Multiplex communications – Communication techniques for information carried in plural... – Combining or distributing information via frequency channels
Reexamination Certificate
1997-04-18
2002-04-09
Kizou, Hassan (Department: 2662)
Multiplex communications
Communication techniques for information carried in plural...
Combining or distributing information via frequency channels
C370S206000, C370S491000, C370S515000, C375S222000, C375S357000, C375S367000
Reexamination Certificate
active
06370156
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a method for transmitting data elements from a transmitter to a receiver, wherein the data elements are modulated on at least one carrier; wherein a pilot carrier is used for synchronisation between the transmitter and the receiver which is multiplexed with the at least one carrier; and wherein the at least one carrier and the pilot carrier are transmitted over a transmission medium interconnecting the transmitter and the receiver.
It is also directed to a transmitter adapted to transmit data elements to a receiver via a transmission medium, the transmitter comprising modulation means to a first input of which the data elements are applied, the modulation means being adapted to modulate the data elements on at least one carrier, and to multiplex the at least one carrier with a pilot carrier used for synchronisation between the transmitter and the receiver; pilot information means, adapted to generate information to identify the pilot carrier, and to apply the information to a second input of the modulation means; and line interface means, coupled between an output of the modulation means and an input of the transmission medium and adapted to condition the at least one carrier and the pilot carrier to be transmitted over the transmission medium.
It is additionally directed to a receiver adapted to receive a signal transmitted thereto by a transmitter via a transmission medium, the receiver comprising: line interface means, coupled to an output of the transmission medium and adapted to condition the signal to be applied to components of the receiver; demodulating means, an input of which is coupled to an output of the line interface means, the demodulating means being adapted to demultiplex in the signal a pilot carrier from at least one carrier whereon data elements are modulated, and to demodulate the data elements from the at least one carrier; and pilot information means, adapted to generate information to identify the pilot carrier, and to apply the information to a second input of the demodulation means.
It is still further directed to a transmission system comprising a transmitter, a receiver and a transmission medium, coupled between an output of the transmitter and an input of the receiver, wherein the transmitter and receiver are of the above described type.
BACKGROUND OF THE INVENTION
Such a method for transmitting data elements, such a transmitter and receiver, and such a transmission system are already known in the art, e.g. from the specifications of the ANSI (
American National Standards Institute, Inc.
) Standard on ADSL, the approved version of which has the reference T1E1.413-1995 and title “
Network and Customer Installation Interfaces, Asymmetric Digital Subscriber Line
(
ADSL
)
Metallic Interface
”. Therein, data elements are modulated on a set of carriers. In case of discrete multi tone (DMT) modulation, these carriers have equidistant frequencies. As is indicated in paragraphs 6.9.1.2 and 7.9.1.2 on pages 46 and 58 of the above cited standard, published in 1995, one of the carriers is reserved as a pilot carrier. This pilot carrier is used for synchronisation between transmitter and receiver and is modulated by a constant signal. In a vector plane, wherein the modulation constellation is represented by a collection of points, the pilot carrier is thus represented by a single point. On the transmission medium, e.g. on a telephone line interconnecting the ADSL transmitter and ADSL receiver in the known system, the pilot carrier thus represents a sine or cosine which does not change in phase, amplitude or frequency in time (in case a guard bond or cyclic prefix is added whose length does not contain an integer number of periods of the pilot tone, the pilot tone might be discontinuous at the edges of the DMT symbol).
A well-known source of narrowbanded or single frequency disturbances is a radio amateur or an AM radio station, which broadcasts radio signals at frequencies close to carrier frequencies. Forward error correction techniques, well-known in the art, can reduce the effect of such disturbances on data carried by the affected carriers. An alternative way to protect data against such interferers, proposed by Peter S. Chow et al. in the article “
A multicarrier E
1-
HDSL Transceiver System with Coded Modulation”
from the authors Peter S. Chow, Noafal Al-Dhahir, John M. Cioffi and John A. C. Bingham published in issue No. 3 May/June 1993 of the Journal of European Transactions on Telecommunications and Related Technologies (ETT), pages 257-266, is bitswapping: bit and energy allocations are updated so that the affected carriers carry less data bits then before. This technique requires an additional communication between transmitter and receiver.
Although data transmitted over the telephone line from the transmitter to the receiver may be protected by one of the above mentioned techniques, the presence of noise or an interferer, for instance a radio amateur signals with a frequency in the vicinity of the frequency of the pilot carrier, may still cause an offset between the received point representing the pilot carrier in the above defined vector plane and the expected point. If this offset in the vector plane is not sufficiently random, it biases the synchronisation mechanism, resulting in a performance degradation. This is e.g. the case if the instantaneous phase of the interferer is very slowly varying in time with respect to the duration of the DMT symbol or if this interferer is constant.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a method for transmitting data elements and related equipment of the known type, but wherein the immunity of the pilot carrier from interference is increased significantly, and consequently wherein the degradation of the synchronisation mechanism between transmitter and receiver is reduced.
According to the present invention, this object is achieved by a method for transmitting data elements from a transmitter to a receiver, wherein the data elements are modulated on at least one carrier; a pilot carrier used for synchronisation between the transmitter and the receiver is multiplexed with the at least one carrier; the at least one carrier and the pilot carrier are transmitted over a transmission medium interconnecting the transmitter and the receiver, characterized in that the pilot carrier is modulated with a non-constant signal before it is transmitted.
It is also achieved by a transmitter, adapted to transmit data elements to a receiver via a transmission medium, the transmitter comprising: modulation means to a first input of which the data elements are applied, the modulation means being adapted to modulate the data elements on at least one carrier, and to multiplex the at least one carrier with a pilot carrier used for synchronisation between the transmitter and the receiver; pilot information means, adapted to generate information to identify the pilot carrier, and to apply the information to a second input of the modulation means; and line interface means, coupled between an output of the modulation means and an input of the transmission medium, and adapted to condition the at least one carrier and the pilot carrier to be transmitted over the transmission medium, characterised in that the modulation means is further adapted to modulate the pilot carrier with a non-constant signal.
It is still further achieved by a receiver, adapted to receive a signal transmitted thereto by a transmitter via a transmission medium, the receiver comprising line interface means, coupled to an output of the transmission medium and adapted to condition the signal to be applied to components of the receiver; demodulating means, an input of which is coupled to an output of the line interface means, the demodulating means being adapted to demultiplex in the signal a pilot carrier from at least one carrier whereon data elements are modulated, and to demodulate the data elements from the at least one carrier; and pilot information means, adapted to generate i
Reusens Peter Paul Frans
Spruyt Paul Marie Pierre
Van der Putten Frank Octaaf
Alcatel
Kizou Hassan
Pezzlo John
Ware Fressola Van Der Sluys & Adolphson LLP
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