FR2885471A1 - METHOD FOR ITERATIVE DECODING OF OFDM / OQAM SIGNAL USING COMPLEX VALUE SYMBOLS, CORRESPONDING COMPUTER DEVICE AND PROGRAM - Google Patents

Abstract

The invention relates to a method for decoding a received OFDM / OQAM type multicarrier signal formed of a temporal succession of symbols consisting of a set of data elements, each of said data elements modulating a carrier frequency of said signal. , and being formed by summation of two real quadrature values each corresponding to a binary word of a source signal, modulated according to a predetermined modulation constellation, so as to transmit information on a real channel and on an imaginary channel of said OFDM symbols According to the invention, such a decoding method is iterative and relies on a successive processing of each of the real and imaginary paths of the signal. It consists more precisely in making an estimate of the source signal associated with one of the channels, then in determining the interference generated by this channel, which is subtracted from the received signal in order to make an estimate of the source signal associated with the channel. other way.

Description

2885471 1

  A method of iteratively decoding an OFDM / OQAM signal using complex value symbols, corresponding device and computer program.

  FIELD OF THE DISCLOSURE The field of the invention is that of the transmission and broadcasting of digital information. The invention relates in particular, but not exclusively, to the transmission and diffusion of digital information with high spectral efficiency over a limited frequency band, for example in a mobile radio environment.

  The invention relates more precisely to information transmission and dissemination techniques using OFDM (Orthogonal Frequency Division Multiplexing) type of multicarrier signals.

  2. Solutions of the Prior Art Several types of multicarrier modulations of the OFDM type are known to date.

  Among these, the most traditional modulation technique includes a particularly simple equalization system, based on the insertion of a guard interval. This guard interval, also called cyclic prefix, ensures good behavior against echoes, at the cost of a loss of spectral efficiency.

  Indeed, during this guard interval, no useful information is transmitted, so as to ensure that all the information received from the same symbol. This effectively combats the different phenomena of echoes due to PIES ("Interference Between Symbols", or in English ISI) and the Doppler effect.

  OFDM / OQAM (Orthogonal Frequency Division Multiplexing / Offset Quadrature Amplitude Modulation) modulation is an alternative to this conventional OFDM modulation, and was designed to avoid this loss in spectral efficiency due to the introduction of a guard interval.

  More precisely, the OFDM / OQAM modulation does not require the presence of a guard interval, or a cyclic prefix, thanks to a judicious choice of the prototype function modulating each of the signal carriers, which must be well localized in the signal. time-frequency space.

  It is recalled that all carriers of a multicarrier modulation form a multiplex and that each of the carriers of this multiplex is formatted using a same prototype function, denoted g (t), which characterizes the multicarrier modulation. By noting the spacing between two adjacent carriers of the multiplex, and the time spacing between two transmitted multicarrier symbols, the transmitted signal, at each instant nt, on the mth sub-band of central frequency vm, is ## EQU1 ## where am, n represent the digital data to be transmitted. The expression of the signal transmitted in baseband (centered around the frequency Mvo) is then: 2M-1 s (t) = a e q ,,, ez; nmvo'g (t nz) m, n 0 n m = 0 It will be noted that the case of a signal having an even number of frequency subbands has been considered for the sake of simplicity. It is of course possible to write the signal more generally in the form: ## EQU1 ## we introduce numerical data a ,,,,,, of zero value on the edges of the spectrum, which modifies the number of terms actually involved in the sum above, and allows for example to be reduced to an even number of carriers .

  The functions gnt, n (t) = e2'n "rv 'g (t n -c0) are called the time-frequency translates of g (t) .To find the information transmitted by each of the carriers, you have to choose g (t) and the phases q, ,,,,, so that the time-frequency translates above are separable A sufficient condition to verify this property of separability is that these translatées are orthogonal, in the sense of a scalar product defined on the set of finite energy functions (which is a Hilbert space in the mathematical sense). (1)

  The space of the finite energy functions admits the two scalar products below: the complex scalar product (xIy) = f x (t) y * (t) dt

