WO1998027683A2 - Traitement de canal retour dans un systeme numerique de transmission de donnees - Google Patents

Traitement de canal retour dans un systeme numerique de transmission de donnees Download PDF

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Publication number
WO1998027683A2
WO1998027683A2 PCT/US1997/022668 US9722668W WO9827683A2 WO 1998027683 A2 WO1998027683 A2 WO 1998027683A2 US 9722668 W US9722668 W US 9722668W WO 9827683 A2 WO9827683 A2 WO 9827683A2
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WO
WIPO (PCT)
Prior art keywords
signal
downstream
upstream
cap
demodulator
Prior art date
Application number
PCT/US1997/022668
Other languages
English (en)
Other versions
WO1998027683A3 (fr
Inventor
Paul Gothard Knutson
David Lowell Mcneely
Original Assignee
Thomson Consumer Electronics, Inc.
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Thomson Consumer Electronics, Inc. filed Critical Thomson Consumer Electronics, Inc.
Priority to AU56942/98A priority Critical patent/AU5694298A/en
Publication of WO1998027683A2 publication Critical patent/WO1998027683A2/fr
Publication of WO1998027683A3 publication Critical patent/WO1998027683A3/fr

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Classifications

    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L5/00—Arrangements affording multiple use of the transmission path
    • H04L5/14—Two-way operation using the same type of signal, i.e. duplex
    • H04L5/1461—Suppression of signals in the return path, i.e. bidirectional control circuits

