FR3004040A1 - METHOD FOR TRANSMITTING A MULTI-CARRIER SIGNAL, TRANSMITTING DEVICE AND CORRESPONDING COMPUTER PROGRAM - Google Patents

Abstract

The invention relates to a method (300) for transmitting an OFDM signal whose pre-transmission processing comprises: a mapping of data representative of a source signal on complex symbols Xn belonging to an extended constellation, 0≤ n <M , n and M being natural, Xn = An + jBn, An and Bn being reals, - a transformation of M symbols Xn into M corrected symbols X'n, - a mapping of M corrected symbols X'n onto M from N carrying an OFDM modulator for generating an OFDM symbol, N being a natural, M≤N, the OFDM signal resulting from the succession of OFDM symbols, such that the transformation comprises: a step (301) of initialization of a 1st accumulator (B), and comprises for each carrier of order n, n ranging from 1 to M-1: - a step (302) of initialization of an intermediate symbol X "n depending on the existence of an extended symbol corresponding to the symbol Xn in the extended constellation, - a step (303) of accumulation in the first acc umulator (B) respectively J samples already present J temporal samples corresponding to J samples of the order carrier n mapped by the intermediate symbol X "n, J being a natural, and comprises: - a step (304) of initialization of a 2nd accumulator (D) with the J accumulated samples in the 1st accumulator, and comprises for each carrier of order n, n ranging from 1 to M-1: - a step (305) of accumulation in the 2nd accumulator (D) respectively to J samples already present of J samples corresponding to J time samples of the order carrier n mapped by a complex correction data dn, a step (306) for detecting P samples among the J output samples of the second accumulator having a power greater than a threshold (α) and zeroing of the other JP samples, P being a natural, - a step (307) of comparing these P samples with the time samples in coincidence among J time samples of the order carrier n, delivering a binary correction control torque (Gn), - a step (308) for selecting the complex correction data item dn from a list of corrections of the intermediate symbol X " n according to the binary correction control torque (Gn) to obtain the corrected symbol X'n = X "n + dn.

Description

Method for transmitting a multicarrier signal, transmission device and corresponding computer program FIELD OF THE INVENTION The field of the invention is that of radiofrequency transmissions for which a carrier modulation, in particular of OFDM type ("Orthogonal Frequency Division Multiplex" in English, for "orthogonal frequency division multiplexing"), is used. . This multi-carrier modulation technique notably makes it possible to overcome the inter-symbol interference generally observed on a multipath channel and is robust to frequency-selective channels. In addition, this technique has a very good spectral efficiency and saves radio spectral resources by the implementation of single-frequency networks. Given the multi-carrier nature, subcarriers of modulation are sometimes referred to to distinguish with the RF carrier. In the application, the carrier and subcarrier terms refer to the same thing when they are associated with the OFDM modulation. Due to its intrinsic robustness to multi-path and frequency-selective channels, OFDM modulation is used in many digital communication systems (ADSL, WiFi, 3GPP LTE of the 3rd Generation Partnership Project and Long Term Evolution ")" and "Digital Audio Broadcasting" (DVB-T), Digital Video Broadcasting - Terrestrial (DVB-T), DVB-T2, DVB-H, DVB-NGH, DRM , etc). A major disadvantage of the OFDM technique is inherent to the large amplitude fluctuations of the envelope of the modulated signal and therefore to the large variations in the instantaneous power.

Indeed, in the time domain, the summation of these multiple independently modulated carriers is carried out in power most of the time, but also in a coherent manner which leads to peaks of instantaneous power that can exceed more than 10 dB the average power of the signal at certain times. The Peak to Average Power Ratio (PAPR) of the transmitted signals, in other words the factor which characterizes the level of these power peaks with respect to the average power of the signal, is thus generally very high and increases with the number N of carriers. The power amplifiers have non-linear characteristics which, coupled with the amplification of so-called high-frequency signals, lead to distortions: spectral rise in the level of the secondary lobes, generation of harmonics, creation of interferences between non-linear symbols, creation of interferences between carriers. Thus, these distortions lead in particular to transmission errors and a degradation of the bit error rate (BER). More precisely, according to a particular embodiment, a B-band OFDM signal consisting of the sum of N orthogonal orthogonal carriers modulated regularly spaced interval of frequency M is used such that: B = NM For a given OFDM block, each carrier is modulated by a symbol Xn belonging to a constellation (QPSK, MAQ16, etc.). The inverse Fourier transform (IFFT) of the band B frequency signal then provides in the time domain the signal x (t) to be transmitted. In the time domain, the duration of an OFDM symbol is N.Te = 11M, with Te being the sampling period, and has the expression: N-1 1 X.eiAf, 13 <t <N. Te 'N / TV x (t) = n = 0 Assuming that the variables Xn are random, statistically independent and centered, we deduce the PAPR from the OFDM signal: E. [Ix (t) 12] With this definition the PAPR can become as large as N in the particular case but also very rare where [Xk}; ', I1j- = 1 considering that x (t) is the IFFT of discrete random variables.

In practice, the peaks of PAPR of a given amplitude occur according to a certain probability of appearance. It is notably unlikely that the amplitude of the signal is as large as N, especially since N is large. As a result, conventionally, to characterize the PAPR of an OFDM system, the complementary cumulative distribution function (CCDF) is used which provides the probability that the amplitude of the signal exceeds a certain threshold. This function is the most used to characterize the reduction systems of PAPR and has for expression: CCDFpApR = Pr [PAPR (XL)> y] 1_ (1_ For example, in the case of a signal comprising 2048 carriers and if the converters analog and / or digital analog and power amplifiers do not work with a dynamic range between average power and peak power of at least 12.2dB, which represents for the amplifier an operating power ratio of 1 to 16, then this equation indicates that the signal can not be correctly transmitted without sample saturation in at least one symbol out of 100. Below this margin of 12.2 dB the signal is clipped or at least strongly distorted with repercussions on transmission and reception conditions PAPR = maxo <t <N IX .Te-ltll30 Prior art In the literature, many techniques have already been proposed to overcome this problem. The common solution is to ensure that the operating range of the amplifier remains limited to a linear amplification zone, which unfortunately limits the efficiency of the amplifier (a few% instead of typically 50%) and therefore an increase. of the transmitter's consumption. This is a very strong constraint for the use of OFDM especially in mobile terminals, knowing that the consumption of the power amplifier can represent more than 50% of the total consumption of a terminal. A second approach is the application of a constraint or coding on the data sequence transmitted to limit the PAPR. This method involves building a code word game that minimizes the PAPR. Several construction techniques for these codes have been proposed. The advantage of this solution lies in the fact that it does not introduce distortion.

