(12) Ulllted States Patent
`Thoumy et al.
`
`(10) Patent No.:
`(45) Date of Patent:
`
`US 7,039,120 B1
`May 2, 2006
`
`US007039l20Bl
`
`(54) DEVICE AND METHOD FOR THE
`DYNAMIC ALLOCATION OF EREOUENCIES
`FOR MULTICARRIER MODULATION
`SYSTEMS
`
`.............. .. 375/260
`5,479,447 A * 12/1995 Chow et a1.
`5,636,247 A
`6/1997 Kamerman et al.
`....... .. 375/260
`
`FOREIGN PATENT DOCUMENTS
`
`(75)
`
`Inventors: Franeois Thoumy, Chevaigne (FR);
`Philippe Le Bars, Nouvoitou (FR);
`Samuel Rousselin, Rennes (FR);
`Lionel Le Scolan, Rennes (FR);
`Frédérique Ehrmann, Rennes (FR)
`
`EP
`Ep
`GB
`W0
`W0
`
`8
`0 905 948 A2
`0 923 034 A2
`2 280 571 A
`W0 97/ 16046
`WO 99/39484
`
`3/1999
`7/I999
`2/1995
`5/1997
`8/1999
`
`(73) Assignee: Canon Kabushiki Kaisha, Tokyo (JP)
`
`( * ) Notice:
`
`Subject to any disclaimer, the term of this
`Patent is extended er adjusted under 35
`U~S~C~ 154(1)) by 0 daY5~
`
`OTHER PUBLICATIONS
`
`“Bit Significance Selective Frequency Diversity Transmis-
`sion,” Kumagai, et al., IEICE Transactions on Communica-
`tions, vol. E8l—B, No. 3, Mar. 1998, pp. 545—552.
`
`(21) Appl. No.: 09/450,716
`(22)
`Filed:
`Nov. 30, 1999
`
`* “ted by e"am‘“er
`
`Foreign Application Priority Data
`(30)
`Nov. 30, 1998
`(FR)
`.......................................... .. 98 15042
`Jan. 29, 1999
`(FR)
`99 0134
`
`.......................................... .. 99 12648
`Oct. ll, 1999
`(FR)
`
`Primary Examl.neriKhai Tran
`(74) Altar/1ey,Age/12, orFirm—Fitzpatrick, Cella, Harper&
`Scinto
`
`(51)
`
`Int. Cl.
`H04L 27/10
`
`(2006.01)
`
`...................................... .. 375/275; 375/260
`(52) U.S. Cl.
`(58) Field of Classification Search ............... .. 375/275,
`375/271, 279, 260, 281, 222, 246
`See application file for Complete Search history.
`
`(55)
`
`References Cited
`U.S. PATENT DOCUMENTS
`>I<
`
`5,313,467 A
`
`5/1994 Varghese et al.
`
`.......... .. 370/468
`
`(57)
`
`ABSTRACT
`
`The method of transmitting data in the form of symbols sent
`over an electromagnetic channel is characterized in that a
`measurement of eignifieanee is attfibutesi to each. grettp of
`data to be transmttteds In that the transmlssten rehablhty of
`the carriers is estimated dynamically, and in that the most
`sigmiicant data are sent over the most reliable carriers at
`each instant, the other data being sent over the carriers of
`decreasing reliability, in decreasing order of significance of
`the data.
`
`101 Claims, 30 Drawing Sheets
`
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`
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`
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`UNIT
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`
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`
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`CONTROL
`UNIT
`
`
`
`FREQUENCY
`
`ALLOCATION
`UNIT
`
`11!