R

  the real scalar product (xly) R = Jie fx (t) y * (t) dt

R

  Two types of multicarrier modulation are thus defined: a multicarrier modulation of complex type, for which the selected function g (t) guarantees an orthogonality in the complex sense of its translatées. In other words, the dot product between two distinct subcarriers is zero on both the real part and the imaginary part. This is the case, for example, of conventional OFDM modulation, also called OFDM / QAM (Orthogonal Frequency Division Multiplexing / Quadrature Amplitude Modulation). For such a modulation, cp, n ,,, = 0 and the data a, n, are complex.

  a real-type multicarrier modulation, for which the function g (t) chosen guarantees an orthogonality in the real sense of its translatées (ie the orthogonality is ensured on the real part of the scalar product between the versions shifted in time and / or in frequency ). This is the case for example OFDM / OQAM modulations. For such a type of modulation, cpn ,, n = (.t / 2) * (m + n) and the data an, .n are real.

  Thus, a transmitted OFDM / OQAM signal can be written as: A! -Is (t) = i'n'e2 vd g (t nr0) (3) m, ng ,,,,, (I) where a, n , n is the actual symbol sent on the sub-carrier mth under the symbol time, M is the number of carriers, vo is the inter-carrier spacing, to represents the duration of an OFDM / OQAM symbol, and g is the function prototype.

  This prototype function g modulating each carrier in OFDM / OQAM must be very well located in the time domain to limit intersymbol interference. In addition, it must be chosen very well located in the domain n m = 0 frequency, to mimic the interference between carriers (due to the Doppler effect, phase noise ...). This function must also guarantee the orthogonality between subcarriers.

  The mathematical functions with these characteristics exist, but the best localized ones provide orthogonality only on real values. In other words, the set of OFDM / OQAM subcarriers forms an orthogonal basis only if we consider only the actual scalar product, ie if we keep only the real part of the dot product between two subcarriers. For this reason, symbols transmitted by OFDM / OQAM modulation must be real valued, to be recovered without interference on reception.

  The orthogonality between the time-frequency translates of the prototype function is guaranteed if: Re (f gm, n (t) Eg ,,,,,,,, '(t) dt = Sm, m'Sn.n' (4) One of the prototype functions satisfying these conditions is the IOTA prototype function, described for example in patent document No. FR 2 733 869, which has the characteristic of being identical to its Fourier transform.

  FIG. 1 presents a time-frequency representation of the OFDM / OQAM modulated real-valued symbols and of complex value symbols transmitted by conventional OFDM modulation, without guard interval.

  In this figure, the triangles represent OFDM / QAM symbols with complex values. Rounds and stars represent OFDM / OQAM symbols with real values. For example, circles correspond to the real part and stars to the imaginary part of a complex symbol from a QAM constellation that is to be transmitted using OFDM / OQAM modulation.

  Indeed, for a conventional complex type OFDM modulation, the real and imaginary parts of a complex derived from the QAM constellation are transmitted simultaneously, all the symbol times Ti,; in the case of real-type OFDM / QAM offset, on the other hand, the real and imaginary parts are transmitted with a temporal offset of one half symbol time (Tu / 2).

  It can be seen in this FIG. 1 that the spectral efficiency of OFDM / OQAM is identical to that of conventional OFDM without guard interval. In fact, for the same inter-carrier spacing vo, one transmits: - in OFDM / OQAM, a real value per carrier all the time intervals io; in classical OFDM without guard interval, a complex value (i.e. two real values) every 2 * io = Tu.

  The amount of information transmitted by these two modulations is therefore identical. However, the need to introduce a guard interval of duration Tg in conventional OFDM has the effect of reducing the spectral efficiency of conventional OFDM with respect to OFDM / OQAM, which is (T0 + 2-ro) / 2t0 more effective.

  Figures 2 and 3 more particularly illustrate the transmission and reception chains of conventional OFDM / OQAM systems.

  A source 20 delivers a succession of binary elements, organized in successive binary words, which undergo a CC channel coding 21, intended to introduce redundancy into the signal, then an II interleaving 22. At the output of the interleaver 22, the interleaved binary words undergo real QAM modulation 23, at the end of which symbols from a predetermined modulation constellation (for example 16-QAM or 64-QAM) are available.