Definitions

  • This invention concerns a digital data system conveying downstream main information and upstream return information.
  • the invention concerns a system for reducing interference associated with the return information in a data link between interface units connecting a user to a large scale network, eg., via FTTC or HFC fiber optic links.
  • Conventional data communications systems typically include a head end source where originating program material is transmitted to a large scale network, and from the network to a user receiver via local data links which may be associated with local telephone lines. Communications from the source to the user is by means of a downstream channel, and communication from the user to the source is by means of an upstream return channel .
  • the subscriber reception equipment typically includes a frequency downconverter/amplifier, a set-top box including a modem and decoder, and miscellaneous interfacing networks and connectors.
  • the set-top box is usually an addressable device which allows a system operator to provide a measure of security as well as the ability to assign a program, a channel, or groups of channels to a particular set-top from a central computer. In this manner, the system operator can remotely regulate the types of services provided to each subscriber location, eg., basic services, premium channels and pay-per-view programming. The system operator can also deactivate one or more set-top boxes as necessary. These practices involve well-known techniques.
  • a downconverter situated at the subscriber's location is used to both receive the downstream signal and to transmit the upstream return signal.
  • the downstream path typically conveys main program material to a subscriber/user, while the upstream path may be used by the user to communicate with the source of the program material, eg., a subscription video operator at the program source.
  • Some downstream and upstream applications include video on demand, home banking, security and home shopping, for example.
  • the main and return information may be conveyed via a land line modem link using conventional telephone lines.
  • the program material may be conveyed from the network to a subscriber's home by means of cabling including fiber optic links and relatively short lengths of conventional unshielded twisted pair copper wire such as is used to carry telephone information.
  • These optical links may be in the form of fiber to the curb (FTTC) where the optical fiber link ends in proximity to the user's home and conventional unshielded twisted pair (UTP) copper wire conveys the information the remainder of the way.
  • the UTP link is typically used to connect an optical fiber interface/modem network to a network including the user's set-top unit.
  • the optical link may also be in the form of hybrid fiber/coax (HFC), where the conduit to the user's home is a combination of optical fiber and copper coaxial cable.
  • HFC hybrid fiber/coax
  • CAP modulation may exhibit various formats. For example, in the case of CAP 16 modulation, each transmitted symbol has 16 different amplitude and phase characteristics, providing for a bit-rate that can be four times the symbol rate.
  • the CAP signal is produced, as is known, by passing I and Q impulse streams through orthogonal bandpass filters centered on the same frequency. Frequency shifting via modulation is not required, whereby a receiver advantageously does not require a carrier recovery tracking loop (since there is no carrier. Additional information about the nature of CAP signal processing is found in "Tutorial on Carrierless AM/PM - Part I Fundamentals and Digital CAP Transmitter" distributed by AT&T Corporation at the UTP Development Forum on September 29, 1991.
  • the upstream return channel is separated from the downstream channel by a small frequency difference.
  • a frequency difference of only 2 MHz is specified by the DAVIC standard for FTTC modems proposed by the digital audio-video council in the United States (see DAVIC spec. 1.0: 1995 (Rev5.0d). Part 8 of the DAVIC specification is particularly relevant in this context.
  • DAVIC digital audio-video council in the United States
  • the return signal exhibits higher power than the downstream signal due to attenuation that may occur in local wiring and cable connections.
  • the UTP link used to connect an input optical fiber interface network to a user's set-top unit modem may exhibit losses of about 25-30 db. at a specified frequency of 19 MHz.
  • the user modem thus receives a downstream main signal which has been highly attenuated by the UTP link, while at the same time a higher power return signal is launched into the same link.
  • the higher power return channel signal is likely to interfere with the lower power downstream signal. The potentially " degrading effect of such interference is reduced or eliminated in accordance with the principles of the present invention.
  • a bidirectional data communication system characterized by a downstream main signal path from a source to a user, and an upstream return signal path from the user to the source, it is recognized as desirable to reduce interference between main signal and the return signal by means of non-frequency selective signal cancellation of interfering information.
  • a user/subscriber "set-top" network in a video system is associated with a modem which is coupled to an input optical fiber interface via a relatively high attenuation unshielded twisted pair (UTP) link.
  • the modem demodulates a UTP-attenuated, CAP 16 modulated downstream main channel signal, and generates a higher energy QPSK (Quaternary Phase Shift Keyed) modulated upstream return signal.
