Pulse or digital communications – Transceivers – Modems
Reexamination Certificate
1998-07-27
2001-10-16
Pham, Chi (Department: 2631)
Pulse or digital communications
Transceivers
Modems
Reexamination Certificate
active
06304594
ABSTRACT:
RELATED PATENT APPLICATION
The present method and Apparatus for Variably Allocating Upstream and Downstream Communication Spectra, by Neil E. Furakawa and Sheldon N. Salinger, filed Dec. 24
th
, 1997, Ser. No. 08/998,237.
FIELD OF THE INVENTION
The present invention relates generally to data communications, and more particularly, to a method and apparatus for automatically detecting the presence of an interfering signal in a data communications receiver and retuning a transmitter to avoid the detected interfering signal.
BACKGROUND OF THE INVENTION
To meet the growing need for ever increasing digital data bandwidth for new subscriber services (e.g., high-data-rate internet service, video telephony, and high definition television) telephone companies are looking towards the use of very-high-speed digital subscriber lines (VDSL). VDSL provides a means to carry such data into the home or business over the existing copper wires used by plain old telephone service (POTS). VDSL carries digital data at bit rates reaching 52 Mbps using carrierless AM/PM (CAP) modulation, which is related to quadrature amplitude modulation (QAM). Modulation contellations can reach 256 symbol points with symbol rates up to 6480 Kbaud. The CAP modulation constellations look the same as the corresponding baseband QAM constellations. The signals are carried within the spectral band up to 30 MHz, which is above the frequency band used by POTS on the same wire pair. Due to the high loss of the copper telephone wires at these frequencies, the VDSL signals are carried on the telephone wires only over the “last mile” (i.e., the last segment of copper wire between a central office and the user's premises).
Within the spectral band of 0.5 MHz to 30 MHz, the VDSL transmissions over the telephone wires are subject to radio frequency interference (RFI) from AM radio broadcasts and amateur radio transmissions, which can be picked up by the telephone wires acting as receiving antenna. This interference can corrupt the received and demodulated CAP/QAM signals, causing symbol and bit errors in the received data. The AM broadcast signals remain indefinitely at the same frequencies in any neighborhood within range of the local AM radio transmitters. The amateur radio signals, however, will vary in frequency throughout the day as the operator tunes his transmitter to optimize the HF-band skywave propagation conditions over the communication links he is attempting to maintain. There are several narrow spectral bands allocated to amateur radio within the 0.5 MHz to 30 MHz band carried by VDSL transmission lines. A VDSL transmission line will normally be interfered with only by amateur radio transmitters within a few hundred feet of the transmission line.
Narrowband interferers, such as AM radio broadcasts and amateur radio signals, are not the only interferers that corrupt the CAP/QAM signals. Wideband noise that extends across a large portion of the VDSL spectrum may also interfere, causing undesirable changes in the carrier-to-noise ratio (CNR) and the bit error rate (BER). Wideband noise can be caused, for example, by electrical machinery, internal combustion engines (e.g., lawn mowers) or fluorescent lights operating in the immediate vicinity of a cable drop to the premises. Narrowband interferers tend to be more predictable and hence somewhat easier to avoid, for example, by detecting the frequency of the interferer and subsequent adjustment of the CAP/QAM VDSL signal for avoidance thereof. Wideband interferers, on the other hand, are somewhat more random, and cannot generally be accounted for prior to data transmission and are therefore more difficult to avoid. Especially difficult is detection and characterization of the interferers without interruption of the data signal for analysis.
Thus, what is needed is a method and apparatus for recognizing the difference between narrowband interferers and wideband noise sources without relying upon spectrum analysis of the received signal except to locate a new transmission band. This is accomplished by the present invention whereby a determination is made of the presence or absence of a narrowband or continuous wave (CW) interferer, and in many cases inferring its carrier frequency without the need for interrupting the transmitted data stream. The transmitted data stream is only interrupted when an inteferer is detected and a spectrum analysis is needed to find a new transmission band. The interferer presence determination can generally be accomplished when the interferer level is low enough that the presence of the interferer cannot be inferred by visual examination of the constellation plot of the demodulated signal. When a narrowband signal is present, the shape of the symbol clusters in the constellation plot changes from a centrally peaked distribution, characteristic of a signal in Gaussian noise, to a ring as shown in
FIGS. 1A and 1B
. The present invention recognizes this transition automatically and at low interference levels where the transistion to a ring is not yet visually apparent. Adjustment of the transmitted symbol rate, bandwidth and carrier frequency is provided in order to avoid the troublesome interferer and other interferers that may be present in the signal environment.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an improved modem that detects and avoids interference signals.
Another object of the present invention is to provide an improved method for recognizing the presence of a continuous wave (CW) interferer in a digital data transceiver.
Still another object of the present invention is to provide a digital data receiver for recognizing narrowband and wideband interferers with either none or little data interruption by using statistical analysis and adjusting the transmitter accordingly to avoid such inteferers.
According to a first embodiment of the present invention, a method of non-intrusively detecting whether an interference signal is present with a received signal of interest is provided. The interference signal is detected by accumulating statistical information associated with the signal of interest. The statistical information is analyzed to determine whether an interference signal is present and is intrusive. If an intrusive interference signal is present, it is analyzed to determine its frequency. An alternate spectral region is next identified that is free of interferers. Lastly, communications are adjusted by retuning or modifying data rates to allow continued communications without intrusive interference.
In another embodiment of the present invention, a modem capable of non-intrusively detecting an unwanted interferer while a receiving of a data signal is provided. The modem includes a transmitter for transmitting information to a second modem and a receiver for receiving the data signal from the second modem. A memory is connected to the receiver for storing statistical information associated with said data signal, including data sample values in each received symbol cluster in the data signal's amplitude phase constellation. A processor measures constellation statistics from the statistical information, determining a carrier-to-noise ratio and bit error rate of said data signal, determining whether the bit error rate exceeds an acceptable limit by testing whether symbol cluster changes from a Rayleigh distribution to a Ricean distribution, and when the bit error rate exceeds said acceptable limit, interupting the received data signal to spectrum analyze a transmission band to find an interference free region of the transmission band and communicating the interference free region to the second modem for adjustment of transmission parameters.
The foregoing and other objects, features, and advantages of the invention will be apparent from the following more particular description of a preferred embodiment of the invention, as illustrated in the accompanying drawing.
REFERENCES:
patent: 3940694 (1976-02-01), Price et al.
patent: 4291277 (1981-09-01), Davis et al.
patent: 4462001 (1984-07-01),
Bayard Emmanuel
General Dynamics Government Systems Corporation
Jenner & Block LLC
Pham Chi
LandOfFree
Interference detection and avoidance technique does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Interference detection and avoidance technique, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Interference detection and avoidance technique will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2617225