`
`
`
`EXTRACTOR
`UNIT
`
`
`
`
`DEMODULATOR
`
`

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`U.S. Patent
`
`May 2, 2006
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`May 2,2006
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`Sheet 13 of 30
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`US 7,039,120 B1
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`U.S. Patent
`
`May 2,2006
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`US 7,039,120 B1
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`Sheet 25 of 30
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`US 7,039,120 B1
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`TRANSMISSION REQUEST
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`
`U.S. Patent
`
`May 2,2006
`
`Sheet 26 of 30
`
`US 7,039,120 B1
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`
`U.S. Patent
`
`May 2,2006
`
`Sheet 27 of 30
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`US 7,039,120 B1
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`

`
`U.S. Patent
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`May 2,2006
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`Sheet 28 of 30
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`US 7,039,120 B1
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`U.S. Patent
`
`May 2,2006
`
`Sheet 30 of 30
`
`US 7,039,120 B1
`
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`US 7,039,120 B1
`
`1
`DEVICE AND METHOD FOR THE
`DYNAMIC ALLOCATION OF FREQUENCIES
`FOR MULTICARRIER MODULATION
`SYSTEMS
`
`BACKGROUND OF THE INVENTION
`
`1. Field of the Invention
`The invention relates to the transmission of information in
`
`the
`the form of an electromagnetic signal. In particular,
`invention concerns the allocation of carrier frequencies for
`transmission systems using multi-carrier type modulation
`and the process for reducing the ratio of peak amplitude to
`mean amplitude usually required in transmission systems
`using multi-carrier type modulation.
`2. Description of the Related Art
`An information transmission system generally sends sym-
`bols serially, where each symbol can be a sequence of binary
`data. Consequentially, the frequency band required to send
`the symbols must be larger than the inverse of the length of
`a symbol. When the symbol transmission rate becomes to
`high, the charmel must have identical amplitude and phase
`characteristics over the entire space of the frequencies which
`constitute the passband. Any distortions will give rise to
`interference between symbols, which must be negated with
`an equalizer.
`One method of avoiding this problem is to distribute the
`signal, formed by a stream of symbols, over a plurality of
`parallel carriers, which are individually modulated at a low
`transmission rate. Because the transmission rate is low for
`
`each carrier, the passband required is smaller and therefore
`the frequency and phase characteristics will more likely be
`identical for all the frequencies constituting this band.
`This technique is generally known as frequency division
`multiplex, and is used to select carriers so as to avoid
`interference. One particular case is Orthogonal Frequency
`Division Multiplex, or OFDM,
`in which the spacing
`between two adjacent subcarriers (the closest subcarriers in
`terms of frequency) corresponds to the inverse of the length
`of a symbol sent.
`As a result of flaws in the transmission charmel, a trans-
`mitted symbol can contain errors on reception, and, if the
`errors are detected, retransmission may be required.
`To improve this situation, it is possible to transmit a series
`of blocks of symbols, where each of these blocks is a
`discrete Fourier transform or inverse Fourier transform of a
`
`corresponding block of information symbols.
`The advantage of this technique is that all of the symbols
`received will be affected by only a small evaluation error in
`the event of a transmission charmel problem. If the proce-
`dure were not applied, a single symbol would be affected by
`a large evaluation error, leading to erroneous detection. It is
`an object of the invention to correctly evaluate all symbols
`using the fast Fourier transforms demodulation technique.
`This technique is also a particular method of OFDM. To
`appreciate the similarity of this technique to OFDM, refer-
`ence can be made to chapter 15 of “Modem Quadrature
`Amplitude Modulation Principles and Applications for
`Fixed and Wireless Charmels,” by W. T. Webb and L. Hanzo.
`OFDM operates as follows. Initially, a complex vector
`comprising 11 components for transmission is transformed
`with an inverse Fast Fourier Transform (IFFT). The complex
`vector can be, for example, complex numbers forming part
`of a whole creating an alphabet, or code adapted to corre-
`spond to the different sequences of data for transmission.
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`The transformation by IFFT may be by a matrix product of
`an inverse Fast Fourier transform matrix, or “Fourier
`Matrix”, into 11 rows and 11 columns by the vector of the 11
`data elements for transmission. The alphabet is generally
`that of the phase and amplitude modulations.
`A vector of n complex numbers, known as a “transformed
`vector,
`is generated from this matrix product. The trans-
`formed vector forms a succession of numbers the amplitudes
`of which are transmitted successively by the device. This
`series of amplitudes, referred to as a baseband OFDM
`symbol corresponds to a sequence of 11 data elements for
`transmission.