On the other hand, the spectral efficiency is penalized without even providing a coding gain. Moreover, to date, its field of application is limited to OFDM modulators with low numbers N of carriers because of too great computational complexity. A third approach, commonly referred to as "TI-CES (Tone Injection - Constellation Extension Scheme"), proposes to increase the number of constellation points that modulate OFDM carriers so that for one point in the constellation The origin can correspond to several possibilities of coordinates in the new constellation. According to this approach, this extra degree of freedom is used to generate a signal of lower PAPR. However, this method has several disadvantages because the constellation extension will lead to an increase in the average signal power since the additional symbols have higher power levels. In addition, the selection of the best possible coordinate for each point requires an increase in the complexity of the calculation implemented, making it unsuitable for a hardware implementation for the signal processing in real time. A fourth approach, commonly called "Constellation Distortion" technique, is also based on a constellation change and is based on the assumption that the output level of the transmit amplification is limited by peaks of higher PAPR and that if the amplitude of these peaks can be decreased then the power output can be increased. According to this technique, for a given distortion rate, a convex optimization problem is solved in order to develop an OFDM signal with a minimum overall PAPR level. However, this method requires a very significant increase in the average power output to compensate for the loss in terms of signal-to-noise ratio. In addition, the calculation complexity implemented increases exponentially when the constellation order becomes high. A fifth technique commonly called "ACE (Active Constellation Extension) technique" is also based on a constellation modification and is based on a displacement in the sense of a distance from the decision axes. However, in the same way as for the two previous methods, this technique is characterized by a lower efficiency for high-order constellations by the increase of the average power of the signal, and by a very high calculation complexity. A sixth method commonly called "TR technique (English" Tone Reservation ") proposes to reserve certain carriers of the OFDM multiplex, which do not carry information but symbols optimized for transmission to reduce the PAPR. The optimization of these symbols can be carried out using, for example, a convex optimization algorithm of the SOCP (Second Order Cone Programming) type. Like the previous method, this solution does not bring any distortion to the transmitted signal, but a major disadvantage. This method is based on the fact that a certain number of carriers must be reserved in order to reduce the PAPR significantly. These carriers are not used to transmit useful information data, which leads to a reduction in spectral efficiency. A seventh technique called "Selected Mapping" consists of applying a phase rotation to each symbol of the sequence to be transmitted. Several phase rotation patterns can be defined. For each pattern applied to the sequence to be transmitted, the operations are performed to obtain a corresponding OFDM signal, and the one with the lowest PAPR is transmitted. Again this technique does not distort, but it requires to communicate to the receiver the rotation sequence used at the emission with a very high reliability, which leads to a reduction of the spectral efficiency and a significant increase in the complexity of the system to route the rotation pattern applied via a dedicated channel. In addition, if this transmission is erroneous, the entire OFDM frame will be lost. It also increases the complexity on the issue, since several treatments must be performed in parallel, then choose the most effective. The other treatments have been carried out unnecessarily, and are not exploited. A last approach is the "clipping" or limiter technique which consists of clipping the amplitude of the signal when it exceeds a predefined threshold. But this clipping is by nature non-linear and introduces a distortion of the emitted signal resulting not only in a degraded BER but also in a rise in the secondary lobes of the DSP (Power Spectral Density). In this particular context, the inventors have therefore identified a need for a new technique to improve the reduction of the PAPR while remaining simple to implement. DISCLOSURE OF THE INVENTION The invention proposes a new solution which does not have all of these disadvantages of the prior art, in the form of a method for transmitting an OFDM signal whose pre-transmission processing comprises: a mapping of data representative of a source signal on complex symbols Xn belonging to an extended constellation, where 0 <n <M, n and M being natural, Xn = An + jBn, An and Bn being reals, a transformation of M symbols Xn in M corrected symbols X 'n, a mapping of the M corrected symbols X'n onto M among N carriers of an OFDM modulator to generate an OFDM symbol, N being a natural, MN, the OFDM signal resulting from the succession of OFDM symbols, such that the transformation comprises: a step of initialization of an accumulator lei, and comprises for each carrier of order n, n ranging from 1 to M-1: a step of initialization of an intermediate symbol X "n depending on the existence of a corresponding extended symbol waving at the symbol Xn in the extended constellation, a step of accumulation in the accumulator lei respectively to J samples already present of J temporal samples corresponding to J samples of the order carrier n mapped by the intermediate symbol X '' n, J being a natural, and comprises: - a step of initialization of an accumulator rd with J accumulated samples in the lei accumulator, and comprises for each carrier of order n, n ranging from 1 to M-1: - a step accumulating in the accumulator row respectively J samples already present of J samples corresponding to J time samples of the order carrier n mapped by a complex correction data dn, a step of detecting P samples from J samples output of the accumulator rd having a power greater than a threshold and zeroing of the other JP samples, P being a natural, a step of comparison of these P samples with time samples coinciding among J time samples of the order carrier n, delivering a bit correction control torque, a step of selecting the complex correction data item dn from a list of corrections of the intermediate symbol X "n as a function of the binary correction control torque to obtain the corrected symbol X 'n = X "n + dn. The invention also relates to a device for transmitting an OFDM signal. According to the invention, the device comprises: a mapping module for mapping data representative of a source signal to complex symbols Xn belonging to an extended constellation, where 0 <n <M, n and M being natural, Xn = An + jBn, An and Bn being reals, a transformation module for transforming M symbols Xn into M corrected symbols X 'n, such that X' n = Xn + dn with dn a complex correction, an OFDM modulator with N carriers to generate an OFDM symbol from the M corrected symbols X 'n mapped onto M among the N carriers, N being a natural, MN, the OFDM signal resulting from the succession of OFDM symbols, the transformation module comprising: an accumulator for accumulating Carrier after carrier with J samples already present respectively J temporal samples corresponding to J samples of the order carrier n mapped by an intermediate symbol X "n whose value is initialized according to the existence of a symbol extended range corresponding to the symbol Xn in the extended constellation, n ranging from 1 to M-1, J being a natural, a 2'd accumulator, initialized with the J samples accumulated in the accumulator lei, to accumulate carrier-carrier J samples corresponding to the J time samples of the order carrier n mapped by a complex correction data dn respectively to J samples already present, n ranging from 1 to M-1, a detection module, carrier-to-carrier, of P samples from J samples at the output of the 2'd accumulator having a power greater than a threshold and zeroing of the other JP samples, P being a natural, a module for comparing these P samples with the coincidence time samples among J time samples of the carrier of order n, delivering a binary correction control torque, a module for selecting the complex correction data item dn from a list of co rrections of the intermediate symbol X "n as a function of the binary correction control torque (Gn) to obtain the corrected symbol X 'n = X" n + dn. Such a transmission device is particularly suitable for implementing the transmission method according to the invention. Thus, the invention is based on a new and inventive approach to the reduction of PAPR of an OFDM signal. More specifically, the present invention makes it possible to improve the reduction performance of PAPR with a low computational complexity in view of the techniques of the prior art. In addition, the present invention is particularly suitable for controlling corrections of the binary type, in particular those imposed by the TI-CES techniques. Indeed, the method according to the invention selects in a controlled manner a duplicated symbol which corresponds to an original symbol or an extended symbol of an extended constellation. By extended constellation is meant an original constellation to which are added so-called extended symbols. By duplicated symbol is meant a symbol which belongs to the original constellation and which corresponds to an extended symbol in the extended constellation. That is, the representative data of the source signal is mapped to either the duplicate symbol or its extended symbol. The correction is called binary because there is no proportionality possible in the displacements of a point of the constellation. When the symbol is not duplicated the selection leads to no correction.

This type of correction is particularly advantageous for constellations of high order unlike corrections of types CD or ACE which lose efficiency with the order increase of the constellation. Indeed, for a correction of type CD, the increase of the order causes the reduction of the distance between two points of the constellation and consequently the decrease of the latitude of displacement. For an ACE-type correction, increasing the order causes the number of peripheral points to decrease relative to the total number of points in the constellation and consequently the decrease in the latitude of movement. After an initialization of the intermediate symbols X "n, the method performs a pre-construction of the time OFDM signal using a frequency-time transform which involves a modulation of the carriers by the intermediate symbols.This pre-construction is implemented by a According to one particular embodiment, the frequency-time transform is a discrete inverse Fourier transform, In a particular embodiment, the inverse Fourier transform is a fast transform (IFFT), followed by the IFFT acronyms denoting the frequency transform. time used by the OFDM modulator for generating the OFDM signal According to a feature of implementation of the pre-construction, the method develops the calculation of a discrete inverse Fourier transform of a block of N carriers, of which M carriers are mapped by the symbols X "n by performing the carrier-carrier calculation and for a given carrier n called by X '' n, by calculating each temporal samples 1, 1 varying between 0 and J -1. The method thus determines the time samples associated with a carrier mapped by X "N. The accumulation then consists in adding the J time samples of the current carrier n respectively to the J previous samples already accumulated concerning the carriers up to the order n This pre-construction takes place in the frequency domain, that is to say before the implementation of the OFDM modulator.This pre-construction initializes a second pre-construction.This second pre-construction then takes into account for the modulation of the carriers that the corrections on the intermediate symbols corresponding to duplicated symbols According to a particularity of implementation, the method develops the calculation of a discrete inverse Fourier transform of a block of N carriers, whose carriers are mapped by dn corrections by carrying out the carrier-to-carrier calculation and for a given carrier n mapped by dn each time samples 1, 1 varying between 0 and J -1. The method thus constructs in parallel the J time samples associated with a carrier mapped by dn. Since the correction only concerns the modified duplicate symbols, the accumulation step in the accumulator rd is simpler than in the accumulator lei. The accumulation in the accumulator 2 'then consists in adding the J time samples of the current carrier n mapped by dn respectively to the J previous samples already accumulated concerning the carriers up to the order n-1. The invention uses, for the reduction of the PAPR, a real-time servocontrol of the correction of a carrier of order n with respect to the previously corrected, that is to say, lower order, carriers of the same OFDM block. . By correction of a carrier of order n is meant a correction of the intermediate symbol X "n by the value dn which makes it possible to determine the symbol X 'n which maps the carrier n of the OFDM modulator during the OFDM modulation. servo-control is notably based on the implementation for correcting the constellation symbol modulating the order carrier n of a detection of P samples whose amplitude exceeds a threshold a, among the J temporal samples representative of the sum of the temporal responses Previously corrected and accumulated carriers Detection occurs after each accumulation In other words, among the J preconstructed time samples, the method detects the peak or peaks of power above a predetermined threshold A. The value of the threshold sets the level. of the final PAPR sought A comparison of the temporal samples resulting from the frequency-time transformation of the carrier n that we seek to correct, which coincide in time with the detected peaks, makes it possible to determine a binary correction control torque. This binary correction control pair is then directly used to select the correction to be made to the real part or the imaginary part of the intermediate symbol and obtain the corrected symbol which modulates the carrier n of the OFDM modulator. The selected correction also maps the order carrier n during the accumulation step of the J temporal samples associated with this order carrier n. The correction is selected from a correction list. This list includes the zero value selected when the intermediate symbol corresponds to an unduplicated symbol. The list further includes the real and imaginary values to be added to the intermediate symbol to obtain the corrected symbol that corresponds to a duplicate symbol or an extended symbol.