  • Return signal interference in the downstream main signal is attenuated by subtracting a delayed, phase adjusted version of the modulated return signal from the main signal before the main signal is demodulated.
  • the attenuation is accomplished by using a subtraction process rather than by using a frequency selective filter.
  • FIG. 1 is a block diagram of modem system with return channel interference cancellation in accordance with the principles of the present invention.
  • Figure 2 depicts amplitude versus frequency characteristics of modulation signal formats suitable for use in the system of Figure 1.
  • FIGS 3-4 show additional details of return channel interference cancellation according to the present invention.
  • a baseband signal source 10 eg., associated with a video/data subscriber network
  • a fiber optic link to an input of an optical interface network 12.
  • This link conveys downstream main information from the source to a channel interface/modem network 12, and conveys upstream return information from network 12 to the source.
  • network 12 is commonly referred to as an optical network unit with an optical fiber input link for providing a CAP output.
  • Network 12 includes an input interface circuit 14 which has an output for providing downstream signals to a CAP 16 modulator 16 and an input for receiving upstream signals from a QPSK demodulator 18.
  • a CAP 16 modulated signal from unit 16 is applied to a downstream signal input of a diplexer 20 of conventional design, and to an input of an optional downstream interference signal canceller 22 as will be discussed subsequently.
  • Diplexer 20 separates the QPSK and CAP signals by means of a highpass filter for selectively passing the QPSK signal, and a lower frequency bandpass filter for selectively passing the CAP signal.
  • Network 12 typically is physically located so as to be able to serve several proximate buildings or user households.
  • Network elements 16 and 18 comprising unit 12 are found, for example, in a technical specification available from Lucent Technologies Corp. "Quad Multipoint Broadband Access (MVA) Network, Transmitter-Receiver” (Feb. 1996).
  • MAA Multipoint Broadband Access
  • T7664 CAP processing network
  • T7665 QPSK processing network
  • a related specification "Quad Multipoint Broadband Access, Network Convergence” describes a network suitable for performing the function of element 14 in Figure 1.
  • An input/output port of diplexer 20 is coupled to an unshielded twisted pair (UTP) copper wire link 24, such as is used to carry telephone signals.
  • Link 24 is typically less than 1000 feet in length for FTTC systems, or more than a mile in length for asynchronous digital subscriber loop (ADSL) systems.
  • Signal canceller 22 also receives an input from an upstream signal output of diplexer 20.
  • This output from diplexer 20 contains return information provided via UTP link 24 from a link interface/modem 30 as will be discussed.
  • the return information is QPSK demodulated by unit 18 before being conveyed via link 24.
  • Optional downstream signal interference canceller 22 subtracts a version of the received CAP 16 modulated signal from the QPSK modulated signal from diplexer 20 using a technique which will be discussed subsequently. This procedure helps assure that the CAP modulated signal does not distort the QPSK modulated return signal, which has been subjected to significant attenuation via UTP link 24.
  • Unit 30 is typically located in a user/subscriber's office or residence.
  • an input/output port of diplexer 32 receives CAP 16 modulated downstream main information via UTP link 24, and sends upstream return channel to interface unit 12 via modulator 38 and UTP link 24.
  • Diplexer 32 is of conventional design and is similar to unit 20.
  • a return signal input of diplexer 32 receives the QPSK " modulated upstream information from QPSK modulator 38.
  • a main signal output of diplexer 32 sends the CAP
  • Signal canceller 34 operates as will be discussed in greater detail subsequently to significantly attenuate or eliminate the QPSK modulated return signal from the main signal path prior to demodulation by unit 36, to prevent the (UTP attenuated) main signal from being distorted by the significantly higher power return signal. This is advantageously accomplished by means of a subtractive process as will also be discussed, which permits the use of economical diplexers.
  • the interference causing such distortion typically is due to the fact that the diplexers do not provide perfect signal isolation, particularly in the case of economical diplexers.
  • the demodulated CAP 16 main signal is applied to a downstream signal input of a subscriber's set-top unit 42, which provides upstream return information to QPSK modulator 38.
  • Set- top unit 42 conveys Output information such as video/television signals to user selected appliances such as a television receiver or video recorder.
  • Set-top unit 42 provides several well-known functions via the return channel, such a pay-per-view programming requests, security functions, billing inquiries,
  • the return information may be generated in a variety of ways, such as by means of a keyboard, menu based display selection, or pushbuttons, for example.
  • Set-top unit 42 of typically includes a CAP (or QAM) digital demodulator, MPEG transport processor, MPEG video and audio processors, an upstream return signal processor including a QPSK modulator as discussed, as well as a central processing unit (CPU) and associated RAM memory.
  • CAP or QAM
  • MPEG transport processor MPEG transport processor
  • MPEG video and audio processors MPEG video and audio processors
  • upstream return signal processor including a QPSK modulator as discussed