`
`This signal can itself modulate a carrier of a higher
`frequency to be able to be transmitted in a transposed band,
`according to a conventional technique.
`Baseband reception or demodulation occurs by multiply-
`ing the received transformed vector with the matrix of the
`direct Fast Fourier transform, or the matrix of the inverse
`transform if the direct Fast Fourier transform was used on
`
`transmission). The received vector is a counterpart of the
`vector obtained from OFDM transmission, although it has
`been subjected to interference, noise addition or partial
`fading.
`therefore, restore the
`OFDM demodulation does not,
`initial components of the complex vector associated with the
`sequence of data for transmission, but rather it approximates
`the components. The information is restored after a decision
`making process which consists of measuring the distance of
`each component calculated after reception at each point of
`the encoding alphabet used for transmission, and of assimi-
`lating the component calculated after reception to the point
`of the alphabet that corresponds to the shortest distance.
`Instead of having most of the data received perfectly and
`a few data elements completely lost, as in conventional
`series transmission,
`the transmission errors are,
`in fact,
`distributed over all of the points, which ensures that it is
`almost always possible to reconstitute the initial information
`in its entirety.
`This conventional mode of transmission using OFDM
`does, however, have a major drawback. Through the effect
`of the matrix product, the discrete Fast Fourier transform
`creates a linear combination of the 11 symbols for transmis-
`sion and a number of critical complex vectors, associated
`with critical sequences of data. This combination can result,
`after the Fourier transform, in transformed vectors wherein
`the succession of amplitudes of the components have local
`maximum values corresponding to signal peaks that are
`substantial, in relation to the mean value for the amplitudes
`of the components of the transformed vector.
`The peak amplitude to mean amplitude ratio of the
`transformed vectors corresponding to these critical
`sequences or critical complex vectors is thus very high.
`Such critical sequences may cause difficulties for the
`downstream devices as, in practice, amplifiers and modula-
`tors may lack the fidelity to process swift amplitude varia-
`tions. As a result clipping, which is the non-transmission of
`the signal peaks, may occur, resulting in the loss of corre-
`sponding information. Furthermore, harmonic distortion,
`one of the major problems of transmission systems, may be
`introduced, and may be impossible to negate.
`Theoretically, maximum amplitude is calculated to be a
`direct function of the length of the sequence of symbols for
`transmission.
`
`It is thus highly desirable to reduce this maximum ampli-
`tude so as to use the full dynamic properties of the amplifiers
`
`

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`US 7,039,120 B1
`
`3
`without causing clipping or distortion. Several solutions
`aimed at alleviating this problem of peaks are known. One
`of these techniques is to exclude sequences of symbols
`creating maximum peak-to-mean amplitude ratio values of
`the OFDM symbol. This is achieved by encoding
`redundancies, resulting in a reduction in the transmission
`rate of useful symbols. One example of implementation of
`this solution is described in U.S. Pat. No. 5,636,247.
`Another solution is to calculate the inverse Fourier trans-
`
`form for the sequences of symbols to be transmitted, and
`then to measure the peak-to-mean ratios for the transformed
`vectors thus obtained, and, by looping, to change the phases
`of the components of the critical complex vectors corre-
`sponding to the peaks. Measurement of these peaks involves
`calculating another discrete Fourier transform. A technique
`of this kind is disclosed in U.S. Pat. No. 5,610,908. A third
`solution is to change the coefficients of the Fourier matrices
`(inverse and direct) so as to avoid or limit the occurrence of
`these peaks. This process induces a slight deterioration in the
`bit error rate. By way of example, one solution of this type
`has been proposed by patent application FR 98.13261.
`All these currently implemented solutions have the draw-
`back either of adversely affecting the bit rate of the
`transmission, impairing the quality of the transmission, or of
`being complicated.