Thus, at the end of the steps of accumulation, detection, comparison and selection mentioned above implemented for each of the M mapped carriers, the method performs the pre-construction in the frequency domain, that is, ie before the OFDM modulation, of the corrected temporal signal consisting of J samples and associated with an OFDM block of N carriers mapped by the corresponding M symbols (MN). It should be noted that the term "pre-construction" means that, even though it is in the frequency domain, temporal samples of the signal response that "could" be obtained after the OFDM modulation are determined. Thus, the term "pre-built" is associated with the temporal samples possibly corrected before implementation of the OFDM modulator. It should be noted that the frequency-time transform which is generally an inverse Fourier transform can operate on a number M of carriers mapped by the M symbols considered, less than N the number of carriers of the OFDM modulator. Indeed, it is common to reserve carriers to insert drivers, to limit spectrum recovery problems by zeroing the edge carriers, to avoid a DC component by zeroing the central carrier. By "complex" is meant "which may have a real value and / or imaginary such that this value is for example defined by v = a + jb". Such a method therefore results in a very simple correction of the OFDM time signal because only the carriers of an OFDM block mapped by a duplicate symbol can be corrected. The correction of the time signal is optimized because the complex displacement of constellation coordinates is selected according to a binary correction control torque of the real part or the imaginary part of the intermediate symbol. According to one embodiment, for a given n, the intermediate symbol X '' n is equal to Xn when Xn is not a duplicated symbol and is equal to Xn punched by its real value or its imaginary value when Xn is a duplicated symbol. According to this embodiment, the intermediate symbols are initialized from the symbols Xn and the modified duplicate symbols of the extended constellation. Each of the duplicated symbols modified is identical to a duplicate symbol whose real or imaginary part has been set to zero (punched). This mode is adapted to an extended constellation which has the advantage of respecting a Gray coding. According to one embodiment, for a given n, the intermediate symbol X "n is initialized with the value of Xn when Xn is not a duplicated symbol and is initialized with the real part of Xn and a given imaginary value or with a the actual value determined and the imaginary value of Xn when Xn is a duplicated symbol According to this embodiment, the intermediate symbols are initialized from the symbols Xn and the modified duplicate symbols of the extended constellation, and each of the duplicated symbols modified is identical to A duplicate symbol whose real or imaginary part has been initialized to a definable parameterizable value The correction list then consists of the real or imaginary values of the duplicate symbols to which the initialization values are subtracted. be determined at the end of simulations or at the end of the experiment This mode may be more advantageous than the previous mode depending on the configuration of the telecommunication system that implements the process. According to one embodiment, the extended constellation is an extended 64QAM comprising the following symbols: Xn with An, E 1 - - -4 4 4 'Xn with An, Bn E 4)' Xn with Xn with Xn with Xn with A = -7 'BE n 4 n 9. 7 and the extended symbol XE = - - + JE 'An = n 4 n' 4 'n 9. 7 E 1 + -1 + -3, + -5} and the extended symbol XE = - + iB Bn = -4, An E n 4 n '7 - 4' - 4 - 4 Bn = (1 3 5 .9 4 'n + 4' + i, + ij and the extended symbol XEn = An - ii, 1 3 5, 9 E + and the extended symbol XEn = An + 7. This extended constellation with 24 duplicated symbols has the advantage of respecting The extended symbols can be obtained by projection on a dual point of the constellation opposite to the axis of the reals or the imaginary of the complex plane, of the duplicated symbols, followed by a shift of an inter-symbol distance The extended symbols can therefore be determined very easily from the duplicated symbols because they have the same real or imaginary part as the corresponding duplicated symbols and an imaginary or real part identical to the near sign and to a near inter-symbol shift. According to one embodiment, the threshold varies as a function of the carrier N. According to this mode, the threshold value is not fixed on the carrier. r all the carriers but variable. This variation may not be linear and can be determined at the end of simulations before or after experiments. This variation makes it possible to obtain a larger reduction in PAPR than with a fixed threshold because it makes it possible to accentuate the correction. According to an advantageous embodiment, the threshold value is lower at the beginning of the treatment than at the end. This makes it possible to process the signal 'in depth': the extreme peaks are not corrected at the beginning but the totality of the peaks which exceed the threshold has evolved rapidly in the direction of a decrease. As a result, a small minority of peaks remain at the end of treatment, which is easily corrected. On the other hand, starting with a higher threshold value, then the extreme peaks are rapidly corrected but at the end of treatment there remains a multitude of weak peaks still present.

According to one embodiment, the threshold is given by the following relation: = Peak Power Target [1 + K. a Average Power According to this mode, the value of the threshold varies continuously according to the carrier n according to a determined relationship to the outcome of experiments. The value of the exponent 5 can be two, K is a constant that can be positive or negative. The Peak Power Target value is the maximum allowable power and the Average Power value is the estimated power average. These values can be determined at the end of simulations before or after experiments. According to one embodiment, the bit correction torque results from a complex correlation between the P temporal samples with P temporal samples of the n-order carrier and a sign detection of the real and imaginary parts of the product of the correlation . The implementation of the complex correlation according to the invention with sign detection advantageously makes it possible to evaluate whether the next accumulation made from the time samples of the order carrier n will amplify the detected peaks or reduce them. The correction torque resulting from this sign detection is directly used to select the correction of the constellation symbol modulating the carrier to be corrected. Thus, the binary correction control torque makes it possible to control the reduction of the PAPR peaks by taking into account the sign correlation between the peaks of the preconstructed temporal samples representative of the previously corrected and accumulated carriers and the complex temporal samples of the current carrier to correct in temporal coincidence with the detected peaks. It is therefore possible to control the correction implemented from one carrier to another in an optimized manner, taking into account the correlation between a current carrier to be corrected and the peaks detected from the accumulation of previously corrected carriers.

According to one embodiment, the number J of time samples derives from an integer factor L, L> 1, of oversampling resulting in J = NL samples. According to this embodiment, the method according to the invention constructs J = LN time samples with L an integer greater than or equal to 1. When L = 1 then J = N. When L> 1, for example when L = 2 or 4 , there is on sampling. Such oversampling advantageously allows to obtain a higher resolution in the reduction of PAPR. According to one embodiment, the step of accumulation in the accumulator implements a carrier-carrier calculated discrete inverse Fourier transform with, for a given carrier n mapped by X "n, a calculation of the J temporal samples. , according to the following equation, for L = 2: N-1 Te x (1.-) = I [A "ncosN + jB" nsin (K.1.111) 1 2 n = 0 with: A "'and B" the real and imaginary components of the intermediate symbol X "n modulating the current carrier of index n, x () the set of time samples resulting from the accumulation in the accumulator lei, Te = -Fe the sampling period and 0 <1 <N. L = 2. N. According to one embodiment, the method further comprises a step of generating the J time samples of the order carrier N. In addition, the detection and comparison steps are performed. implemented with respect to the step of generating J time samples of the order carrier n, with a delay of determined duration According to a particular aspect of the invention, the detection step on the J samples resulting from the n-order accumulation step is carried out with respect to a step of generating J time samples associated with the current carrier of index n with a predetermined time delay corresponding to a clock cycle at Fe. The implementation of this delay according to a predetermined clock cycle makes it possible in particular to synchronize obtaining the complex correction control information so that it is delivered just when taking into account the current carrier of order n mapped by the correction during the accumulation step to the order n.