  Serial / parallel conversion 24 makes it possible to generate N symbol streams 25 in parallel from the symbols coming from the QAM modulator 23, to which a fast inverse Fourier transformation IFFT 26 is subjected, and then a polyphase filtering 27, starting from a adapted prototype function, such as IOTA. Then OFDM / OQAM type symbols are available which, after parallel / serial conversion 29, form an OFDM / OQAM type multicarrier signal whose symbols are real values and which is ready to be transmitted on a transmitting antenna 30.

  This OFDM / OQAM multicarrier signal, after passing through a transmission channel, is received on an antenna 31 of the reception channel of FIG. 3. It is subjected to a serial / parallel conversion 24, which makes it possible to extract from the multicarrier signal. OFDM / OQAM 32 symbols feeding a polyphase filter module 33, for example by means of the IOTA prototype function. After FFT fast Fourier transform 34, then equalization 35, we obtain N symbol streams 36 which, after parallel / serial conversion 29, constitute a received signal r representative of the modulated source signal.

  3. Disadvantages of the Prior Art The invention is more particularly in the context of a new type of OFDM / OQAM modulation, according to which information is simultaneously transmitted on the real and imaginary paths of the OFDM / OQAM symbols, which are therefore complex values, and no longer real. This new technique for encoding complex value symbol OFDM / OQAM multicarrier signals is the subject of a joint patent application filed on the same day as the present patent application, on behalf of the same holders and entitled "Coding process of an OFDM / OQAM type multicarrier signal using symbols with complex values, signal, devices and corresponding computer programs ".

  It should be noted that symbols with complex values are understood here and throughout this document as symbols obtained by summation of two real values in quadrature, as opposed to symbols with real values which convey information only on a single dimension (this unique dimension corresponding to the so-called real path of the complex value symbols of the present invention).

  It should be noted that this particular approach of the OFDM / OQAM coding goes against the technical prejudices of the skilled person, for whom such transmission of symbols with complex values in OFDM / OQAM has always been considered impossible, in because of the intrinsic interference generated between the real and imaginary parts of the symbols that would be sent, even in the absence of a transmission channel.

  The OFDM / OQAM reception and decoding technique of the prior art, as illustrated in FIG. 3, is not suitable for receiving and decoding such an OFDM / OQAM multicarrier signal with complex value symbols.

  Indeed, a first disadvantage of this reception technique of the prior art is that the decoding performance obtained would be very poor, because of the strong disturbances induced by the intrinsic interference generated by the use of the imaginary part of the symbols.

  Another disadvantage of this reception technique of the prior art is that it allows only the real part of the received symbols to be considered. The information transmitted, on transmission, on the imaginary part of the symbols would therefore be lost in reception.

  4. OBJECTIVES OF THE INVENTION The object of the invention is notably to overcome these disadvantages of the prior art.

  More precisely, an object of the invention is to provide a technique for receiving and decoding OFDM / OQAM type multicarrier signals which is adapted to a new type of OFDM / OQAM modulation with complex value symbols, in which information is transmitted, not only on the real part, but also on the imaginary part of the symbols.

  The purpose of the invention is therefore to provide such a technique for decoding OFDM / OQAM multicarrier modulations with complex value symbols which makes it possible to reliably estimate the source information conveyed by such modulations, even in the presence of intrinsic IES interference. .

  Another objective of the invention is to propose such a decoding technique that is simple and inexpensive to implement.

  It is another object of the invention to provide such a technique that makes it possible to make the most of the diversity of the transmission channel.

  5. DISCLOSURE OF THE INVENTION These objectives, as well as others which will appear subsequently, are achieved by means of a method of decoding a received OFDM / OQAM-type multicarrier signal formed of a succession temporal representation of symbols consisting of a set of data elements, each of said data elements modulating a carrier frequency of said signal.