`A conventional multicarrier transmission system of stan-
`dard type (see FIG. 1) has a data source 910 a serial-to-
`parallel converter 920 connected to a stream of subcarriers,
`and a multi-carrier modulator 950 which transmits the data
`
`to an RF transmitter 960. In a standard system of this type,
`the data are distributed sequentially over the different sub-
`carriers. For example, for a system using eight subcarriers,
`the data bearing the numbers 0, 8, 16, 24 will be transmitted
`over the subcarrier 930 of frequency 001, the data bearing the
`numbers 1, 9, 17, 25 will be transmitted over the subcarrier
`frequency 002 etc.
`In a conventional device not according to the invention,
`this stream of the “serial” type is converted into a “parallel”
`stream by the serial-to-parallel converter 920, so as to reduce
`the transmission rate of the modulating signals. This parallel
`stream is then sent to the multi-carrier modulator 950, which
`effects the modulation necessary for the transmission over
`the chosen transmission charmel.
`
`In the example presented, the serial stream is transformed
`into a parallel stream in eight bits. In this case,
`if the
`transmission rate of the binary source is D, the rate of each
`stream at the output of the serial-to-parallel converter 920
`will therefore be D/8.
`
`Each of these stream then modulates a subcarrier by virtue
`of the subcarriers 930 to 937. The modulation can be of
`
`different types: phase, amplitude or frequency modulation,
`according to conventional techniques.
`An adder 940 next adds all the modulated subcarriers so
`
`as to obtain the global signal S(t), which is then transmitted
`to the RF transmitter 960.
`
`.
`.
`It is significant to note that the binary data X01, X1, X2, .
`X7 issuing from the serial-to-parallel converter 920 and used
`for modulating the subcarriers, can consist of several bits.
`They will then more generally be referred to as “symbols”.
`In this case the modulations employed can be complex (for
`example according to types known to persons skilled in the
`art as QPSK, 8PSK, 16QAM, 64QAM etc) in order to
`improve the spectral efficiency.
`These elements constitute a conventional multicarrier
`
`device, known to persons skilled in the art. It will therefore
`not be detailed any further in the present description.
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`The majority of the transmission channels, or “radio”
`channels used have transmission characteristics, such as
`attenuation, noise, or phase displacement, which vary
`depending on the carrier frequency used. Certain charmels
`have characteristics which vary over time, because of “mul-
`tipat ” effects, such as the presence of elements entering the
`channel.
`
`FIG. 2 depicts an example of a symbolic representation of
`the transmission quality, quantified by a signal to noise
`ration, or “SNR”, on each of the subcarriers in the case of
`eight subcarriers, at two different times, Time t1 and Time
`t2. The transmission characteristics for each frequency vary-
`ing with time, it is found in the example that the data item
`X6 is correctly transmitted at time tl, but may be erroneous
`at time t2.
`
`The concept of efficiency of such a multicarrier transmis-
`sion is then related to the resolution of the following
`problem: with what power P must transmission be carried
`out in order to ensure the transmission of a certain output of
`data D with a quality Q in a given physical transmission
`channel?
`
`This efficiency can be defined as the ratio
`(transmission rate x quality)/emitted power
`The solution generally adopted for this problem of trans-
`mission efficiency is a compromise between on the one hand
`the energy emitted during transmission over the transmis-
`sion charmel and on the other hand the acceptable error rate
`for the transmitted data.
`
`The operating principle of the majority of existing devices
`is to increase the transmission power in order to counteract
`the degradation of the transmission channel and to transmit
`all the data with guarantee of an error rate below a prede-
`termined threshold.
`
`Several techniques have been disclosed for improving the
`efficiency of transmission.
`These techniques are based on a different coding for the
`data considered to be the most significant, before sending
`over the transmission channel.
`
`A technique disclosed in U.S. Pat. No. 5,425,050 intro-
`duces a concept of pyramidal coding, in which two classes
`of data requiring two different transmission quality levels
`are created.
`
`U.S. Pat. No. 5,467,132 describes a method for coding the
`data differently according to their significance.
`Other techniques are based on a dynamic estimation of the
`transmission quality on each subcarrier, and on a modifica-
`tion of number of bits per symbol transmitted in order to take
`account of this variation in transmission quality. U.S. Pat.
`No. 5,479,447 describes one example of this technique.