According to one embodiment, the step of accumulation in the accumulator 2'd implements a carrier-carrier calculated discrete inverse Fourier transform with, for a given carrier n mapped by dn, a calculation of the J time samples, according to the following equation: N-1 Te n, = [dnen'iTi'l n = 0 with: x '() the set of time samples resulting from the accumulation in the 2nd accumulator, Te = -Fe the period d sampling and 0 <1 <N. L = 2 N. The use of a fast discrete inverse Fourier transform allows an extremely efficient implementation with a very simple pre-construction of the corrected temporal signal.

According to one embodiment, the transformation module further comprises: a module for generating the J time samples associated with the current carrier of index n. The invention further relates to a computer program comprising instructions for the implementation of a transmission method according to the invention when the program is executed by a processor. The invention further relates to an information medium comprising program instructions adapted to the implementation of the steps of the method according to the invention when said program is loaded and executed in an OFDM transmission device.

The invention further relates to an OFDM signal obtained by the implementation of a method according to the invention. List of Figures Other features and advantages of the invention will appear more clearly on reading the following description of a particular embodiment, given as a simple illustrative and nonlimiting example, and the appended drawings, among which: Figure 1 illustrates the principle of the signal processing on which the invention is based; Figure 2 is a block diagram of the processing of an OFDM signal to reduce the PAPR implemented by a transmission system; Fig. 3A is a flowchart of the main steps of a transmission method according to an embodiment of the invention; FIG. 3B is a diagram of a device for reducing the PAPR according to the invention; FIGS. 4A and 4B respectively represent a 64QAM constellation of origin and the extended constellation according to an example; FIG. 5 illustrates the structure of a transmission device according to the invention; Figs. 6A and 6B respectively represent an original 64QAM constellation and the extended constellation according to one example; FIG. 7 represents a curve of the evolution of the parameter a as a function of the index n of the current carrier according to an example; FIG. 8 represents curves of the modulus of the time signals without and with reduction processing of PAPR according to the invention for a equal to 6 dB; FIG. 9 represents a CCDF reference curve and the curve for a equal to 6 dB. DESCRIPTION OF EMBODIMENTS OF THE INVENTION FIG. 1 illustrates the principle of signal processing on which the invention is based. The reduction of PAPR must apply to the time signal S (t) which would be transmitted without correction at the output of the OFDM transmission device. As the time signal S (t) is not known from the method according to the invention, the latter pre-constructs a digital signal representative of the dynamic point of view and peak values of the analog signal at the output of the transmission device. In other words, this pre-construction consists in obtaining an "image" of the analog signal at the output of the transmission device and in progressively correcting, carrier-borne, each constellation symbol modulating a carrier in order to obtain a preconstructed and corrected signal whose PAPR is reduced.

The time signal S (t) is obtained at the end of a processing shown in a simplified manner in FIG. 1. The symbols Xn are taken by input block, called OFDM block, and after processing the time signal S is obtained ( Each constellation symbol Xn is defined by a pair of real values (An, Bn) that define the coordinates of the constellation symbol Xn in the complex plane: Xn = An + j.Bn (1). It is understood that a complex signal can be represented by its real and imaginary components in the form An + j.Bn or in the form Cnei`P = Cncoscp + jCnsirup. The processing includes a frequency-time transform 41 at N frequencies called carriers, considered for purposes of illustration as being a fast inverse Fourier transform (IFFT). This transform calculates the time samples from the modulation of the carriers by respectively the symbols of the OFDM block considered. This calculation can be decomposed into two calculations carried out in parallel, one for the real components (An) and one for the imaginary components (Bn) of the constellation symbols Xn. In general, it can be considered that only M carriers are modulated by symbols, the rest of the carriers being reserved, M <N, M being an integer. In particular, the first and last carriers of an OFDM block are generally not modulated in order to avoid problems of recovery of the frequency spectrum associated with each OFDM block. For simplicity, it is considered that the N carriers are modulated by symbols Xn, M = N, where n is an integer such that CKri N-1. The signal obtained can be expressed in the form: .2 j.n..Lik.Te X (k. Te) = EnNlâ Xn. e (2) with Te = 1 / Fe the sampling period, t = k. Te, 0 <k <K = N and K. Te = T the duration of the OFDM block considered and therefore of an OFDM symbol. The processing is followed by a baseband transposition 42 with the frequency Fe / 2: y (k Te) = Eiej-cos (k Tc). Xn. e (4) the Fe / 2 carrier expressing itself as: 2.j.ir .-- -. kTe = cos (k. Tc) (3) Then an oversampling 43 of a factor L at a frequency equal to L.Fe: n1 y (1.Te,) = COS (1.11). X e (5) L with t = 1. Te / L and 0 1 <J = LN. At the end of an ideal digital-analog conversion 43 the signal can be considered as the limit of y (1 -Te) when L tends to infinity: z (t) = limL, 'y (1.-7 (6) The actual modulated signal 44 on a carrier frequency is expressed as: S (t) = Re [z (t) p (0) (7) or in the form with ar g [z (0 ] = (t): S (t) = lz (01Re [eie (t) e 2. j.nvti (8) - The reduction of the PAPR amounts to limiting the amplitude variations of the OFDM signal: IS (t) I This constraint is checked if: N-1 nl 2.jn..k.Te An .e lx (iT) 1 <a VL L n = 0 Indeed, if lx (1.1) 1 <a VL then ly (1.1) 1 <a VL since y (t) is identical to x (t) but transposed into baseband If l (1.7) 1 <a VL then I z (t) I <a considering that z ( t) = limLy If I z (t) I <then has a fortiori I z (t) I cos (0 (t) + 2Kvt) <a Vv since cos (0 (t) + 2Kvt) <1. As a result, The stress to check on the IFFT output signal n1 nN = - also written under has the form: v N-1 n C (1. = r N-1 V-01 = 0 P 12 n, 1] L r = L ^ n0 12 <a VL with: LI = Re [x (1.-Te) 1 EnN; êi Pn, 1 (9) 15n N = 0, in, 1 IM [X (1 -Te)] L (10) n 1 n 1) 1 Pn, 1 = [(An. COS 2. T C. -Nr) - Bn. sin (2. K.-NT (11) Qn, 1 = [Bn COS (2. T (.11-1) + An. sin (2. K.11-1) 1 (12) NLNL To limit the PAPR signal OFDM S (t) transmitted, the method according to the invention pre-built the time signal x (k.Te) and applies on this signal the amplitude constraint: 20 lx (1.7- ÷ e, = P12 + [ Mg Qn, i.] 2 <a VL and thus the time signal OFDM S (t) satisfies the constraint of PAPR: IS (t) I <a Vv as demonstrated above, in particular with over-sampling L = 2, the relations (11) and (12) become: Pn, 1 = [An. COS (7 '-Nn 1) -An.cos (tr.-Nn 1) 1 (13) Qn, 1 [Bn. COS (7 'C.11 1) + An. Sin (7r.L'1) 1 (14) In this case, to pre-construct the time signal x (k.Te), the pre-built process 2N temporal samples for each real and imaginary component.

