• US 6,052,412 A
• Filed: 06/04/1999
• Issued: 04/18/2000
• Est. Priority Date: 10/30/1998
• Status: Expired due to Fees

• Current Assignee: Terayon Communications Systems Incorporated (Motorola Solutions, Inc.)
• Original Assignee: Tyco Electronics Corporation (TE Connectivity Ltd. (United Kingdom))
• Inventors: Ruether, Peter Gade, Ford, Carolyn Gaye
• Primary Examiner(s): Bocure, Tesfaldet
• Application Number:
  US09/326,355
• Time in Patent Office: 319 Days
• Field of Search: 375/222, 375/254, 375/242, 375/285, 375/231, 375/346, 375/219, 375/350, 379/93.01, 379/93.05, 379/93.06
• US Class Current: 375/231
• CPC Class Codes: H04B 14/04 using pulse code modulation
• CPC Class Codes: H04L 25/4927 using levels matched to the quantisation levels of the channel

Abstract

A codec supports PCM modem communications (e.g., V.90, X2, or K56flex protocols) over a universal digital loop carrier (UDLC) having an analog interface to the central office (CO). The codec includes an analog-to-digital converter (ADC) that is synchronized to the CO clock, and an equalizer that has been trained to provide a transfer function to compensate for the distortion introduced by the CO line interface reconstruction filter and connecting circuits, to minimize resampling error. These features allow the codec to output a PCM stream that accurately reproduces the PCM data at the central office, at data rates sufficient to support PCM modem communications, as well as voice and conventional modem communications. The equalizer can be trained using a remote training unit that transmits a predetermined sequence of training signals to the equalizer. The connection to the training unit can be over a local central office telephone call or over a wide area network that can be accessed through a switched connection. In one embodiment, the equalizer comprises a feed-forward linear equalizer followed by a decision feedback equalizer.

Claims

1. A codec supporting quantization level sampling (QLS) communications with a PCM modem over a digital loop carrier (DLC) having an analog connection to a central office (CO) line interface;

said CO line interface having a digital-to-analog converter (DAC), a clock controlling the DAC, and a reconstruction filter;

said codec also being switchably connected over the CO line interface to a training unit transmitting a predetermined sequence of training signals to said codec;

said codec comprising;

an analog-to-digital converter (ADC) synchronized to the CO clock for converting analog signals from the CO line interface into corresponding digital signals;

an equalizer having an operational mode in which said equalizer receives said digital signals from said ADC and provides a transfer function to substantially compensate for distortions introduced by said CO line interface reconstruction filter and the signal path from said CO line interface reconstruction filter to said ADC;

said equalizer also having a training mode in which said equalizer is connected to the training unit, receives the training signals, and adjusts predetermined equalizer parameters to more accurately compensate for said distortions; and

a PCM converter converting the output of said equalizer to output digital data over said DLC that reproduces the digital data entering the CO line interface.

2. The codec of claim 1 wherein said equalizer comprises a feed-forward linear equalizer.

3. The codec of claim 2 wherein said feed-forward linear equalizer comprises a fractionally-spaced equalizer.

4. The codec of claim 2 wherein said equalizer further comprises a decision feedback equalizer.

5. The codec of claim 1 wherein said training unit further comprises a modem, and wherein said codec further comprises a modem for control and data communications between said training unit and said codec.

6. The codec of claim 1 wherein said equalizer comprises a plurality of equalizers, each trained to process one PCM symbol in a repeating group of PCM symbols from said ADC.

7. A telecommunications system comprising:

a PCM modem;

a central office (CO) switch communicating with said PCM modem over a digital network, said CO switch having a CO clock;

a CO line interface connected to said CO switch having;

(a) a digital-to-analog converter (DAC) controlled by said CO clock converting digital data received from said PCM modem into an analog signal; and

(b) a reconstruction filter limiting the bandwidth of said analog signal output by said DAC;

a training unit transmitting a predetermined sequence of training signals through said CO line interface; and

a digital loop carrier (DLC) having a central office terminal (COT) with an analog connection to said CO line interface, said COT including a codec having;

(a) an analog-to-digital converter (ADC) synchronized to the CO clock for converting analog signals from the CO line interface into corresponding digital signals;

(b) an equalizer having an operational mode in which said equalizer receives said digital signals from said ADC and provides a transfer function to substantially compensate for distortions introduced by said CO line interface reconstruction filter and the signal path from said CO line interface reconstruction filter to said ADC;

said equalizer also having a training mode in which said equalizer is connected to said training unit, receives the training signals, and adjusts predetermined equalizer parameters to more accurately compensate for said distortions; and

(c) a PCM converter converting the output of said equalizer to output digital data over said DLC that reproduces the digital data entering the CO line interface.

8. The system of claim 7 wherein said equalizer comprises a feed-forward linear equalizer.

9. The system of claim 8 wherein said feed-forward linear equalizer comprises a fractionally-spaced equalizer.

10. The system of claim 7 wherein said equalizer further comprises a decision feedback equalizer.

11. The system of claim 7 wherein said equalizer comprises a plurality of equalizers, each trained to process one PCM symbol in a repeating group of PCM symbols from said ADC.

12. The codec of claim 7 wherein said training unit further comprises a modem, and wherein said codec further comprises a modem for control and data communications between said training unit and said codec.

13. A method for supporting communications with a PCM modem over a digital loop carrier (DLC) having an analog connection to a central office (CO) line interface, said CO line interface having a digital-to-analog converter (DAC), a clock controlling the DAC, and a reconstruction filter;

said CO line interface further providing a connection to a training unit transmitting a predetermined set of training signals through said CO line interface;

said method comprising;

in an operational mode;

(a) converting analog signals from the CO line interface into corresponding digital signals at a sampling rate synchronized to the CO clock;

(b) equalizing said digital signals using an adaptive equalizer to process said digital data and provide a transfer function to substantially compensate for distortion introduced by said CO reconstruction filter and the signal path from said CO reconstruction filter over said analog connection; and

(c) sampling the output of said adaptive equalizer to output digital data over said DLC that reproduces the digital data entering the CO line interface; and

in a training mode;

(a) establishing a communications link between said training unit and said adaptive equalizer;

(b) transmitting said training sequence from said training unit to said adaptive equalizer; and

(c) adjusting predetermined equalizer parameters to more accurately compensate for said distortion.

14. The method of claim 13 wherein said adaptive equalizer comprises a feed-forward linear equalizer.

15. The method of claim 14 wherein said feed-forward linear equalizer comprises a fractionally-spaced equalizer.

16. The method of claim 13 wherein said adaptive equalizer further comprises a decision feedback equalizer.

17. The method of claim 13 wherein said adaptive equalizer comprises a plurality of adaptive equalizers, each trained to process one PCM symbol in a repeating group of PCM symbols from said analog-to-digital conversion step.

18. The method of claim 13 wherein said communications link between said training unit and said codec is established by a dial-up connection over a public switched telephone network.