`In summary, the conventional solutions to this problem of
`multicarrier transmission efficiency are:
`increasing the transmission power so as always to trans-
`mit with a sufficient signal/noise level,
`testing the transmission charmel and eliminating the sub-
`carriers most interfered with,
`adding redundancy to the data by coding,
`modifying the number of bits per symbol for the subcar-
`riers interfered with.
`All these solutions result in an increase in the emitted
`
`energy for transmitting the same data stream with a constant
`quality.
`
`BRIEF SUMMARY OF THE INVENTION
`
`The present invention relates to an improved method of
`efficiently transmitting information using multi-carrier
`modulation.
`
`

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`US 7,039,120 B1
`
`5
`According to a second objective of the invention, noisy
`carriers may be utilized, even noisy carriers normally
`rejected by conventional techniques.
`According to another objective, the invention proposes a
`dynamic transmission method which makes it possible to
`preserve optimum efficiency when there are variations in
`characteristics of the transmission channel.
`
`Another objective of the invention is to guarantee correct
`transmission of the most significant data, simply to a suffi-
`cient SNR.
`
`Another object of the method is to reduce the emitted
`energy during transmission, compared with the existing
`techniques, for equal efficiency.
`To this end, the invention proposes under a first aspect a
`method of transmitting data using a modulation of the
`multicarrier type, comprising operations of:
`extraction from received data of a first signal representing
`the transmission quality on each sub-carrier observed
`and transmitted by a remote device;
`allocation of transmission data to the sub-carriers in an
`
`order based on significance of the transmission data and
`the first signal representing the transmission quality,
`and
`
`insertion in transmission data of a second signal repre-
`senting the order in which the transmission data are
`allocated to the sub-carriers based on the significance
`of the transmission data and the first signal;
`It will be understood that this method takes account of the
`
`significance of the data, not at the time of coding, but at the
`time of allocation of a transmission frequency, in an adaptive
`fashion, when the reliability of the carriers (error rate during
`transmission) varies, and by taking advantage even of the
`very noisy carriers, for transferring data of lesser signifi-
`cance. This possible use of the noisy carriers affords a gain
`in efficiency compared with conventional
`techniques,
`in
`which these carriers would have been avoided, and in which
`a single error rate common to all the data transmitted is
`defined in advance.
`
`In addition, the dynamic character of the method guaran-
`tees that, in the event of high degradation of the reliability
`of the transmission channel, the most significant data will
`always be transmitted as a priority (that is to say over the
`most reliable channel) with the greatest possible quality at
`this moment.
`
`Taking into account the significance of the data gives a
`good saving in energy compared with current methods since
`in this way it is possible to reduce the transmission power
`without impairing the quality of transmission of the signifi-
`cant data, and therefore without any risk of loss of the
`substance of the message.
`This is an improvement compared with the current meth-
`ods in which transmission is carried out with sufficient
`
`transmission power to guarantee a given SNR for all fre-
`quencies used.
`Similarly, the invention proposes a method of receiving
`data using a modulation of a multicarrier type, comprising
`operations of
`analysis of transmission channel so as to supply a signal
`representing transmission quality of each sub-carriers
`in a return direction;
`extraction from received data of a signal representing an
`order in which the transmission data are arranged by a
`transmission device on the sub-carriers; and
`formation of the received data according to the signal
`representing the order in which the transmission data
`are arranged by the transmission device.
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`According to a preferred implementation, the received
`data are serialized in said formation operation according to
`the signal representing the order.
`The invention also relates to a device for transmitting data
`to a remote device, comprising:
`means for allocating the transmission data to the sub-
`carriers in an order based on significance of the trans-
`mission data and transmission quality of the sub-
`carriers; and
`means for inserting in the transmission data of a signal
`representing the order in which the transmission data
`are allocated on the sub-carriers based on the signifi-
`cance of the transmission data and the transmission
`quality of the sub-carriers.
`According to a preferred implementation of this device,
`said allocating means allocates the transmission data to the
`sub-carriers in the order based on the transmission quality of
`the sub-carriers observed and transmitted by a reception
`device.