An example of the processing of an OFDM signal for reducing the PAPR implemented by a transmission system is described below in relation to the diagram of FIG. 2. According to this example, the processing of the OFDM signal comprises a succession of steps: TX transmission: generation 201 of source data; encoding and interleaving 202 said data delivering interlaced data; modulating said interleaved data 203 for example according to a QAM modulation of mapping the interleaved data representative of the source signal to complex symbols Xn belonging to the constellation; insertion of pilot carriers; correction of the symbols Xn of an OFDM block for reducing the PAPR according to the method of the invention; OFDM modulation 206 implementing, for example, an inverse fast Fourier transform after mapping the corrected symbols on the carriers of the OFDM modulator to generate an OFDM symbol, the succession of OFDM symbols forming the OFDM signal S '(t) to be transmitted; transmission 207 of the OFDM signal on a transmission channel Ch, this transmission is generally accompanied by noise, for example a Gaussian white noise awgn; and on reception by the receiver RX: reception 210 of a signal said to be received; OFDM demodulation 211 of the received signal implementing according to a particular mode a Fast Fourier Transform (FFT) delivering a transformed received signal; channel estimate 212; demodulation 213 of the transformed received signal delivering a demodulated signal; de-interleaving and decoding 214 of the demodulated signal; determination of the bit error rate (BER). The invention therefore proposes a specific correction technique 205 that makes it possible to effectively reduce the PAPR while being simple to implement. The correction according to the invention is implemented only on transmission and does not require modification of existing receivers. According to the illustration of FIG. 2, the method according to the invention takes place after the insertion 204 of pilot carriers. This insertion can according to other embodiments intervene nested with steps of the method according to the invention or successively to the steps of the method. To simplify the description, it is considered that the method according to the invention takes into account a block m of M Xn symbols whose size is equal to the total number N of OFDM modulator carriers (hence M = N). The block of M symbols can equally well be less than N to take account of reserved carriers whose value is fixed elsewhere, or of unmodulated carriers on the edges of the spectrum. The method and the device for reducing PAPR according to the invention are described with reference to the block diagram of FIG. 3A and according to the diagram of FIG. 3B. The steps of the transmission method according to the invention are implemented in the frequency domain between the conventional modulation 203 and OFDM modulation steps 206. The method according to the invention corresponds to a feedback control system of the retroactive type. English "Feed-Back").

This method operates in real time according to a clock timing at a frequency Fe, for example the sampling frequency of the source data. In addition, this method is non-iterative, in other words a correction on a block of N carriers (N also corresponding to the size of the Fourier transform and the inverse Fourier transform) is entirely calculated in a duration. N samples at the Fe frequency.

The method consists in "pre-constructing" before the OFDM modulation 206, the real time signal x (1.TelL) from the different OFDM block carriers mapped by the complex symbols and constraining its amplitude peaks to act on the PAPR of the time signal S (t). The transmission method of the invention makes it possible to apply a correction of the modulation constellation in order to reduce the PAPR of the transmitted signal. The method according to an embodiment illustrated in FIG. 3A comprises the following steps 301-308. Step 301 of initialization of the accumulator. According to one embodiment, the initialization consists of zeroing the accumulator. The step 302 of initialization of the intermediate symbols X "n corresponding to the current block M. For each Xn of the block m, the value of the intermediate symbol X" n is determined as a function of the existence of an extended symbol corresponding to the symbol Xn in the extended constellation. n is a natural integer, n = 0 to N-1. This step does not introduce a delay. According to a first embodiment, when Xn is not a duplicated symbol the intermediate symbol X "n is initialized to the value of Xn, and when Xn is a duplicated symbol, the intermediate symbol X" n is initialized to Xn punched from its real value or its imaginary value. In other words: For n = 0 to N-1, X "n = Xn = An + j.Bn if Xn is not a duplicated symbol and X" n = AEn = An or X "n = jBEn = j.Bn if Xn is a duplicate symbol.

According to a variant: X "n = Xn = An + j.Bn if Xn is not a duplicated symbol and X" n = AEn + jBEn = An + jB0 or X "n = AEn + jBEn = AO + j.Bn if Xn is a duplicate symbol with AO and BO parameterizable values For example, the extended constellation is an extended 64QAM shown in Figures 4A and 4B Figure 4A is the original 64QAM constellation and Figure 4B is the extended constellation The extended constellation includes the following symbols: Xn with An, B, E {± -1 4, + -4, - ± 7 Xn with An, B, E {± -4}, Xn with An = -7 , Bn - E 1 + -1, - + -3 + -5} and the extended symbol XEn = - -49 + jB ', 444, 4 (AE, E {--9, -7}), Xn with An = - -7, B, E -3, + - 5} and the extended symbol XEn = 719 + jBn, 4 - 4 -4 -4 (AE, E Xn with B, = I, A, E ï, - ± and the extended symbol XEn = An - j, (BE, E 1-.4, Xn with B, = - -7 '- - A, E 1 + 1-, + 2, + 2) and the extended symbol XEn = An + j, 44 -4 -4 (B In E {-9, -7}) 4 4 According to the first embodiment of the initialization step, the symbols are intermediate The diaries X "n are thus initialized as follows: X" n = Xn if An, Bn E1, +0, X "n = Xn if An, B, EX" n = jBEn = jBn if Xn with An = - 7 and Bn E 4 X "n = jBEn = jBn if Xn with An = - -7 and B, E 4 - 7 X" n = AEn = An if Xn with B, = -4 and An EX "n = AEn = An if Xn with Bn = - -7 and AE 1 + -1 + -3 -5 4 n - 4 - + 4, - 4} - The corrected symbol X'n used by the OFDM modulation is obtained from X " not.

X'n is either identical to a symbol Xn when it is not a duplicate symbol, or identical to a duplicate symbol Xn or its extended symbol and obtained by adding AE, or jBE, to the symbol X "n. step 303 of accumulating complex temporal samples in the accumulator corresponding to J samples of the order carrier n mapped by the intermediate symbol X "n, J being a natural. The accumulation step for the n order carrier amounts to simultaneously calculating all the real samples Pnj (11) and imaginary Q ', 1 (12) of its temporal response which could be obtained after IFFT if this mapped subcarrier by the intermediate symbol X "n was transformed into the time domain in isolation and added to the already existing samples 2. The initialization and accumulation steps are repeated for each value of n, n = 0 to N- 1. After the last accumulation, the output signal corresponds to: N-1 n1 B [0, ... 1, ... = LN-1] = X "n. n = 0 The step 304 of initialization of a 2'd accumulator with the J samples accumulated in the lei accumulator. This step occurs after the end of the last accumulation in the accumulator lei: Dc, [] = BN_i []. At its initialization, the module D contains all the responses of the sub-carriers mapped by the intermediate symbols X "n These intermediate symbols coincide with the symbols Xn or with the symbols Xn punched by the real or imaginary values respectively AEn or BEn when Xn is a duplicated symbol The method according to the invention restores, during the step 305 of accumulation, these real or imaginary mapping values, having previously made a choice, according to their polarity, between the two possible values for AEn or BEn The step 305 of accumulating in the 2'd accumulator J samples corresponding to J complex time samples of the order carrier n mapped by a complex correction data dn.A each symbol Xn, the pre-built method progressively following the J samples of each real and imaginary component of the temporal response of the parallel signal Dril I to determine the new mapping X 'n, resulting from the choice between the two values restored for AEn or BEn. The temporal response of the parallel signal Dril I may be in accordance with the series x '(1.Te / 2) obtained after OFDM modulation. However, the real and imaginary temporal responses of all the subcarriers mapped by the intermediate symbols X "n have already been loaded into the pre-construction module D at its initialization and the operation is linear. calculating and accumulating in the module D the complex temporal components of the sub-carriers mapped only by dn which contains the real or imaginary component chosen for respectively AEn or BEn, or which contains zero when the current symbol Xn can not be duplicated, that is to say, is not a duplicated symbol.The accumulation of the J samples amounts to adding them respectively to the J samples already present.The J samples form the complex signal vector Dn [] = DdPn [] + jDdQ [ 1. The real and imaginary components d1) ', 1 and dQ', 1 respectively of the two signal vectors DdPnl I and DdQn11 have the following expressions: dPn, i = 0 and dQn, i = 0 or dPn, i = [AEn. Cos (tr LIN 1) 1 and dQn, i = [AEn sin (tr LIN 1) 1 (15) or dPn, i = [B En. sin (tr LIN .1) 1 and dQn, i = [BEn. cos (tr LIN. (16) At each clock stroke and starting from C [loaded at initialization in module D, all the real and imaginary samples of the signal x '(/, Te / 2) are updated progressively accumulating the current results of equations (15) and (16) in module D. At the end of the last accumulation, the output signal corresponds to: N-1, nor Dn [0, ... 1,, (I = LN) - 1] = Do [] + dn. E`'Pe'W, n = 1 Step 306 is an operation for detecting P samples from J samples at the output of the accumulator rd having a higher power. at a threshold a, that is to say such that: 1Dn [1] 12> a2 The P samples are kept and the other JPs are set to zero The threshold dc is a parameter P is a natural. in this step, all the samples of the signal Dn [between -cc and cc are thus set to zero, the value of cc sets the desired final level of PAPR.According to one embodiment, the detection is made on the samples s real and on the imaginary samples. Detection provides two signals: EPn [] = Discr (DPn []) EQn [] = Discr (DQn []) with: Discr (DPn, i []) = DP ', / if ° Discr (DQn, i [] ) = DQn, i, [(DPn, l) U) Qn, l) 21 otherwise If RDPn, l) 2 U) Qn, l) \ otherwise 21 a2 (17) (18) An example of vectors for which the samples 0, 3, ..., (2.N-4), and (2.N-2) are signal peaks of amplitude greater than cc obtained after detection is given below: DP ', 0 D ## EQU1 ## According to one embodiment, the value of the parameter a varies according to the progress of the algorithm in the OFDM block, for example: Peak Power Target a =. [N n81 1 + K. (Average Power N) or K can be a positive or negative constant and 8 = 2 for example. According to one embodiment, the parameter a is defined as a function of the order n of the current carrier, this function can be defined in intervals of n.

