Multiplex communications – Generalized orthogonal or special mathematical techniques – Quadrature carriers
Reexamination Certificate
1998-04-17
2001-06-12
Patel, Ajit (Department: 2662)
Multiplex communications
Generalized orthogonal or special mathematical techniques
Quadrature carriers
C370S535000, C370S536000, C370S537000, C375S260000
Reexamination Certificate
active
06246664
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
This invention relates to a modulator for modulating digital signals, and to a demodulator.
2. Discussion of Related Art
It is known, for example from the journal “Der Fernmeldeingenieur”, 8 and 9/1993, Verlag für Wissenschaft und Leben Georg Heidecker GmbH, Erlangen, pages 51 to 56, to transmit digital signals by the single-carrier method or by the multicarrier method. Both methods use one carrier frequency per transmission channel or per subchannel of predetermined bandwidth to transmit digital signals. Quadrature amplitude modulation (QAM), for example, is applied to the digital signals, and the result is assigned to the carrier frequency of the respective transmission channel.
In an article entitled “Multicarrier Modulation for Data Transmission: An Idea Whose Time Has Come”, IEEE Communications Magazine, May 1990, pages 5 to 14, the problems associated with the overlapping of spectra of a transmission channel of predetermined bandwidth and a neighboring transmission channel are considered. If the overlap of the spectra is such that their −3 dB frequencies lie one above the other, the spectra of the individual transmission channels can still be detected at the receiver end by means of suitable filters.
The transmission of digital signals is thus limited by the bandwidth of a transmission channel. The alignment of transmission channels or subchannels in terms of frequency is limited by the overlap of the spectra into neighboring channels.
It is therefore an object of the invention to increase the transmission capacity of digital signals without changing the bandwidth of a transmission channel.
SUMMARY OF INVENTION
According to the invention, this object is attained by a modulator characterized in that it is adapted to assign digital signals to be transmitted to several carrier frequencies in a transmission channel. For example, in the simplest case, a two-bit sequence is assigned to two carrier frequencies of a transmission channel in such a way that on the occurrence of the bit sequence 01, for example, the first carrier frequency is not transmitted and the second carrier frequency is transmitted, or that on the occurrence of the bit sequence 11, both carrier frequencies are transmitted. In contrast to the prior art, where only one carrier frequency is provided per transmission channel and where consequently, on the occurrence of a bit sequence 11, the carrier frequency would have to be transmitted twice in succession, in the present example, the transmission capacity is increased by the modulator according to the invention by a factor of 2.
If two closely spaced carrier frequencies are transmitted simultaneously, overlapping of the spectra and beating between the carrier frequencies takes place, which requires at the receiver end a demodulation different from the prior art.
According to the invention, therefore, the above object is also attained by a demodulator as claimed in claim
10
. This demodulator is characterized in that there is provided at least one means adapted to detect at least three carrier frequencies to generate a bit combination in dependence on the result of the detection and on the number of possible carrier frequencies, and to combine several bit combinations generated successively in time into at least one bit stream.
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“A Discrete Multitone Transceiver System for HDSL Applications”, J. Chow et al, IEEE Journal on Selected Areas in Communications, vol. 9, No. 6, Aug. 1991, pp. 895-908.
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Alcatel
Nguyen Hanh
Patel Ajit
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