  According to the invention, each of said data elements is formed by summation of two real quadrature values each corresponding to a binary word of a source signal, modulated according to a predetermined modulation constellation, so as to transmit information over a real path and on an imaginary channel of said OFDM / OQAM symbols. The decoding method of the invention then comprises: an OFDM / OQAM demodulation phase of said received signal delivering an equalized complex signal; a processing phase of said complexed equalized signal comprising at least one iteration of the steps of: selecting one of said channels of said equalized complex signal, called first channel; processing said first channel for delivering an estimate of said source signal corresponding to said first channel, referred to as the first estimated source signal; estimating an intrinsic interference generated by said first channel and affecting the other of said channels, called second channel, delivering an estimated interfering signal; subtracting said estimated interfering signal from said equalized complex signal so as to obtain an improved complex signal; selecting said second channel of said enhanced complex signal; processing said second channel making it possible to deliver an estimate of said source signal corresponding to said second channel, called the second estimated source signal.

  Thus, the invention is based on a completely new and inventive approach to the reception and decoding of OFDM / OQAM type multicarrier signals. Indeed, the technique of the invention makes it possible to decode OFDM / OQAM signals whose symbols are of complex values, i.e. they convey information on their real part, but also on their imaginary part.

  Such a decoding technique runs counter to the technical prejudices of the skilled person, who has always considered it impossible to reliably estimate source information conveyed in the form of complex value OFDM / OQAM symbols. because of the intrinsic interference that would then be generated to the transmission between the symbols.

  Ignoring these prejudices, the technique of the invention is based on an alternative and iterative decoding of the two real and imaginary paths of the received OFDM / OQAM symbols, after demodulation. More precisely, the invention consists in subtracting from the signal obtained at the output of the OFDM / OQAM demodulator an interference calculated from one of the two real or imaginary paths. From the signal obtained, the interference generated by the other channel is then calculated, and it is removed from the signal coming out of the OFDM / OQAM demodulator.

  Against all expectations, the results are satisfactory after only 3 or 4 iterations, and it is even possible to completely eliminate the interference in the case where we know, in reception, the data transmitted on the one of the two real or imaginary paths, as well as the transmission channel.

  Preferably, said first channel is the channel with the highest power. We begin by estimating the most powerful route, which is generally received in better conditions.

  Advantageously, said processing phase also comprises steps of: estimating an intrinsic interference generated by said second channel and affecting said first channel, delivering a second estimated interfering signal; subtracting said second estimated interfering signal from said equalized complex signal.

  Therefore, to estimate the source information of each channel, the intrinsic interference generated by the other channel is taken into account, which makes it possible to increase the quality of the estimate.

  Advantageously, said estimated first and / or second source signals are signals representative of at least one binary value and at least one associated confidence information. We are still talking about "soft" binary signals, from the English "loft bits". Such confidence information is an indication of the reliability of the estimate made, and therefore the confidence to be granted. More precisely, these signals convey relative integers, whose sign indicates whether the binary value conveyed is +1 or -1, and whose absolute value represents the information of confidence: the higher this absolute value, the more we can confirm with certainty that the binary value conveyed is indeed given by the sign of the relative integer transported.

  According to an advantageous characteristic of the invention, said first and second source signals having undergone, on transmission, channel coding, interleaving and real QAM modulation, said processing steps of said first and second channels implement, for each of said channels, sub-steps of: real QAM demodulation for converting a received Mayor signal on said first or second channel into a signal representative of at least one binary value and at least one associated confidence information, called a binary signal; deinterleaving said binary signal delivering a deinterleaved binary signal; channel decoding, making it possible to estimate, from said de-interleaved binary signal, a signal representative of at least one binary value and at least one associated confidence information, called an improved binary signal, and a decoded binary signal.

  In an advantageous embodiment of the invention, said steps for estimating an intrinsic interference implement successive sub-steps of: - OFDM / OQAM modulation of an improved Maire signal obtained by interleaving and converting said improved binary signal, delivering a modulated complex signal; OFDM / OQAM demodulation of said modulated signal, delivering a demodulated complex signal; selecting the imaginary part of said demodulated complex signal, delivering an estimated interfering signal.

  Thus, the intrinsic interference can simply be evaluated by conventional OFDM / OQAM modulation / demodulation, then selecting the imaginary part of the obtained signal.