A step 307 for comparing these P samples with coincident time samples among J time samples of the order carrier n, delivering a binary correction control torque Gn, (GAEn, GBEn). According to one embodiment, the comparison implements a complex correlation product between the COSn [] and sinus SIN [] cosine samples of the n-order subcarrier and the real EPn [] and imaginary EQn [ ] from peak detection. The correlation product has the following expression: Fn = CorrEc n (0) = E ÏLN, - 1 In (1). C 1, '(1) (19) with: Re (F) = EI 1 [EPn, 1. COSn SINn, 1] (20) Zsm (F) = EI: N0-1 [EQn, /. COS, / - EPnj. S / Nnj] (21) Cn [] = COSn [] + j. SINn [] and In [] = EPn [] + j.EQn [] The correction control torque Gn (GAEn, GBEn) provides the signs of the real and imaginary parts: GAEn = Sign [9e (Fn)] (20) GBEn = Sign [Zsm (Fn)] (21) The correction control torque Gn makes it possible to choose between the two possible values for AEn or for BEn when the symbol can be duplicated. Thus, this advances the reactualization of the signal in the direction of the overall decrease of the signal peaks. A step 308 of selecting the complex correction data dn of the intermediate symbol X "n to obtain the corrected symbol X'n = X" n + dn. The selection is made from a list of corrections as a function of the binary correction control torque Gn. According to the first embodiment the list comprises: 0, +/- jBEn, +/- AEn. According to the variant, the list comprises: 0, (+/- AEn-A0n), j (+/- BEn-B0n). Thus, when the polarity GAEn (or GBEn) of the real (or imaginary) part of the correlation product is positive, then a correction by selection of the negative value of AEn (respectively BEn) must be performed; conversely, if the polarity is negative, a correction by selection of the positive value of AEn (respectively BEn) must be applied to obtain A'n (respectively B'n) the new abscissa or ordinate of the corrected mapping symbol X 'n . One embodiment of a PAPR reduction device according to the invention is described with reference to the diagram of FIG. 3B. The device comprises the means adapted to implement SINn [] = implement a method according to the invention. According to the invention, the pre-construction is obtained by means of an accumulation module B, an accumulation module D, a detection module E, a comparison module F, G and a selection module H. The device further comprises the modules A, A ', C, I and SW.

The module C generates time symbols of a subcarrier n. At each clock stroke, the module C delivers the sequence of the 2x2.N time samples (N = Size IFFT and L = 2) in cosine and sine of the subcarrier in correspondence with the symbol Xn in the OFDM block of size N. These samples, stored in ROM memory or computed algorithmically, are then exploited by the different modules B, D, F. If the cosine and sine samples of the sub-carriers at the order n, constitute the elements of the vectors COSn [] and SINn [], these are expressed in the following way: 1 cos cos ((2. N-2), 7E, ili) cos ((2. N-1) .7E. ili) sin ((2. 2. N - 2) 7E sin ((2. N-1) .7E.) _ Module A initializes 302 intermediate symbols X "n, According to a first embodiment, when Xn is not a symbol duplicated the intermediate symbol X "n is initialized to the value of Xn and when Xn is a duplicated symbol, the intermediate symbol X" n is initialized to Xn punctured by its actual value or its value ima In other words: 1 3 5 Xn if An, Bn E 1 + - + - + -I or An, Bn E - 4 1 3 5 and Bn 7 An E 1- ± -41 and Bn symbols Xn of a block of index m. This module is a delay line of duration the size N of a block. Thus, the first symbol XO of the block of subscript m emerges from the delay line when the module D has just been reinitialized and contains the sequence of the samples of the two real and imaginary components of the signal Bril I corresponding to the same block of index m. The module A 'is similar to the module A. The module A' transforms the symbols Xn into intermediate symbols X "n The accumulation module B accumulates 303 carrier after carrier J time samples corresponding to J samples of the current carrier mapped by an intermediate symbol X '' n, J is a natural (natural integer) Accumulation occurs for each of the carriers n, n ranging from 1 to M-1 The accumulation results in the addition of J samples to respectively J samples already present Before the accumulation operation, the module Xun = An if j.Bn if the module I receives the accumulation input can be initialized 301 to zero If the current carrier n has to be mapped by a symbol Xn not duplicated then the intermediate symbol X "n is initialized 302 with the value of Xn. If the current carrier n is to be mapped by a duplicate symbol Xn then the intermediate symbol X "n is initialized 302 to a modified value of Xn.

In other words, for each carrier of the OFDM modulator modulated by a symbol, the lei accumulation module B simultaneously calculates all the J samples of the temporal response of this carrier mapped by the symbol X "n which could be obtained after OFDM modulation, if this carrier was transformed in the time domain in isolation, then from carrier to carrier, the different temporal responses are accumulated by the accumulation module B to the order carrier N. The accumulation module B is recorded at the frequency Fe which is represented by a black vertical bar.Further said, at each edge of this clock, a new input, namely an intermediate constellation symbol X "n is loaded and a new output is updated. This new output corresponds to the J samples of the current carrier mapped by the intermediate symbol X "n respectively accumulated to the samples already accumulated with the previous orders, the corresponding values of the temporal samples being then maintained during a clock cycle. In this embodiment, the accumulation module B accumulates in parallel on the one hand the real samples Pn [] and on the other hand the imaginary samples Qn []. The output signal Bn [] of the block is expressed in the form: Bn [] = BPn [] + jBQn [] At initialization the accumulators of block B are set to zero: BPoll = [0] BQoll = [0] And at order n, 0 <n <N, l 'accumulation gives at the output of the block: BPn [] = 1PH BQn [] Qill i = 0 The values of the vector BPn [] are given by the equation (11) and the values of the vector BQfl [] are given by the equation (12), for 1 = 0, ... (J = LN) -1.

According to an embodiment L = 2, the values of the vector BPn [] are given by equation (13) and the values of the vector BQn are given by equation (14), for 1 = 0,. = 2.N) -1. At the end of the accumulation, the result of the accumulation is used to initialize 304 the 2'd accumulation module D. At its initialization, the module D contains all the responses of the sub-carriers mapped by the symbols These intermediate symbols coincide with the symbols Xn or with the symbols Xn punched by the real or imaginary values, respectively AEn or BEn when Xn is a duplicated symbol.

According to one embodiment, the initialization 304 occurs following the switchover of a switch SW which relates the outputs of the lei accumulation module B with the inputs of the accumulation module R D. According to one embodiment, the failover occurs after accumulation to order n = N-2, the last J samples are ignored. This mode simply makes it possible to switch to the same symbol time, that is to say in the total time of the current block. The accumulation module D accumulates 305 carrier after carrier J samples corresponding to the J time samples of the current carrier n mapped by a complex correction data dn to J samples already present respectively. n is from 1 to M-1.