  • CPU central processing unit
  • RAM memory random access memory
  • Figure 2 illustrates the amplitude-versus-frequency spectrums of CAP and QPSK signals which may be associated with
  • Figure 2 illustrates the Class B FTTC spectrum associated with the DAVIC standard. Also shown for comparison is a POTS ("Plain Old Telephone Signal") spectrum.
  • the POTS signal is a relatively narrowband signal in the lowermost end of the frequency spectrum.
  • the downstream CAP is a relatively narrowband signal in the lowermost end of the frequency spectrum.
  • the 16 modulation spectrum occupies a bandwidth of from 1MHz to 17 MHz.
  • the amplitude of the illustrated CAP 16 spectrum represents that of the CAP 16 signal after attenuation by UTP link 24.
  • the QPSK return path modulation spectrum exhibits a much narrower bandwidth and a significantly larger amplitude prior to being applied to UTP link 24 from diplexer 32.
  • the relatively high power QPSK return signal is located adjacent in frequency to the weaker CAP downstream main signal.
  • the inventors have recognized that, with this frequency proximity, keeping the downstream CAP signal separated from the upstream QPSK return channel signal by means of filter-based separation may be difficult since the return channel signal may be 30 db to 50 db larger than the main signal.
  • set-top unit 42 Since set-top unit 42 generates the return signal, its character is known, and it is generated with a clock that is related to the downstream symbol rate. Specifically, the upstream return path symbol rate is a sub-multiple of the downstream symbol rate.
  • phase and amplitude adjusted version of the locally generated return signal may be subtracted from the CAP signal applied to demodulator 36 to reduce return signal signal interference in the downstream path prior to unit 42. This is preferably accomplished in the digital domain, but analog domain processing is also possible.
  • FIG 3 is a block diagram illustrating the process by which the return signal interference is reduced or eliminated.
  • elements 36 and 38 correspond to similarly numbered elements in Figure 1, and signal canceller 34 of Figure 1 is shown in greater detail.
  • a received signal from diplexer 32 ( Figure 1) is applied to a non-inverting input of a subtractive combiner 56 and to an input of a digital correlator 60.
  • Another input of correlator 60 receives the QPSK modulated output signal from unit 38.
  • a FIR digital filter 64 receives the QPSK modulated signal from unit 38 and responds to a control signal from correlator 60.
  • the output signal from filter 64 is applied to an inverting (-) input of subtractive combiner 56, where the QPSK modulated upstream return signal from filter 64 is subtracted from the downstream signal from diplexer 32.
  • the output signal from subtractor 56 as applied to demodulator 36 exhibits reduced or eliminated return signal interference.
  • phase adjustment of the QPSK return signal is accomplished by correlating (via unit 60) the corrected receiver diplexer output with the transmitted QPSK signal from unit 38.
  • the results of the correlation provided by correlator 60 are used to determine the coefficients of FIR filter 64.
  • the following expressions apply:
  • R(w) is the received signal output of diplexer 32
  • Q(w) is the QPSK signal from unit 38
  • Q*(w) is the complex conjugate of Q(w);
  • H(w) is the frequency response of diplexer 20
  • S(w) is the received CAP 16 signal applied to 56, 60, and
  • a digital least-mean-square (LMS) filter may be used, as shown in Figure 4, to cancel the undesired signal in a feedback configuration.
  • an adaptive LMS filter 72 receives the undesired QPSK signal from unit 38, and provides an output signal to an inverting (-) input of a subtractive combiner 74.
  • the non-inverting (+) input of combiner 74 receives the desired signal (CAP) plus the undesired signal (QPSK) to be attenuated or cancelled.
  • the output signal of combiner 74 is the desired CAP signal with return channel interference removed.
  • Th e output of combiner 74 is applied to a coefficient control input of adaptive LMS filter 72, which also receives as its signal input the (undesired) QPSK signal from unit 38.
  • the output of adaptive filter 72 is the inverse of the undesired component of the desired + undesired signal input to combiner 74, whereby the undesired (QPSK) component is cancelled or attenuated. Additional information concerning the operation of an adaptive filter such as filter 72 may be found in chapter 12 of the text Adaptive Signal Processing by Widrow and Stearns (Prentice Hall).
  • the interference cancellation technique described above can also be applied to interface unit 12 prior to UTP link 24 by using downstream canceller unit 22.
  • downstream CAP modulated signal from unit 16 would be considered as the interferor signal to be attenuated in the return path, and the QPSK modulated signal conveyed from diplexer 20 to demodulator 18 after attenuation by UTP link 24 would be considered as the desired signal.
  • the components of unit 22 would be similar to those discussed in connection with Figures 3 and 4, but would be arranged to achieve this result in accordance with the principles discussed in connection with Figure 3.
  • HFC cable modems have a similar return channel interference problem, which could be resolved using the interferor signal cancellation techniques according to the present invention as described above.
  • the disclosed system is also useful in association with systems using the asynchronous digital subscriber loop (ADSL) data transmission format with HFC and HTTC links.
  • ADSL asynchronous digital subscriber loop