  Preferably, said first and second source signals being correlated signals obtained from the same source data, account is taken of said confidence information associated with each of said first and second estimated source signals, to determine a combination of said first and second source signals estimated to optimize the decoding of said source data.

  Thus, if the first estimated source signal can be given great confidence, and the second estimated source signal is weakly trusted, the first source signal will be preferred to determine the source data that has been transmitted. If, on the other hand, the confidence information weighing each of the first and second source signals is substantially equal, the same shall be taken into account of each of these two signals to decode the source data.

  For example, the combination of the estimated first and second source signals implements an addition of their metrics. This addition can be weighted according to the trust information.

  In a variant of the invention, one of said estimated first and second source signals is used only when the reception of said received OFDM / OQAM multicarrier signal satisfies a predetermined quality criterion. For example, it is conceivable to systematically use the estimated source signal from the real channel, and to use the estimated source signal from the imaginary channel only if the reception is of good quality.

  The invention also relates to a device for decoding a received OFDM / OQAM type multicarrier signal formed of a temporal succession of symbols consisting of a set of data elements, each of said data elements modulating a carrier frequency of said signal, and being formed by summation of two real quadrature values each corresponding to a binary word of a source signal, modulated according to a predetermined modulation constellation, so as to transmit information on a real channel and on an imaginary channel of said symbols OFDM / OQAM. Such a decoding device comprises: OFDM / OQAM demodulation means of said received signal delivering an equalized complex signal; means for processing said complexed equalized signal comprising means for: selecting one of said channels of said equalized complex signal, called first channel; processing said first channel for delivering an estimate of said source signal corresponding to said first channel, referred to as the first estimated source signal; estimating an intrinsic interference generated by said first channel and affecting the other of said channels, called second channel, delivering an estimated interfering signal; subtracting said estimated interfering signal from said equalized complex signal 2.5 so as to obtain an improved complex signal; selecting said second channel of said enhanced complex signal; processing said second channel making it possible to deliver an estimate of said source signal corresponding to said second channel, called the second estimated source signal.

  The invention also relates to a computer program product downloadable from a communication network and / or stored on a computer readable medium and / or executable by a microprocessor, comprising program code instructions for the implementation of the method of decoding a multicarrier signal described above.

  6. List of Figures Other features and advantages of the invention will appear more clearly on reading the following description of a preferred embodiment, given as a simple illustrative and nonlimiting example, and the accompanying drawings, among others. which: - Figure 1, already commented in relation to the prior art, is a time-frequency representation of complex value symbols transmitted according to a conventional OFDM modulation and real-valued symbols transmitted according to an OFDM / OQAM modulation of the prior art; FIG. 2 shows a block diagram of a transmission chain of a multicarrier signal of the OFDM / OQAM type of the prior art; FIG. 3 shows a block diagram of a reception chain of a multicarrier signal of the OFDM / OQAM type of the prior art; FIG. 4 describes the technique of coding OFDM / OQAM multicarrier signals with complex value symbols in the form of a mimic of a transmission chain; FIG. 5 shows the general structure of a decoding device of the invention; Figure 6 details the operating principle of the iterative processing module of the device of Figure 5; FIG. 7 shows, in the form of simulation curves, the performances of the iterative decoding technique of the invention; FIG. 8 presents a simplified block diagram of the hardware architecture of the decoding device of FIG. 5.

  7. DESCRIPTION OF AN EMBODIMENT OF THE INVENTION The invention is part of the particular context of the use of symbols with complex values in an OFDM / OQAM type modulation, making it possible to transmit information on the actual channels. and imaginary OFDM / OQAM symbols. Such use of OFDM / OQAM symbols with complex values generates intrinsic interference related to the fact that the OFDM / OQAM carriers form an orthogonal base, and are therefore separable, only by considering the actual dot product between two carriers.

  In this context, the general principle of the invention is based on the alternative and iterative decoding of the real and imaginary channels of the received signal, by subtracting from the signal obtained at the output of an OFDM / OQAM demodulator, an interference calculated from the one of the ways. From the signal obtained, the interference generated by the other channel is calculated and it is removed from the signal coming out of the OFDM / OQAM demodulator.