The accumulation operation is similar to that performed by the lei accumulation module. The output signal is denoted D '[]. At each symbol Xn, the method progressively pre-builds the sequence of the J samples of each real and imaginary component of the temporal response of the parallel signal Dnl I to determine the new mapping X'n, resulting from the choice between the two values restored for AEn. or BEn. The time response of the parallel signal Dnl I must be in accordance with the series x '(1.Te / 2) obtained after OFDM modulation. Now the real and imaginary temporal responses of the set of sub-carriers mapped by the intermediate symbols X "n have already been loaded into the pre-construction module D at its initialization and the operation is linear. and to accumulate in the module D the complex temporal components of the sub-carriers mapped only by the real or imaginary component chosen, respectively AEn or BEn, or to do nothing when the current symbol can not be duplicated. embodiment, from the first symbol X "0 of the OFDM block of index m provided by the module A ', the module D then receives, successively at the rhythm of the clock, the mapping choices AEn or BEn or 0 ( unduplicated symbol) for each following symbol X "n as well as the sequence of the 2x2.N (L = 2) time samples in cosine and sine of the subcarrier which corresponds to it in the OFDM block, generated by the module C. These 4.N ec Sine and cosine antillons are delivered by the module C to the modules B and D identically and synchronously because the delay between the operations involving the same components corresponds to the period of an OFDM block exactly (module I). According to one embodiment, the accumulation module D accumulates in parallel on the one hand the actual samples dPnll and on the other hand the imaginary samples dQri [l. The output signal D [] of the block is expressed as: D '[] = dP' [] + jdQ '[]. When switching over, the block D is initialized with the output of block B. According to one embodiment, the switching occurs after the accumulation at order N-2 in block B: N-2 N-2 dPo BPN-2 [= Pi [dQ0E1 = BQN-2 [] = QLE i = 0 t = 0 And at the order n, 0 <n <N, the accumulation gives at the output of the block D: DPn [] = dPo [] DQn [] = cl (20 [] i = ii = t The vectors dPj [] and dQi [] correspond to the 2.J temporal samples of the current carrier i mapped respectively by the real component and by the imaginary component of the correction data complex di.

The detection module E detects 306 P samples among the J samples at the output of the accumulator rd which have a power greater than a threshold (a) and sets the J-P other samples to zero. P is a natural. Detection intervenes carrier after carrier, that is to say after each accumulation. The comparator module F, G compares these P samples with the coincidence time samples among the J time samples of the n order carrier and delivers a pair of bit correction control Gn. The comparison intervenes carrier after carrier, that is to say after each detection. The J time samples of the order carrier n result from the frequency-time transform of the carrier n. The selection module H selects 308 the complex correction data item dn from a list of corrections of the intermediate symbol X "n as a function of the bit correction control torque Gn to obtain the corrected symbol X 'n = X" n + dn. According to one embodiment, the list comprises: 0, jBEn, AEn. According to a second embodiment the list comprises: 0, AEn-A0n, j (BEn-B0n). At the rhythm of the clock, the block H provides the choice of mapping AEn or Ben or 0 to correct the current carrier of the block D.

Such a complex correction has the effect at the next accumulation step of reducing the amplitude of the detected peaks compared to what it could have been without correction. This next accumulation will therefore produce a new value of the preconstructed time samples in parallel with a regression constraint on the P samples that exceed the threshold.

The method according to the invention is reproduced for each new constellation symbol. As the corrected carriers, new peaks to correct can appear and are corrected. The method according to the invention ends at the end of a current OFDM block once the set of carriers that constitutes it have been traversed. The lei accumulation module D is then reset to process the next OFDM block of index m + 1. The simplified structure of a device for transmitting an OFDM signal implementing a transmission technique according to the invention is described in relation to FIG. 5. Such a transmission device comprises a storage module 60 comprising a buffer memory M , a processing unit 61, equipped for example with a microprocessor / IP, and driven by the computer program 62, implementing the transmission method according to the invention. At initialization, the code instructions of the computer program 62 are for example loaded into a RAM memory M before being executed by the processor of the processing unit 61. The processing unit 61 receives as input complex symbols Xn on which data representative of a source signal have been mapped. The microprocessor of the processing unit 61 implements the steps of the transmission method described above, according to the instructions of the computer program 62, to perform a correction of the modulation constellation to reduce the PAPR of the transmitted signal S ( t). For this, the transmission device comprises: a mapping module for mapping the data representative of the source signal to the complex symbols Xn, 0 <n <M, belonging to a constellation, n and M being integers, a transformation module for transforming M symbols Xn into M corrected symbols X'n, such that X'n = Xn + dn with dn a complex correction, an OFDM modulator with N carriers to generate an OFDM symbol from the M corrected symbols X'n mapped to M carriers, with MN, N being a natural integer, the OFDM signal resulting from the succession of OFDM symbols.

The transformation module comprises: a 1 ei accumulator B to accumulate carrier carrier J samples already present respectively J time samples corresponding to J samples of the order carrier n mapped by an intermediate symbol X "n whose value is initialized in function of the existence of an extended symbol corresponding to the symbol Xn in the extended constellation, n ranging from 1 to M-1, J being a natural, a storage accumulator D, initialized with the J accumulated samples in the accumulator, to accumulate carrier after carrier J samples corresponding to J time samples of the order carrier n mapped by a complex correction data dn respectively to J samples already present, n ranging from 1 to M-1, a detection module E, carrier after carrier , of P samples out of J samples at the output of the accumulator rd having a power greater than a threshold (a) and of z er of the other JP samples, P being a natural, a comparison module F, G of these P samples with coincidence time samples among J time samples of the n-order carrier, delivering a binary correction control torque (Gn ), a selection module H of the complex correction data item dn from a list of corrections of the intermediate symbol X "n as a function of the binary correction control torque (Gn) to obtain the corrected symbol X'n = X" n + dn. According to one embodiment, the transformation module further comprises: a module for generating J complex time samples associated with said current carrier of index n, J being an integer. These means are controlled by the microprocessor of the processing unit 61. An implementation of the invention with a simulation chain is described below. This chain includes: a pseudo-random generator for performing error rate readings. a Turbocoder-Decoder. maqr mapping / mapping blocks according to Gray's coding. a couple of interlacing / deinterlacing mapping symbol. a couple of IFFT / FFT functions. a Gaussian noise generator. visualization and measurement interfaces: BER, power, CCDF. The configuration for the implementation implemented is as follows: 64-QAM modulation - FFT size: 256 output coding 1/2 symbol interleaving: 8 OFDM blocks correction type: OCS (CES). number of duplicate points: 16 The initial MAQ64 constellation is shown in Figure 6A and the MAQ64 constellation extended in Figure 6B. The method according to the invention is parameterized as follows: initialization to zero of the components AEn and BEn of the intermediate symbols, variation of the parameter a during the course of the process, that is to say as a function of the index n of the current carrier.

The evolution of the parameter a as a function of the index n of the current carrier is shown in FIG. 7. The function of n has been experimentally defined in pieces with a final value of 6 dB. As long as the final value exceeds 6 dB, it determines the level of PAPR that is output after OFDM modulation. When the final value falls below 6 dB, there is no longer any gain progression and, on the contrary, a re-increase of the PAPR can be observed below 5.5 dB. The curves of FIG. 8 represent the modulus of the time signals without and with PAPR reduction processing according to the invention for a equal to 6 dB. The signals were acquired over a depth of 100000 samples, that is to say a little more than 195 OFDM FFT256 symbols, comprising 512 samples per symbol at 2.Fe, L = 2. The comparison of the two curves shows that the peak signal peaks have been resorbed to achieve, after correction, a reduced and relatively uniform maximum level.

One way to quantitatively characterize the effectiveness of the PAPR reduction algorithm is to record the CCDF curve which provides the probability that the signal amplitude exceeds a certain threshold (PARO) relative to the average signal power. The reference curve of CCDF and the curve for parameterized according to the figure are represented in FIG. 9. The reference curve corresponds to an OFDM signal FFT256 and MAQ64. The difference between these two curves shows that the algorithm has a good efficiency since for Prob (PAR> PARO) = 10-2 the reduction of PAPR is about 2.7 dB. The 10-2 CCDF is generally used as a benchmark for comparing the performance of PAPR reduction systems. Despite the dilation of the constellation to obtain an extended constellation with extreme points of greater dynamics, the simulations show that the increase in average power is low and remains of the order of 0.35dB. In addition, the simulations have shown that the invention with or without coding does not bring Equivalent Noise Degradation (DEB) Gaussian channel compared to a conventional modulation of symbols.

