Pulse or digital communications – Systems using alternating or pulsating current – Plural channels for transmission of a single pulse train
Reexamination Certificate
1999-06-28
2004-06-01
Vo, Don N. (Department: 2631)
Pulse or digital communications
Systems using alternating or pulsating current
Plural channels for transmission of a single pulse train
Reexamination Certificate
active
06744821
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a multicarrier receiver for receiving a sequence of cyclically extended multicarrier symbols.
Such a multicarrier receiver is already known in the art, e.g. from the article ‘
A Multicarrier E
1-
HDSL Transceiver System with Coded Modulation
’ from the authors Peter S. Chow, Naofol Al-Dhohir, John M. Cioffi and John A. C. Bingham. This article was published in Vol. 4, No. 3, May-June 1993 of the Journal of
European Transodions on Telecommunications and Related Technologies
(
ETT
), pages 257-266. Therein,
FIG. 5
represents a block scheme of a multicarrier receiver which is able to receive a sequence of cyclically extended multicarrier symbols, the so called discrete multi-tone (DMT) symbols. The effect of intersymbol interference due to transmission of the DMT symbols over a channel between multicarrier transmitter and multicarrier receiver can be removed by adding a cyclic extension to each DMT symbol with a length superior to the channel impulse response length. The data rote however reduces linearly proportionally to the length of the cyclic prefix that is added to the DMT symbols so that the length of the cyclic extension of DMT symbols has to be limited to an acceptable number. If the channel impulse response is larger than the cyclic extension, remaining intersymbol interference (ISI) will depend from the part of the impulse response exceeding the cyclic extension length. To compensate for this remaining intersymbol interference (ISI), the received DMT symbols are equalised by a time domain equaliser TEQ which is an adaptive traditional linear equaliser that allows to reduce the length of the cyclic prefix of DMT symbols to an acceptable number of bits by flattening the transmission line impulse response. After being equalised in the time domain, the DMT symbols are serial-to-parallel converted, their cyclic extension is removed, and the non extended DMT symbols are applied to the input of a fast Fourier transformer FFT which demodulates the DMT symbols by converting the symbols from time domain to frequency domain.
Although different carriers or tones may be affected differently when transmitted over the channel, the time domain equaliser proposed by Peter S. Chow et al. in the above mentioned article equalises all carriers of the multicarrier symbol in the same way and as a result limits the performance of the multicarrier system unduly. Indeed, since the known equaliser cannot be optimised individually per carrier, this equaliser is not able to fully optimise the capacity of the system. Carriers which are more affected than others for instance are not equalised more intensively. If for instance part of the carriers are unused, equalisation thereof, although not necessary, is not avoided. The equalisation is performed for all carriers, because of the structure, and by no means it is possible not to equalise groups of carriers, for example the unused ones. Equivalently, in the known system the equalisation complexity does not reduce if part of the carriers ore unused and the equalisation effort cannot be concentrated on equalisation of the more affected carriers with as consequence that the performance of the known system is not fully optimised.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a multicarrier receiver of the above known type, but whose performance is increased whilst its complexity is kept at the some level, or even smaller levels.
According to the invention, this object is achieved by the multicarrier receiver defined in claim
1
.
In this way, by replacing the known time domain equaliser with a per carrier frequency domain equaliser acting on the output of a sliding Fourier transformer, channel equalisation for one carrier is made independent from channel equalisation for the other carriers. The taps for equalisation of a carrier can be set independently from the tap settings for equalisation of other carriers so that performance can be individually optimised per carrier. Furthermore, if the tapped delay lines used for equalisation of more affected carriers include more taps than tapped delay lines used for less affected carriers, equalisation effort is concentrated on the most affected carriers. In particular, if the number of taps in a tapped delay lines used for equalisation of an unused carrier is made zero, no effort is wasted to equalise such an unused carrier.
It is remarked that although at first glance, replacing the fast Fourier transformer of the known multicarrier receiver with a sliding Fourier transformer significantly increases the complexity of the multicarrier receiver, efficient implementation of this sliding Fourier transformer, increases the complexity only in a negligible way, or even allow smaller complexities as it is explained further in this application.
It is to be noticed that the term ‘comprising’, used in the claims, should not be interpreted as being limitative to the means listed thereafter. Thus, the scope of the expression ‘a device comprising means A and B’ should not be limited to devices consisting only of components A and B. It means that with respect to the present invention, the only relevant components of the device are A and B. In this respect, it is noticed for instance that the multicarrier receiver according to the present invention may be equipped with a windowing unit as described in the European Patent Application EP 0 802 649, entitled ‘Method and windowing unit to reduce leakage, Fourier transformer and DMT modem wherein the unit is used’.
Similarly, it is to be noticed that the term ‘coupled’, also used in the claims, should not be interpreted as being limitative to direct connections only. Thus, the scope of the expression ‘a device A coupled to a device B’ should not be limited to devices or systems wherein an output of device A is directly connected to an input of device B. It means that there exists a path between an output of A and an input of B which may be a path including other devices or means.
An additional feature of the multicarrier receiver according to the present invention is defined in claim
2
.
Thus, the gain in performance compared to the known system is even more increased in case the per-carrier frequency domain equaliser is provided with adaptive taps. Indeed, whereas in an adoptive version of the known multicarrier receiver, adaptation of the equaliser taps inevitably had an influence on all carriers, the taps of a tapped delay line equalising one carrier according to the present invention can be adapted independently from the taps of other tapped delay lines equalising other carriers.
A further feature of the present invention is defined in claim
3
.
Hence, in a preferred embodiment of the invention, the signal to noise ratio for transmission of multicarrier symbols over a channel between a multicarrier transmitter and the multicarrier receiver of the present invention is maximised via a mean square error criterion which allows to determine the complex tap values of the per-carrier frequency domain equaliser in which an error function expressing the mean squared difference between received and expected carrier's QAM (Quadrature Amplitude Modulation) symbols is minimised.
Another advantageous feature of the multicarrier receiver according to the present invention is defined in claim
4
.
In this way, the gain in performance compared to the known system is yet more increased, and the search for the optimal equaliser is simplified. Indeed, whereas in an advanced version of the known mulicarrier receiver, the number of equaliser taps is adjustable for all carriers in the same way, the number of taps of the tapped delay lines in the present multicarrier receiver can be increased or decreased independently from each other so that equalising effort can be concentrated on the most affected carriers whilst less affected carriers or unused carriers can be equalised slightly. If a carrier suddenly becomes more affected, the number of taps in the tapped delay line associated with this carrier is
Leus Geert Jozef Thérèse
Moonen Marc Suzanne Paul
Pollet Thierry
Van Acker Kathleen Peggie Florimond
Van de Wiel Olivier René Christian
Alcatel
Vo Don N.
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