  Firstly, in relation with FIG. 4, the principle of encoding an OFDM / OQAM type multicarrier signal with non-real but complex value symbols is recalled. Throughout the remainder of this document, a particular embodiment of the invention is described in which the prototype function used to format the OFDM / OQAM symbols is the IOTA function. Any other OFDM / OQAM prototype function can of course also be used.

  With respect to the transmission chain of FIG. 2, two sources of useful information 40, and 402, which may or may not be dependent on one another, are now considered. Thus, these two sources 40, and 402 can deliver the same binary words. They can also be completely decorrelated from one another, so that there is no relationship between the binary words from each of these two sources 40, and 402. Finally, the binary words from one of both of these sources may correspond to redundancy information from the other source.

  As in FIG. 2, the binary words coming from each of the two sources 40, and 402 undergo, after CC channel coding 41 ,, 412 and interlace H 42, 422, real QAM 43, and 432, so as to be transformed into real-valued symbols of a QPSK, 16-QAM constellation, etc. The M-ary signals from each of the two modulators 43, and 432 are then weighted, respectively by a real coefficient X, and by a complex coefficient (where X, and? 2 are scalars), then summed 44, in order to form a succession of complex value OQAM symbols having a flow rate of Ni; symbols per second.

  As in FIG. 2, a series-to-parallel conversion 24 is then carried out making it possible to obtain N streams of complex-valued OQAM symbols, each having a rate of 1 / -r0 symbols per second. These N fluxes 25 undergo an inverse fast Fourier transform or IFFT 26, then are shaped by IOTA 27 polyphase filtering, so as to generate OFDM / OQAM symbols 28, at a rate of 1 / -ro symbols per second. After parallel-to-serial conversion 29, the complex-symbol symbol OFDM / OQAM type multicarrier signal of the invention, which can be transmitted by the antenna 30, is then obtained.

  It is preferable to carry out, on each of the two channels (indices 1 and 2), channel coding, interleaving and real-QAM modulation operations that are independent from one channel to another, for the sake of diversity seeking.

  It will be noted that the complex signal formed by weighted summation of the symbols of each of the two channels is modulated via a conventional OFDM / OQAM modulator (same numerical references as for the different modules of the conventional modulator of FIG. 2) before being transmitted.

  It is recalled that the transmission in OFDMIOQAM of complex and non-real data causes intrinsic interference to the transmission between the symbols. The nature of this interference is known to the receiver and can therefore be eliminated in whole or in part at the reception, as presented in more detail later in connection with FIGS. 5 and 6.

  The receiver of the invention, presented in FIG. 5, comprises two modules referenced 51 and 52. The first module 51 carries out a conventional OFDMIOQAM demodulation of the multicarrier signal received on the antenna 50. However, unlike an OFDM / OQAM demodulator of the prior art, which only considers the real part of the received signal, it retains, at the output of the first demodulation module 51, the entire demodulated signal r, namely the real part and the imaginary part of the symbols received.

  The second module 52 implements an iterative process of recovery, from the demodulated complex signal r, of an estimate di (P) and d2 (P) of the information transmitted on the real and imaginary paths of the signal OFDM / OQAM, where the index p designates the rank of the iteration considered.

  More precisely, the first OFDM / OQAM demodulation module 51 carries out the following operations, as described previously with reference to FIG. 3: Parallel serial conversion 24; - FFI '34; - Polyphase filtering 33; - Equalization 35; - Series parallel conversion 29.

  As indicated above, however, the output of the first module 51 at the same time retains the real part and the imaginary part of the signal.

  The operation of the second iterative processing module 52 is now presented in greater detail in connection with FIG. Its principle consists in subtracting from the signal obtained at the output of the OFDM / OQAM demodulator 51 an interference calculated from one of the two channels, preferably the most powerful channel. From the signal obtained, the interference generated by the other less powerful channel is calculated and it is removed from the signal coming out of the OFDM / OQAM demodulator 51. This is repeated again until the process converges, which intervenes after only three or four iterations. The selection of each channel is done by taking the real or imaginary part of the signal. It will be noted that the receiver knows which channel was sent with the highest power.