Claims (16)

REVENDICATIONS1. A method (300) for transmitting an OFDM signal whose pre-transmission processing comprises: a mapping (203) of data representative of a source signal on complex symbols Xn belonging to an extended constellation, 0 <n <M, n and M being natural, Xn = An + jBn, An and Bn being reals, a transformation (205) of M symbols Xn into M corrected symbols X 'n, a mapping (206) of the M corrected symbols X'n onto M among N carrying an OFDM modulator for generating an OFDM symbol, N being a natural, MN, the OFDM signal resulting from the succession of OFDM symbols, such that the transformation comprises: a step (301) of initialization of a accumulator (B), and comprises for each carrier of order n, n ranging from 1 to M-1: - a step (302) of initialization of an intermediate symbol X "n depending on the existence of a extended symbol corresponding to the symbol Xn in the extended constellation, a step (303) of accumulation in the accumulator 1 (B) respec J samples already present of J temporal samples corresponding to J samples of the order carrier n mapped by the intermediate symbol X "n, J being a natural, and comprises: - a step (304) of initialization of a 2nd accumulator (D) with the J samples accumulated in the first accumulator, and comprises for each carrier of order n, n ranging from 1 to M-1: a step (305) accumulation in the 2nd accumulator (D) respectively with J samples already present of J samples corresponding to J temporal samples of the order carrier n mapped by a complex correction data item dn, a step (306) for detecting P samples among J samples at the output of the 2'd accumulator having a power greater than a threshold (a) and zeroing of the other JP samples, P being a natural, a step (307) of comparing these P samples with the coincidental time samples among J ech of the order carrier n, delivering a binary correction control torque (Go), a step (308) for selecting the complex correction data item from among a list of corrections of the intermediate symbol X "n as a function of the binary correction control torque (Gn) to obtain the corrected symbol X'n.X "ni-dn. 2. Method (300) of transmission according to claim 1, wherein, for a given n, the intermediate symbol X "n is equal to Xn when Xn is not a duplicate symbol and is equal to 30 Xn punched of its value real or its imaginary value when Xn is a duplicate symbol. A transmission method (300) according to claim 1, wherein, for a given n, the intermediate symbol X "n is initialized with the value of Xn when Xn is not a duplicate symbol and is initialized with the part real of Xn and a given imaginary value or with a specific real value and the imaginary value of Xn when Xn is a duplicate symbol. A transmission method (300) according to claim 1, wherein the extended constellation is an extended 64QAM comprising the following symbols: Xn with An, B, E {± 4 !, ±}, Xn with An, Rn t ± Xn with A, = B, E ± !, ±} and the extended symbol XE, = -4-9 + jB ,, Xn with An = -, BE {± -1;, ±!, ± -45} and the extended symbol XE, = + jB ,, Xn with B, = An E {±!, ± -4-3, ± -451 and the extended symbol XE, = A - j Xn with B '= -1, A, E 1 ± 1, ±!, ± and the extended symbol XE '= A, + j :. 4 15 5. Method (300) of transmission according to claim 1, wherein the threshold varies depending on the carrier n. 6. Transmission method (300) according to the preceding claim, wherein the threshold is given by the following relationship: Peak Power Target a =. [1 + K. Average Power 20 Transmission method (300) according to claim 1, wherein the bit correction torque (Gn) results from a complex correlation between the P temporal samples with P temporal samples of the n-order carrier and a detection sign of the real and imaginary parts of the product of the correlation. A transmission method (300) according to claim 1, wherein the number of time samples is derived from an integer factor L, L> 1, of oversampling resulting in J = NL samples. 9. Transmission method (300) according to the preceding claim, wherein the step of accumulation in the first accumulator implements a carrier-based discrete inverse inverse Fourier transform with, for a given carrier n mapped by X "n , a calculation of the J time samples, according to the following equation, for L = 2: N-1 T ex (1-2) pc., (1.N + jB "nsin (n..1. -N) 1 n = with: 3 004 (14 0 31 A "and B" 'the real and imaginary components of the intermediate symbol X "n modulating the current carrier of index n, - x () the set of samples time resulting from accumulation in the accumulator, 5 Te = - the sampling period and 0 1 <N, L = 2 N. Fe The transmission method (300) of claim 1, further comprising a step of generating the time samples of the n-order carrier and wherein the detection and comparison steps (306, 307) are implemented. with respect to the step of generating the J time samples of the order carrier n, with a delay of determined duration corresponding to a clock cycle at Fe. The transmission method (300) of claim 1, wherein the step of accumulating in the second accumulator implements a carrier-derived, discrete inverse inverse Fourier transform with, for a given carrier n mapped by dn, a calculation of the J temporal samples, according to the following equation: N-1 Te n = [en'Til)] n = 0 with: - x '() the set of temporal samples resulting from the accumulation in the 2nd accumulator, Te = -Fe the sampling period and 0 <1 <N. L = 2. N. An OFDM signal transmission device (TX), characterized in that it comprises: a mapping module (203) for mapping data representative of a source signal to complex Xn symbols belonging to an extended constellation , 0 n <M, n and M being natural, Xn = An-EjBn, An and Bn being reals, a transformation module (205) for transforming M symbols Xn into M corrected symbols X'n, such that X ' n = Xn dn with dn a complex correction, - an OFDM modulator with N carriers for generating an OFDM symbol from the M corrected symbols X'n mapped on M among the N carriers, N being a natural, MN, the OFDM signal resulting from the succession of OFDM symbols, the transformation module (205, B, D, E, F, G, H) comprising: a first accumulator (B) to accumulate carrier carrier to J samples already present respectively J time samples corresponding to the J samples of the order carrier n mapped by a symbol i the intermediate X "n whose value is initialized according to the existence of an extended symbol corresponding to the symbol Xn in the extended constellation, n being from 1 to M-1, J being a natural, an accumulator (D), initialized with the J samples accumulated in the accumulator, to accumulate carrier carrier J samples corresponding to J time samples of the order carrier n mapped by a complex correction data dn respectively J samples already present, n ranging from 1 to M -1, a detection module (E), carrier after carrier, P samples from the J samples 2'd accumulator output having a power greater than. a threshold (a) and zeroing of the other JP samples, P being a natural, a comparison module (F, G) of these P samples with coincident temporal samples among J time samples of the n order carrier , delivering a binary correction control torque (Gn), a selection module (1-1) of the complex correction data item dn from a correction list of the intermediate symbol X "n as a function of the binary correction control torque ( Gn) to obtain the corrected symbol X'n = X "n + dn. 13. Device (1X) for transmitting an OFDM signal according to claim 11, such that the transformation module further comprises: a module (C) for generating the J time samples associated with the current carrier of index n. 14. Computer program comprising instructions for implementing a transmission method according to claim 1 when the program is executed by a processor. 15. Information medium comprising program instructions adapted to the implementation of the following steps when said program is loaded and executed in an OFDM transmission device: a mapping of data representative of a source signal on complex symbols Xn belonging to an extended constellation, 0 <n <M, n and M being natural, Xn = An + jBn, An and Bn being reals, a transformation of M symbols Xn into M corrected symbols X'n, a mapping of M corrected symbols X'n on M among N carriers of an OFDM modulator for generating an OFDM symbol, N being a natural, 11 / K \ l-, the OFDM signal resulting from the succession of OFDM symbols, such that the transformation comprises: a step (301) of initialization of a first accumulator (B), and comprises for each carrier of order n, n ranging from 1 to M-1: a step (302) of initialization of a symbol intermediate X "n depending on the existence of an extended symbol corresponding to the Ymbole Xn in the extended constellation, a step (303) of accumulation in the accumulator (B) respectively to J samples already present of J temporal samples corresponding to J samples of the order carrier n mapped by the intermediate symbol X " n, J being a natural, 33 and comprises: a step (304) of initialization of a 2nd accumulator (D) with J samples accumulated in the accumulator, and comprises for each carrier of order n, n ranging from 1 to M-1: a step (305) of accumulation in the second accumulator (D) respectively to J samples already present of J samples corresponding to J temporal samples of the order carrier n mapped by a correction data complex dn, a step (306) for detecting P samples among the J samples at the output of the accumulator having a power greater than a threshold (00 and zeroing J-10 P other samples, P being a natural, astep (307) of comparing these P samples with coincident temporal samples among J time samples of the order carrier n, delivering a binary correction control torque (On), a step (308) for selecting the data complex correction system dn from a list of 15 corrections of the intermediate symbol X "n as a function of the binary correction control torque (Gn) to obtain the corrected symbol X 'n = X" ni-dn. 16. Signal (S '(t)) OFDM obtained by the implementation of a method according to claim 1.