Pulse or digital communications – Transceivers – Modems
Reexamination Certificate
2000-03-09
2004-03-02
Phu, Phoung (Department: 2631)
Pulse or digital communications
Transceivers
Modems
C375S219000
Reexamination Certificate
active
06700927
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to methods for establishing and adapting communication link parameters, including symbol rate and transmission modulation density, in xDSL transmission systems.
BACKGROUND OF THE INVENTION
Basic telecommunications services such as analog telephony, or Plain Old Telephony Service (POTS), currently serve as a main source of revenue for local exchange carriers and other telephone companies. However, there is a growing demand for high speed data services, such as digital video and high speed Internet access. Although there has been relatively little deployment of high speed data services to date, local exchange carriers and other telecommunications service providers are working to find cost-effective ways of meeting the growing demand for those services.
The part of the telecommunications network that connects a telephone central office to subscriber premises is known as the local loop. Local loops are still largely comprised of twisted wire pair links. Such links, which were originally installed to provide narrowband POTS, usually employ copper wire as the electrical transmission medium. Some twisted wire pair links have either been upgraded or replaced by the installation of fiber optic links, but the majority of local loop twisted wire pair links have been in place for many years.
Telecommunications service providers are currently exploring a number of technologies for providing high speed data services, including wireless technology, Hybrid Fiber Coax (HFC), Fiber-To-The-Curb (FTTC), Fiber-To-The-Home (FTTH), Asymmetric Digital Subscriber Line (ADSL), Symmetric Digital Subscriber Line (SDSL), High-speed Digital Subscriber Line (HDSL), and Very-high-speed Digital Subscriber Line (VDSL) technology. Although many of these technologies are likely to play some role in service providers' long-term business strategies, many providers are seeking to meet the growing demand for high speed data services by using switched wireline infrastructures based on Fiber-To-The-Neighborhood (FTTN) and xDSL. In an FTTN local loop, optical fiber connects the central office to a cross-connection device, and the last portion of the local loop—from the cross-connection device to the subscriber premise—is comprised of a twisted wire pair link. The last portion of the local loop can also be comprised of other types of transmission facilities, as known to those of skill in the art.
As used in this specification, the term “xDSL” refers to ADSL, SDSL, HDSL, VDSL, and other digital subscriber line technologies known to those of skill in the art. xDSL technology enables telephone companies to provide high speed data services over local loops comprised in whole or in part of twisted wire pair links (or other transmission facilities), as in FTTN networks.
In a typical xDSL installation, the transmission facility that connects a subscriber residence to the rest of the telecommunications network is terminated by a subscriber-side xDSL termination unit (SS-XTU) on the subscriber side, and a line-side xDSL termination unit (LS-XTU) on the line or central office side. The LS-XTU receives signals from the upstream portion of the telecommunications network, and converts the signals from the format employed by the telecommunications network into xDSL format. The SS-XTU receives the xDSL-modulated signals, demodulates them, and passes them on to various telecommunications devices at the subscriber residence. The SS-XTU also transmits information in the upstream direction, where it is received by the LS-XTU, reformatted, and passed on to the telecommunications network.
In digital communication systems, the signal being transmitted is generally distorted by channel impairments, which results in intersymbol interference (ISI). A general means to reduce the error rates resulting from ISI is to use an equalizer. Thus, in most embodiments involving digital communication, the SS-XTU and the LS-XTU contain an equalizer designed to compensate for or reduce ISI.
One method of equalization is based on the use of previously detected symbols to suppress ISI in the present symbol being detected. An equalizer that implements such a method is called a decision feedback equalizer (DFE).
DFEs contain both feedback and feed-forward filters. The filter coefficients are commonly adjusted according to the mean-square error (MSE) criterion, which seeks to minimize the mean-square value of the error between the information symbol being transmitted and the estimate of that symbol at the output of the equalizer. The basic idea is to move the set of equalizer coefficients closer to the optimum set corresponding to the minimum MSE. Other criteria such as peak distortion can be used to optimize filter coefficients. Such criteria, as well as other types of equalizers and equalization methods, are known to those of skill in the art.
A typical approach in establishing communication between a LS-XTU and a SS-XTU is to transmit a signal from the LS-XTU to the SS-XTU at a predetermined symbol rate, and to attempt to determine a solution—i.e., to converge the equalizer coefficients on an optimum solution—for the transmitted signal. However, if the symbol rate is high—as it needs to be for xDSL transmission—such “blind” acquisition may cause the equalizer solution in the SS-XTU to diverge, especially if the transmission facility between the LS-XTU and SS-XTU contains bridged taps. In those situations, communication may not be established between the LS-XTU and SS-XTU, or, if established, the communication may be poor. Consequently, there is a need for an improved method of establishing communication between xDSL line-side and subscriber-side termination units.
Before a telecommunications company can provide xDSL-based high speed data services to a subscriber, it needs to select a symbol rate and modulation density to use for communication between xDSL termination units.
Telecommunications companies are likely to provide various high speed data services, such as digital video, Internet access, ethernet service, and telephone service, which differ in terms of bandwidth requirements. The territories of telecommunication companies (or service areas within a single territory) may also differ in terms of the length and quality of the transmission facilities between the line-side and subscriber-side termination units. Individual local loops are thus likely to differ in terms of their signal-to-noise ratios and bit error rates. As a result, no single combination of a particular symbol rate and a particular transmission modulation density is likely to be optimal for all high speed data services and deployment scenarios. Consequently, there is a need for an efficient method to determine a symbol rate and modulation density combination that is capable of supporting a bit rate sufficient to provide desired high speed data services on individual local loops.
In addition, telecommunications companies need to maintain their ability to provide basic telecommunications services, such as POTS, Integrated Services Digital Network (ISDN), and Universal Digital Channel (UDC) service, which constitute primary sources of revenue, over the same facilities used to provide newer xDSL-based services. Consequently, a method for determining how to provide xDSL service should also specify parameters, such as an upstream and a downstream frequency range, that avoid or at least reduce the risk of interference with basic telecommunications services that are being provided either on the same link or on adjacent links.
SUMMARY OF THE INVENTION
The present invention provides a method for establishing and adapting communication link parameters in xDSL transmission systems. The overall method includes the steps of establishing communication between xDSL termination units, and determining a maximum transmission modulation density that can be used for each of a plurality of predetermined symbol rates. The invention provides additional, specific methods for performing those two steps: a symbol rate stepping method for establishing communi
Esliger Jamie
Say Sabit
Covington & Burling
Next Level Communications, Inc.
Phu Phoung
LandOfFree
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