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  • Engineering & Computer Science (AREA)
  • Signal Processing (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Radio Relay Systems (AREA)
  • Two-Way Televisions, Distribution Of Moving Picture Or The Like (AREA)

Abstract

Dans un système de télévision par satellite, un coffret (42, 44) d'utilisateur/abonné est associé à un modem (30) couplé à une interface d'entrée (12) en fibre optique, par l'intermédiaire d'un câble (24) à paires torsadées (UTP) non blindées d'atténuation relativement élevée. Le modem démodule (36) un signal principal aval d'atténuation UTP modulé en phase d'amplitude sans porteuse (CAP16), puis génère (38) un signal de retour amont de plus haute énergie modulé en QPSK. L'interférence du signal de retour dans le signal principal reçu est atténuée (34) par soustraction d'une configuration différée, réglée en phase, du signal de retour modulé à partir du signal principal avant démodulation du signal principal. L'élimination de l'interférence s'effectue au moyen d'un procédé de soustraction plutôt que d'un procédé de filtrage sélectif en fréquence. L'élimination de l'interférence peut également s'effectuer (22) au niveau de l'interface (12) précédant la liaison UTP.
PCT/US1997/022668 1996-12-19 1997-12-10 Traitement de canal retour dans un systeme numerique de transmission de donnees WO1998027683A2 (fr)

Priority Applications (1)

Application Number Priority Date Filing Date Title
AU56942/98A AU5694298A (en) 1996-12-19 1997-12-10 Return channel processing for a digital data communications system

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
US77381596A 1996-12-19 1996-12-19
US08/773,815 1996-12-19

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WO1998027683A2 true WO1998027683A2 (fr) 1998-06-25
WO1998027683A3 WO1998027683A3 (fr) 1998-08-20

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Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP1069739A2 (fr) * 1999-07-14 2001-01-17 Fujitsu Limited Elimination d'une tension en mode commun dans un récepteur différentiel

Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0229524A1 (fr) * 1985-12-23 1987-07-22 Fujitsu Limited Modem à multiplexage de fréquence en duplex avec annulation de l'écho
WO1995017046A1 (fr) * 1993-12-17 1995-06-22 Bell Communications Research, Inc. Compensateur d'echo ameliore pour systeme a multitonalite discrete
DE29605116U1 (de) * 1996-03-19 1996-05-30 Siemens Ag Übertragungssystem zur Übertragung von hochbitratigen Digitalsignalen in Frequenzgetrenntlage über symmetrische Cu-Doppelader-Leitungen

Patent Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0229524A1 (fr) * 1985-12-23 1987-07-22 Fujitsu Limited Modem à multiplexage de fréquence en duplex avec annulation de l'écho
WO1995017046A1 (fr) * 1993-12-17 1995-06-22 Bell Communications Research, Inc. Compensateur d'echo ameliore pour systeme a multitonalite discrete
DE29605116U1 (de) * 1996-03-19 1996-05-30 Siemens Ag Übertragungssystem zur Übertragung von hochbitratigen Digitalsignalen in Frequenzgetrenntlage über symmetrische Cu-Doppelader-Leitungen

Cited By (8)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP1069739A2 (fr) * 1999-07-14 2001-01-17 Fujitsu Limited Elimination d'une tension en mode commun dans un récepteur différentiel
EP1069739A3 (fr) * 1999-07-14 2004-04-21 Fujitsu Limited Elimination d'une tension en mode commun dans un récepteur différentiel
US6826390B1 (en) 1999-07-14 2004-11-30 Fujitsu Limited Receiver, transceiver circuit, signal transmission method, and signal transmission system
EP1594274A2 (fr) * 1999-07-14 2005-11-09 Fujitsu Limited Elimination d'une tension en mode commun dans un récepteur différentiel
EP1594274A3 (fr) * 1999-07-14 2005-11-16 Fujitsu Limited Elimination d'une tension en mode commun dans un récepteur différentiel
US7389097B2 (en) 1999-07-14 2008-06-17 Fujitsu Limited Receiver, transceiver circuit, signal transmission method, and signal transmission system
US7822403B2 (en) 1999-07-14 2010-10-26 Fujitsu Limited Receiver, transceiver circuit, signal transmission method, and signal transmission system
US7991359B2 (en) 1999-07-14 2011-08-02 Fujitsu Limited Receiver, transceiver circuit, signal transmission method, and signal transmission system

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Publication number Publication date
WO1998027683A3 (fr) 1998-08-20
AU5694298A (en) 1998-07-15

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