Pulse or digital communications – Transceivers – Modems
Reexamination Certificate
1998-06-19
2001-12-11
Pham, Chi (Department: 2631)
Pulse or digital communications
Transceivers
Modems
C375S346000, C702S191000
Reexamination Certificate
active
06330275
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
This invention generally relates to a method and apparatus for overcoming periodic disturbances in digital subscriber loops and, more particularly, to a method and apparatus which measures and/or records periodic-disturbance waveforms in premise wiring and which generates an inverse of the periodic-disturbance waveform which is used to cancel the periodic-disturbances.
BACKGROUND OF THE INVENTION
It is well known in the communications industry that noise or disturbances in a communications channel can adversely affect the performance of data communications equipment (DCE) using the channel. For example, it is well known that dial-up voice-frequency modems are constrained by noise in the channel to operate at less than their full capacity. This is why dial-up frequency modems do not always operate at their advertised maximum data rates. The performance of xDSL modems is also adversely affected by noise. In accordance with the present invention, it has been determined that periodic disturbances in premise wiring may adversely affect data communications equipment at the customer premises. Periodic impulse noise at the 60 Hz rate, which may be produced by radiating sources such as light dimmers, vacuum cleaners, blenders, etc., adversely affects data communications equipment at the customer premises. While these impairments may not adversely affect dial-up modems to a great extent, it is believed that they significantly adversely affect high-speed modems, such as DSL modems, because these types of modems utilize higher frequencies where pickup from radiating sources in the 60 Hz range may be greater.
Many DSL line coding techniques, such as discrete multi-tone (DMT), asymmetric digital subscriber line (DMT-ADSL), carrier less amplitude and phase modulation ADSL (CAP-ADSL), integrated services digital network (ISDN), and 2B1Q high-bit rate DSL (2B1Q-HDSL), do not attempt to communicate with general premise wiring but either (1) use a POTS splitter or filter to prohibit premise wiring disturbances above approximately 20 KHz from coupling into the DSL modem or (2) terminate the connection to the subscriber line at a single DSL modem such that the DSL modem does not communicate with any premise wiring. Therefore, coupling of premise wiring disturbances with frequencies above 20 KHz into the DSL communications channels is not a major problem. With respect to dial-up modems, coupling of periodic disturbances above approximately 4 KHz is not a problem because dial-up modems which operate over general premise wiring generally do not operate above 4 KHz. However, some DSL systems communicate over general premise wiring without a POTS splitter. In these cases, periodic disturbances coupled into the general premise wiring can significantly degrade performance.
If periodic disturbances contain only a relatively small number of frequency components within the data communication bandwidth, e.g., 1 to 15 disturbing frequency components, techniques such as precoding and data-feedback-equalization (DFE) can be used to improve performance. However, if there are many disturbing components within the data communication bandwidth, preceding and DFE techniques are impractical and can lead to other problems, such as long signal delays. With respect to periodic impulse disturbances in the 60 Hz range, these impulses imply a spectral content with energy at frequencies separated by 60 Hz, which is far too fine a separation to be dealt with by standard preceding or DFE techniques. At this frequency, the impulse period is about 16.67 milliseconds, which is too short to consider transmitting only between the impulses, especially since the impulses are likely to be spread over perhaps a millisecond.
Accordingly, a need exists for a method and apparatus which measures periodic impulse disturbances at the customer premises and which produces a counter measure which overcomes the disturbances to improve performance of data communications equipment located at the customer premises. Furthermore, in accordance with the present invention, it has been determined that canceling the periodic disturbance at one customer premise modem receiver will effectively cancel the disturbance on all customer premise modem receivers, due to the very close physical proximity of the modems relative to the frequency band of communication. Also, since both the periodic disturbance at issue and the transmitted countermeasure are generated nearly concurrently at the premise modem, the cancellation will be equally effective at the remote end of the subscriber loop, i.e., at the central office.
SUMMARY OF THE INVENTION
In accordance with the present invention, an impulse canceler is located at the customer premises for measuring and/or recording impulse disturbances produced by radiating sources at the customer premises and for generating an impulse disturbance countermeasure which effectively cancels the impulse disturbances on all premise modem receivers operating over the general premise wiring.
The impulse canceler may be implemented separately from any of the modems at the customer premises or, alternatively, may be incorporated into one of the DCEs at the customer premises. In accordance with the preferred embodiment of the present invention, the impulse canceler of the present invention is implemented within one of the DCEs at the customer premises. Alternatively, the impulse canceler of the present invention may be implemented at the central office, or remote-terminal end, if the periodic impulse disturbances are generated at the remote-terminal end instead of at the customer premises. However, it is expected that the periodic impulse noise will be generated at the customer premises rather than at the central office or remote-terminal end. Consequently, the impulse canceler of the present invention preferably is implemented at the customer premises.
In accordance with the preferred embodiment of the present invention, the impulse canceler of the present invention is incorporated into the DCE coupled to the first tributary at the customer premises. This DCE is designated at the “main” DCE. The DCE at the remote-terminal end is designated as the “master” DCE. Any other DCEs at the customer premises are connected to their respective tributaries and, in the event that the main DCE tributary drops off, the DCE at the next higher-number tributary will be designated as the new main DCE.
At the beginning of a data communication session, or at the beginning of any retrain session, a silence period is provided with a duration suitable to measure several periodic impulse disturbances that might exist. For 60 Hz disturbances, the duration of this period may be, for example, 200 milliseconds. During the silence period, all DCE transmitters are silenced and all line terminals are placed into their normal data communication mode. The main DCE then measures and/or records any impulse disturbances. Representations of the average waveform, or a suitable facsimile of the average waveform, and a representation of the period are then stored in memory. This may be accomplished by, for example, directly storing digital samples of the waveform or by analyzing the waveform and storing digital parameters which can be used to regenerate the waveform. The impulse canceler will then use these representations periodically to cancel the periodic disturbances. If the disturbances are time-stationary, then the cancellation will not need to be adaptive. However, if the disturbances are not time-stationary, the impulse canceler will adaptively alter the representations of the impulse waveform and period by tracking the amplitude and period of the disturbance waveform.
In accordance with an alternative embodiment of the present invention, the main DCE will develop a representation of the impulse disturbances by measuring the period of the impulse disturbance train, by creating a single pulse at the measured period which is input into a finite impulse response (FIR) equalizer, and by adapting the FIR equalizer to match and cancel the i
Paradyne Corporation
Pham Chi
Phu Phuong
Thomas Kayden Horstemeyer & Risley LLP
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