Pulse or digital communications – Systems using alternating or pulsating current – Plural channels for transmission of a single pulse train
Reexamination Certificate
2000-03-28
2003-11-11
Pham, Chi (Department: 2631)
Pulse or digital communications
Systems using alternating or pulsating current
Plural channels for transmission of a single pulse train
C375S257000, C370S201000, C379S417000
Reexamination Certificate
active
06647067
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method and a device for reducing crosstalk interference in a transmission system which makes use of frequency translated signals, particularly a discrete multi tone (DMT) modulated transmission system or an OFDM (Orthogonal Frequency Division Multiplex) transmission system, in. which the modulation may be effected using a fast inverse Fourier transform (IFFT). The invention relates further to the transmission system itself, and to a transceiver of said transmission system.
DESCRIPTION OF RELATED ART
Digital data duplex transmission systems are currently being developed for high-speed communication. Standards for high-speed data communication over twisted-pair phone lines that have been developed include Asymmetric Digital Subscriber Lines (ADSL) and Very High Speed Digital Subscriber Lines (VDSL).
A standardized ADSL system (ANSI T1.413-1995, ATIS Committee T1E1.4), providing transmission at rates up to 8 Mbit/s over twisted-pair phone lines, defines the use of a discrete multi tone (DMT) system that uses 256 carriers or sub-channels, each 4.3125 kHz wide, in the downstream direction. In this context the downstream direction is defined as transmission from a central office (typically owned by a telephone company) to a remote location such as an end-user (i.e. residence or business user). The standard defines also the use of an oppositely directed (i.e. in the upstream direction) signal at a rate of 16 to 800 kbit/s, which is considerably lower than in the downstream direction.
A corresponding VDSL standard is intended to provide transmission up to 25.96, and preferably up to 51.92 Mbit/s, in the downstream direction and requires generally shorter phone lines than what is permitted using ADSL. Another system that is similar to VDSL is referred to as Fiber To The Curb (FTTC).
Several modulation schemes have been proposed for use in the standards mentioned above, of which most use frequency division multiplexing of the upstream and downstream directions. Other modulation systems proposed for the VDSL and FTTC systems, including multi-carrier transmission schemes such as DMT and single carrier transmission systems such as Quadrature Amplitude Modulation (QAM), use non-overlapping periodic synchronized upstream and downstream communication periods separated by a silent period. Such a system is referred to as a “ping pong” based data transmission system.
A common feature of all the above mentioned systems is that twisted-pair phone lines are used at least a part of the transmission medium that connects a central office (e.g. telephone company) to users (e.g. residence or business). It is difficult to avoid twisted-pair wiring from all parts of the interconnecting transmission medium. Even though fiber optics may be available from a central office to a curb near a user, twisted-pair phone lines are used to bring signals from the curb into the users residence or business.
The twisted-pair phone lines are grouped in a binder. While the twisted-pair phone lines are within the binder, the binder provides reasonably good protection against external electromagnetic interference. However, within the binder, the twisted-pair phone lines, being located close together, induce electromagnetic interference on each other. This type of electromagnetic interference is known as crosstalk interference. As the frequency of transmission increases, the crosstalk becomes substantial. As a result, the data signals being transmitted over the twisted-pair phone lines at high speeds can be significantly degraded by the crosstalk caused by other twisted-pair phone lines in the binder. As the speed of data transmission increases, the problem gets worse.
Conventional crosstalk cancelers have been used to reduce crosstalk. The difficulty with such conventional crosstalk cancelers is that they are very complex and consume large amounts of resources. For instance, the approach described in M. L. Honig et al., “Suppression of Near- and Far-end Crosstalk by Linear Pre- and Post-filtering, “IEE Journal on Selected Areas in Communication, Vol. 10, No. 3, pp. 614-629, April 1992, requires so much processing to implement the filtering that its benefits are overshadowed by the processing burdens.
PCT application WO 98/10528 (inventor J. F. Cioffi) proposes a system for removing crosstalk by adaptively estimating the crosstalk as induced by other of the interfering lines and canceling the crosstalk by using the estimated crosstalk. The adaptive scheme avoids processing when it is not justified in view of its processing costs. The document does not, however, address problems associated with the computational complexity of the reduction as such.
Hence, the problem of using twisted-pair phone lines at high frequency data transmission rates, such as those available using ADSL and VDSL, is that crosstalk, particularly NEXT from other lines in a binder, becomes a substantial impediment to proper reception of the transmitted data signals. Conventional NEXT cancelers are complex and do need considerable processing power for implementing the reduction.
Also in orthogonal frequency division multiplexed (OFDM) transmission systems, crosstalk may occur which systems correspondingly also needs large amounts of processing power to handle the crosstalk.
SUMMARY OF THE INVENTION
It is consequently an object of the present invention to provide a method for reducing crosstalk interference in a transmission system which makes use of frequency translated signals, particularly a discrete multi tone (DMT) modulated transmission system or an OFDM (Orthogonal Frequency Division Multiplex) transmission system, that uses less computational power as compared with known practice.
It is a further object of the invention to provide the estimated crosstalk interference at a fast rate.
It is yet a further object of the invention to provide the reducing of crosstalk interference as being implemented at least partly in specific hardware.
These objects among others are, according to one aspect of the invention, fulfilled by a method for reducing crosstalk interference induced on a signal on a first line by a signal on a second line, comprising following steps. A complex coupling factor for the crosstalk interference, which when multiplied by the signal on the second line estimates the induced crosstalk interference, is estimated, the complex coupling factor is multiplied by the signal on the second line through an approximation method operating on the signal on the second line including pre-rotation, scaling and multiplication by a complex number according to the coupling factor, whereby the complex number is chosen from a bank of predetermined complex numbers so as to obtain the best approximation possible. Finally, the product, i.e. the estimated induced crosstalk interference, obtained is subtracted from the signal on the first line.
The number of complex number provided in the bank may be calculated from a maximum acceptable error in the computation.
According to a second aspect of the present invention, there is provided a method for reducing crosstalk interference induced on a signal S
N
on a first line N by signals D
1
, . . . , D
N−1
, each on a respective line
1
, . . . , N−1.
The method comprises following steps. A coupling factor &agr;′
1
—
N
, . . . , &agr;′
N−1
—
N
is associated with the respective line
1
, . . . , N−1, said coupling factor &agr;′
j
—
N
, 1≦j≦N−1, being a complex number and when multiplied by the signal D
j
, 1≦j≦N−1, on its associated line j, 1≦j≦N−1, estimating the crosstalk interference I
j
—
N
induced on the signal S
N
on the first line N by the signal D
j
on its associated line j. The crosstalk interference I
N
on the signal S
N
on the first line N is reduced by subtracting an estimated crosstalk interference I′
N
from the signal S
N
on the first line N, said estimated crosstalk interference I′
N
being computed from said coupling factors &agr;′
1
—
Hjelm Mikael
Rudberg Mikael
Burns Doane Swecker & Mathis L.L.P.
Pham Chi
Telefonaktiebolaget LM Ericsson (publ)
Tran Khanh C.
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