Pulse or digital communications – Synchronizers – Phase displacement – slip or jitter correction
Reexamination Certificate
2000-02-29
2004-04-20
Tran, Khai (Department: 2631)
Pulse or digital communications
Synchronizers
Phase displacement, slip or jitter correction
Reexamination Certificate
active
06724849
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to digital subscriber line transceivers for communicating using telephone subscriber loops, and more particularly, to techniques for performing clock frequency synchronization (timing recovery) for asymmetric digital subscriber line transceivers under TCM-ISDN cross-talk.
2. Description of Background Art
FIG. 1
depicts in a block diagram a relationship between a single transmitter
102
(central office (CO)) and single receiver
104
(customer premises equipment (CPE)) that use digital subscriber line (DSL) communications over copper telephone wires
106
. The wider bandwidth needed for DSL transmission generates cross-talk interference among copper wire pairs bundled in the same cable binder. The level of cross-talk varies for different cable structures and materials. Some countries such as Japan and Korea use telephone cables with a paper-based “pulp” insulator rather than the plastic insulated cables (PIC) used in the United States. These pulp cables have high level of cross-talk between different services over copper wires bundled in the same cable binder. ISDN service has been deployed widely over copper wires. Cross-talk caused by ISDN service is one of the major interferences to other newly deployed DSL services since portions of the transmission band for ISDN service overlap with portions of the transmission band for DSL services.
In countries such as Japan, where the noisy pulp cables are installed, a special TCM-ISDN system is deployed. This system is described in the International Telecommunication Union Telecommunication Standardization Sector (ITU-T) specification G.961, Appendix III. The G.961 Appendix III system reduces cross-talk interference by switch synchronizing ISDN cards at the central office using Time Compression Multiplexing (TCM). TCM provides for ISDN signal transmission and reception during different time periods to reduce near-end cross-talk between ISDN services.
ITU-T ADSL standards G.992.1 and G.992.2 Annex C (hereafter “ADSL Annex C”) describe the operation of DSL modems under TCM-ISDN interference. Signal transmissions from DSL modems are switch synchronized to a 400 Hz TCM Timing Reference (TTR) generated at the central office. The TTR signal is the master clock signal for determining when the central office modem (the “CO
102
modem”) and the customer premises equipment (the “CPE
104
modem”) should transmit and receive ISDN and DSL signals.
Within the same cable binder, TCM generates a time varying noise environment. During the first half period of the TTR signal, the CO modem is dominated by near end cross-talk (NEXT) interference, and roughly speaking, during the second half period the CO modem is dominated by far end cross-talk (FEXT) interference. The reverse is true for the CPE
104
modem.
FIG. 2
is a diagram illustrating the relationship between TTR, ISDN, and G.992.2 timing.
The TCM-ISDN cross-talk environment can be different depending on the length of the subscriber loop. On long subscriber loops, because the received signal is heavily attenuated, the NEXT interference is large compared to the received signal. The channel capacity in the NEXT period can be greatly reduced, sometimes be zero. On the other hand, in the FEXT period, the channel typically has good signal-to-noise ratio (SNR) because the FEXT interference is much weaker than NEXT, and small relative to the received signal.
FIG. 3
illustrates the relationship between the TTR signal, the ISDN NEXT/FEXT interference, and ADSL Annex C transmit frames. A “Sliding Window” operation specified in G.992.1 and G.992.2 Annex C defines the procedure for transmitting symbols under ISDN interference synchronized to the TTR signal. The FEXT
R
symbols are symbols completely inside the FEXT
R
period. The NEXT
R
symbols are symbols inside any portion of the NEXT
R
period. Thus, there are more NEXT
R
symbols than FEXT
R
symbols, as shown in FIG.
3
. The CO modem
102
decides if a particular symbol is a FEXT
R
symbol or NEXT
R
symbol according to the sliding window and transmits the symbol according to bit maps corresponding to FEXT
R
and NEXT
R
symbols. Similarly, the CPE
104
modem decides if a particular symbol is a FEXT
C
symbol or NEXT
C
symbol and transmits the symbol according to bit maps corresponding to FEXT
C
and NEXT
C
symbols. The bit map for NEXT symbols can be all zero. In that case, only one bit map is used in each direction for FEXT symbols only. Although the exact symbol time is sliding relative to the TTR signal, the pattern is fixed by ADSL Annex C to be periodic with the period
345
symbols long, which is hereafter referred to as a “hyperframe.”
Referring to
FIG. 4
, there is shown the 345 training symbols that make up a hyperframe, and its relationship to the TTR signal including the mapping of NEXT
R
/FEXT
R
symbols. The only significant difference between the NEXT
R
and FEXT
R
symbols is the additive TCM-ISDN interference. Any symbol that is partially affected by NEXT interference is treated as NEXT symbol. The FEXT symbols represent a signal treated as transmitted entirely during the FEXT period. The remaining training symbols are treated as though they were transmitted during the NEXT period From
FIG. 4
, it is observed that the TTR signal and the CO modem symbols are not aligned. However, over a period of 345 symbols, the TTR signal spans 32 or 34 periods, depending on the cyclic prefix selected by the CO modem. This least common multiple period is used by ADSL Annex C to define the hyperframe.
ADSL Annex C specifies a Discrete Multi-tone (DMT) system, which includes a plurality of tones having different carrier frequencies, each of which is modulated with different data. Tone
64
is used to transmit a “pilot tone” which enables synchronization of the clocks of the CO and CPE modems. The pilot tone is transmitted by the CO modem
102
(master) and synchronized to by the clock of the CPE modem
104
(slave). Using a conventional pilot tracking technique, the CPE modem checks the received pilot signal continuously to control the CPE modem clock. However, under the TCM-ISDN interference environment, use of a pilot tone transmitted during both NEXT and FEXT periods can lead to inaccurate synchronization of the clocks of the CO and CPE
104
modems because the pilot signal during NEXT period may be badly corrupted by the TCM-ISDN NEXT.
What is needed is a method and apparatus that improves the synchronization of the clocks of the CO and CPE under TCM-ISDN cross-talk.
SUMMARY
One embodiment of the present invention includes an apparatus used to perform timing recovery, namely, synchronize the frequency of the receiver clock with a reference clock of a remote transmitter. For example, in this embodiment, the frequency of the reference clock or a representation thereof is transmitted to the receiver. When this embodiment is used in an environment such as ADSL, near end cross talk periodically disrupts the integrity of the reference signal. In this embodiment, a computer implemented method is provided that includes the following acts. First, receiving a first pilot tone symbol from the transmitter. Second, receiving a second pilot tone symbol from the transmitter later. Third, determining a phase error between the first and second pilot tone symbols. Fourth, selectively setting the phase error to zero based on the cross talk environment of the second pilot tone symbol. The TCM-ISDN cross talk environment is predictable and periodic. Fifth, adjusting the frequency of the receiver clock based on the phase error. In this embodiment, the act of setting the phase error to zero occurs if the cross talk is near end type. By setting phase error to zero, clock adjustment is skipped. Alternatively, in this embodiment, the act of setting the phase error to zero occurs if the cross talk is far end type, the receiver is in early training mode, and the pilot tone symbol is a boundary symbol adjacent to a NEXT symbol.
One embodiment of the present invention includ
Bar-Ness Yaron
Hung Chin N
Long Guozhu
Centillium Communications Inc.
Fenwick & West LLP
Tran Khai
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