`According to a preferred implementation,
`comprises premodulator means including:
`inputs of the
`a means of presenting,
`to the different
`modulator, each input corresponding to a subcarrier,
`different data to be transmitted according to a classifi-
`cation of their significance as well as the transmission
`quality level of each subcarrier in the “outward” direc-
`tion A—>B,
`a means of inserting in the data to be transmitted a signal
`representing the transmission quality observed in each
`subcarrier in the “return” direction B—>A,
`and a means of inserting, in the data, a signal representing
`the order in which there are arranged the different data
`to be transmitted at the input of the premodulator,
`and the device also has:
`
`the device
`
`a post-demodulator means including:
`a means of extracting, from the signal issuing from the
`demodulator, an FCD signal representing the transmis-
`sion quality observed by the remote device B on each
`subcarrier in the “outward” direction A—>B, said signal
`being generated by the remote device B,
`and a means of analyzing the transmission channel so as
`to supply the signal representing the quality of the
`transmission of each subcarrier in the “retum” direction
`B—>A,
`a means of extracting, from the signal issuing from the
`demodulator, a signal representing the order in which
`there were arranged the different data to be transmitted
`at the input of the premodulator of the remote device B,
`and a means of serialising the data received as a function
`of the DP signal representing the order in which there
`were arranged the different data to be transmitted at the
`input of the premodulator of the remote device B.
`According to a preferred implementation, the premodu-
`lator means also includes a data classification unit and a
`
`frequency allocation unit.
`According to a particular characteristic, the unit for clas-
`sifying data to be transmitted has means adapted to generate
`a DS signal representing the significance of each data item
`supplied by the source.
`According to another particular characteristic, the fre-
`quency allocation unit has means adapted to generate a data
`allocation command signal (determining the distribution of
`the data over the different subcarriers), from data including
`the DS and FCD signals A B and means adapted to generate
`a signal representing the order in which there are arranged
`the different data to be transmitted at
`the input of the
`premodulator.
`
`

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`US 7,039,120 B1
`
`7
`the frequency
`According to a particular characteristic,
`allocation unit has means adapted to perform operations of:
`initialization, in which the frequency allocation unit reads
`the information contained in the FCD, DS and storage
`signals,
`classification of the subcarriers by order of interference
`and storage in the table thus obtained,
`classification of the indices of the data to be transmitted
`
`in order of significance, using the information con-
`tained in the DS signal, and storage of the result of this
`classification,
`transmission of the signal of the relative positions of the
`data with respect to each other, to the unit for insertion
`in the data to be transmitted,
`transmission of the data allocation command signal to the
`data allocation unit,
`this DAC signal being in fact
`composed of pairs (subcarriers, index of the data),
`testing to check whether all the pairs have been supplied,
`so that, if the test is negative, the following pair is
`supplied, and if the test is positive, the initialization
`step is returned to.
`According to yet another particular characteristic, data
`allocation unit has means adapted to transfer each data item
`supplied by the source to the subcarrier defined by the
`frequency allocation unit in the data allocation command
`signal.
`According to a particular characteristic, the device for the
`transmission of data from a device A to a remote device B
`via a transmission channel, has a CPU calculation unit, a
`temporary data storage unit, a program storage unit, char-
`acter entry means, image reproduction means and means
`allowing inputs and outputs.
`Under a second aspect,
`the present invention aims to
`provide a novel process for optimizing an information
`transmission system using multi-carrier modulation, the said
`process offering improved transmission efficiency.
`For this purpose, the process for transmitting groups of
`data elements over a transmission channel using multi-
`carrier type modulation is wherein a significance is attrib-
`uted to each data element or group of data elements for
`transmission, and the most important data is transmitted
`after modulation favoring a minimum bit error rate, the other
`data being transmitted after modulation favoring a maxi-
`mum data rate.
`
`It will be appreciated that, generally speaking, the inven-
`tion aims to optimize the perceived quality of a transmission
`using OFDM modulation by exploiting knowledge of the
`importance of the data for transmission.
`According to the nature of the data transmitted, an
`improvement in perceived quality can be obtained, in fact,
`either