Pulse or digital communications – Transceivers – Modems
Reexamination Certificate
1999-07-16
2003-11-18
Chin, Stehen (Department: 2734)
Pulse or digital communications
Transceivers
Modems
C375S219000, C379S090010
Reexamination Certificate
active
06650697
ABSTRACT:
BACKGROUND TO THE INVENTION
This invention relates, in general, to a wireline communication system and an associated method of allocating frequencies for use in traffic communications. More particularly, but not exclusively, the present invention is applicable to bi-directional wireline communication systems that support digital subscriber line (xDSL) communication protocols, which wireline communication systems inherently suffer from the undesirable effects of cross-talk interference.
SUMMARY OF THE PRIOR ART
Telecommunication systems that interconnect wireline subscriber terminals are being developed to support broadband data communication. More particularly, recent developments in broadband communication protocols allow broadband data to be overlaid on narrowband voice or integrated service digital network (ISDN) traffic. Specifically, the interconnection of broadband modems located at the subscriber terminal and at an exchange allow current broadband access systems to communicate on spare spectrum (i.e. spare frequency channels) of a twisted pair communication resource; the spare frequency channels being isolated from conventionally encoded voice signals by a suitable filter. In this respect, and depending upon the complexity of the xDSL coding scheme, overlaid broadband systems can support data rates in excess of two Megabits per second (Mbps), although this rate is dependent upon the physical parameters of the connection, e.g. the overall length of the twisted pair and its composition and configuration.
Asymmetric Digital Subscriber Line (ADSL) and High-speed Digital Subscriber Line (HDSL) protocols, for example, can support data rates of 2 Mbps over distances of approximately three kilometers, while more complex schemes (such as VDSL) can support data rates of 8 Mbps and above over distances of, typically, less than two kilometers. Line codes such as discrete multi-tone (DMT), which can be used for Very high-speed Digital Subscriber Line (VDSL), utilise multiple sub-channel carriers, e.g. in a DMT environment, to provide an adaptive system that mitigates the effects of cross-talk by selectively ignoring noise-effected sub channel carriers or reducing the number of bits supported by the sub-channels. As will be appreciated, DMT provides a comb of frequency carriers that are each separated modulated and then combined to generate a composite signal envelope. As such, information (both control information and traffic) is distributed across a number of different frequency carriers.
Presently, some xDSL systems (and the like) utilise a time division duplex transmission scheme in which a communication resource (such as a dedicated channel within frequency limits) has a time-split use for unlink and down-link transmissions between line termination equipment (LTE) and customer premises equipment (CPE). More specifically, the up-link and down-link may have different traffic capacities, i.e. there is a fixed but potentially asymmetric symbol capacity (or number of time slots) between the up-link and the down-link assigned for the entire duration of a call. For example, in an Internet-type environment, It is usually beneficial to have a higher down-link capacity since information download is the dominant data flow, whereas voice traffic generally requires equal traffic capabilities in both directions.
In frequency division duplex (FDD) systems, spectrum is allocated between the up-link and down-link.
In relation to bundles of wireline communication resources, it is also important to consider the potentially undesirable effects associated with cross-talk interference. Specifically, with bi-directional communication, the relative location of the lines, for example, between twisted copper-pair causes cross-talk interference to be induced into proximately located wireline communication resources (principally by the mechanisms of capacitive and inductive coupling and by radiation arising from the imperfect nature and performance of the cabling). Moreover, where symmetrical and asymmetrical service are simultaneously required on pairs in the same bundle, cross-talk becomes a significant problem, as will readily be appreciated.
For completeness, it will be understood that near-end cross-talk (NEXT) occurs when electromagnetic interference is induced into a wireline resource that is communicating information in an opposing direction, e.g. down-link (or downstream) information appears as noise in an unlink (or upstream) path. NEXT is undesirable because near-end generated interference is at a level that can potentially swamp data signals received from a remote terminal, which data signals have previously been subjected to attenuation through the transmission path. Furthermore, NEXT increases significantly at the higher frequency components and so is even more undesirable in high frequency data-over-voice wireline systems, such as VDSL. To avoid the harmful effects of Near-End Cross-Talk (NEXT) in a TDD system, an ensemble of collated communication resources must have synchronised and aligned transmissions. However, in a mixed symmetrical-asymmetrical system, NEXT often occurs where the two opposing schemes have either different frequency allocations (in frequency division duplex, FDD) or different time slot allocations (in TDD).
The skilled artisan will appreciate that the partitioning of symbols (or time slots) between up-link and down-link transmissions must also take into account the form of traffic that is to be sent in the respective directions, and this is reflected by the present schemes of fixing symbol (or time-slot) allocation for the entire duration of a call. Specifically, for voice communication (as opposed to data transmissions) it will be appreciated that the nature of verbal expression requires regular information to be conveyed in order to support a coherent understanding. In contrast, data can be of a bursty nature since a reception pattern for information in somewhat irrelevant and there can, in fact, be a reordering of information at a receiver. As such, voice communication in any telecommunication scheme generally requires the fixed provisioning of sufficient capacity/bandwidth.
With regard to Far End Cross-Talk (FEXT), this form of cross-talk affects non-addressed ports of a remote terminal in other words, FEXT occurs when electromagnetic interference (i.e. noise) is induced into a wireline resource that is communicating information in a similar direction, e.g. upstream (or up-link) information appears as noise in another upstream wireline resource to an extent that performance on a given pair is limited. The effects of FEXT are correspondingly reduced by the attenuation path of the wireline resource However, when multiple separate modem links exist (as supported by a multiplicity of different copper pairs proximally located towards the exchange LTE as a bundle of pairs in the access network and then fanned out to individual drops serving particular CPEs), crosstalk between the numerous signals at (or towards) the exchange presently generates noise that limits the data-rate performance of both a given pair and the entire wireline system, in general. In synchronised systems, FEXT is inherent.
FEXT on adjacent pairs can be severely exacerbated from increased signal strengths at a receiver modem of the exchange LTE for the pair causing the FEXT. More especially, where these adjacent pairs have shorter reaches (i.e. shorter cable lengths), the attenuation of the signal in the wireline resource from such relatively closely located modems (as opposed to remotely located modems) is relatively little and, correspondingly, FEXT induced into adjacent wireline resources can be relatively large. In other words, in instances when FEXT from a relatively closely located modem is introduced into a wireline resource serving a distantly located modem, the FEXT interference effects can be catastrophic and corruption of the data from the distant modem absolute. For this reason, it is accepted that there is a need to “back-off” the power transmitted by the transmitting (CPE) modem in the
Czajkowski Igor Kajetan
Humphrey Leslie Derek
Tate Christopher Neville
Ahn Sam K.
Barnes & Thornburg
Chin Stehen
Nortel Networks Limited
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