Multiplex communications – Generalized orthogonal or special mathematical techniques
Reexamination Certificate
1996-11-29
2001-03-13
Luther, William (Department: 2664)
Multiplex communications
Generalized orthogonal or special mathematical techniques
C370S208000, C370S210000, C370S343000, C370S480000
Reexamination Certificate
active
06201785
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process for transmitting a digitized signal according to OFDM (Orthogonal Frequency Division Multiplexing) type multicarrier modulation using N orthogonal channels. The present invention also relates to a transmitter and a receiver for the implementation of this process.
2. Discussion of Background
In the international Patent Application PCT/FR 89/00546 filed in the name of THOMSON-CSF, there is described a particularly high-performance process for transmission using OFDM type multicarrier modulation making it possible to transmit, with a high density, a digitized signal. In the transmission process described, a plurality of orthogonal frequencies is used and an (amplitude/phase) or (real part/imaginary part) pair is transmitted on each frequency, the amplitude/phase or real part/imaginary part pair being one-to-one equivalent to the information to be sent. Moreover, in order to recover at the receiver the information sent according to this process whilst taking into account multiple echoes, various techniques are used such as the associating with the useful transmission interval of a transition interval or guard interval, the use of synchronization signals and especially the use of test-packets which, on reception, enable equalization of the channels to be performed. However, when wishing to use a very large number N of channels, for example 1024 channels, there is in this case a very large spectral occupancy. Indeed, the frequency band used is at least 16 MHz. This poses a number of problems in regard, for example, to the frequency of operation of the signal processing circuits.
SUMMARY OF THE INVENTION
The purpose of the present invention is to propose a novel process for transmitting a digitized signal according to OFDM type multicarrier modulation which enables the frequency band used to be narrowed.
The purpose of the present invention is also to propose a process for transmitting a digitized signal according to OFDM type multicarrier modulation using N orthogonal channels, which can be used equally well with modulation of the type described in international Patent Application PCT/FR 89/00546 as with other OFDM type multicarrier modulations.
Accordingly, the subject of the present invention is a process for transmitting a digitized signal according to OFDM (Orthogonal Frequency Division Multiplexing) type multicarrier modulation using N orthogonal channels, characterized in that the channels are divided into two, N/2 channels corresponding to a first frequency band being sent via a first transmission track according to a first polarization and the remaining N/2 channels being brought back into the same frequency band and sent via a second transmission track according to a second polarization crossed with respect to the first.
By using this novel transmission process it is possible to have a much larger number of channels for the same frequency band. Moreover, independence between channels is achieved owing to the fact that each channel is orthogonal to the other channels. Thereby, the maximum amplitude of a channel corresponds to the zero of all the other channels as explained in the abovementioned PCT application. Accordingly, if there is coupling between the two polarizations each transporting half of the channels, on decoding, its influence on the channels is minimized.
According to another characteristic of the present invention, the digitized signal transmitted on each track includes, periodically, test-packets allowing the send channel to be equalized on reception.
This feature makes it possible to compensate in particular for the attenuation of one polarization with respect to the other, from the moment when the variations in the conditions of propagation of a polarization are slower than an equalization cycle.
According to a particular embodiment, two successive test-packets are transmitted per track and the test-packets are chosen in such a way that:
t
A
1
(
2
q
)
≠
0
;
t
A
2
(
2
q
)
=
0
;
t
B
1
(
2
q
)
=
0
;
t
B
2
(
2
q
)
≠
0
t
A
1
(
2
q
+
1
)
=
0
;
t
A
2
(
2
q
+
1
)
≠
0
;
t
B
1
(
2
q
+
1
)
≠
0
;
t
B
2
(
2
q
+
1
)
=
0.
The present invention also relates to a transmitter for the implementation of the above process, characterized in that it principally includes a circuit for calculating an inverse fast Fourier transform (FFT
−1
) for N samples, a first send track for N/2 samples and a second send track for the remaining N/2 samples, the two tracks allowing sending in one and the same frequency band but with cross-polarizations.
The subject of the present invention is also a receiver for receiving signals transmitted according to the above process, characterized in that it includes two reception tracks each able to receive the signals corresponding to N/2 samples, transmitted in one and the same frequency band but with cross-polarizations and a circuit for calculating a fast Fourier transform (FFT) enabling N samples to be processed or, according to a variant embodiment, a circuit for calculating a fast Fourier transform (FFT) enabling N/2 samples to be processed, connected at the output of each track and a circuit for coupling the two circuits for calculating a fast Fourier transform (FFT).
REFERENCES:
patent: 3742149 (1973-06-01), Yoshida et al.
patent: 4112370 (1978-09-01), Monsen
patent: 4385378 (1983-05-01), Kreutel, Jr.
patent: 4521878 (1985-06-01), Toyonaga
patent: 4811021 (1989-03-01), Yoshimoto et al.
patent: 5357502 (1994-10-01), Castelain et al.
patent: 5471464 (1995-11-01), Ikeda
patent: 9004893 (1990-05-01), None
Couasnon Tristan de
Fouche Yvon
"Thomson-CSF"
Luther William
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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