  The different steps implemented successively by the second module 52 are thus as follows: - selection of one of the two channels on the equalized signal r. Preferably, it is the most powerful channel (in the example of FIG. 6, the real channel has been selected, retaining the real part 60 of the signal Re (r)); conversion of the signal Maire to a flexible binary signal (i.e. representative of at least one binary value and at least one associated confidence information) comprising a confidence calculated for example from the channel estimate; deinterleaving Ir '62 of the soft bit signal; DC channel decoding 63 which makes it possible to calculate, from the flexible binary signal (ie representative of a binary value and associated confidence information) deinterleaved, an improved binary signal x, also comprising a confidence measurement , as well as a decoded bit signal d, the decoded bit signal d, is the information that the receiver will deliver at the end; - interleaving H 64 of the enhanced binary signal x, - conversion 65 of the enhanced binary signal interleaved into a signal Maire improved soft (ie associated with a confidence) by real QAM modulation; - calculation 66 of the intrinsic interfering term generated by the selected channel (here the real channel) delivering an interfering signal y (1 ()); - subtraction 67 of the interfering signal i * v (J ')) affecting the second channel to the equalized signal r to deliver an improved received signal Note that the second channel is here the imaginary channel, that is why we multiply by i the interfering signal y (x, ") before subtracting this term interferes with the demodulated signal r; selecting the other channel of this improved signal obtained after subtraction 67.

  In Figure 6, it is the imaginary channel, which is selected by taking the imaginary part 68 of the improved signal; -Barninary conversion 61 of the signal of the imaginary channel by real MAQ demodulation to obtain a flexible binary signal (ie a signal representative of a binary value and associated confidence information, "soft bits") which may also include a confidence calculated for example from the channel estimate; deinterleaving IT-62 of the soft bit signal; - CC channel decoding 63, for calculating, from the de-interlaced flexible binary signal, an enhanced binary signal x2 also including a confidence measurement, and a decoded binary signal d2. The decoded binary signal d2 constitutes the other part of the information that will be delivered by the receiver at the end - interleaving II 64 of the enhanced binary signal x2; converting the interleaved enhanced binary signal into a soft enhanced (i.e. associated with confidence) signal. x2 ') by actual QAM modulation; calculation of the intrinsic interfering term generated by the selected channel (here the imaginary channel) delivering an interfering signal y (z,); subtraction 67 of the interfering signal y (e) affecting the first channel to the equalized signal r to deliver an improved signal.

  The set of successive steps above form the first iteration of the iterative process implemented by the second processing module referenced 52. At the end of this first iteration, one can then loop back on the first step, and select again. the complementary path of the last channel processed (namely to select 60 the actual channel in the case considered), so as to start a second iteration of estimation.

  In a preferred embodiment of the invention, the calculation 66 of the intrinsic interference generated by the real channel can be done by applying OFDMIOQAM modulation and demodulation to the signal considered, then selecting the imaginary part of the signal obtained. It is then not necessary to perform the multiplication of the interfering signal y (x ,, ") by i.

  An exemplary detailed embodiment of the invention is described below for an OFDMIOQAM modulation using the IOTA waveform. A simulation was carried out from the following parameters: - Size of the FTE: 512; 2885471 19 - Number of modulated carriers: 345; - Channel: AWGN; - Error correction coding: convolutional code of constraint length K = 7 of output 1/2.

  The flows on the 2 channels (real and imaginary) are independent. The power adjustment coefficients are: = 1 and? = 0.6.

  Two Offset-QPSK (Offset Quadrature Phase Shift Keying) constellations are used with 1/2 code efficiencies on both real and imaginary paths. For comparison, a 16-offset-QAM constellation (only on the real path) encoded by the same convolutional code and with the same coding efficiency is simulated, which therefore represents the technique of the prior art where the OFDM / OQAM symbols are with real values. A 16-Offset-QAM constellation is used to obtain the same spectral efficiency in both cases, provided that two OQPSK constellations are equivalent to a 16-OQAM constellation.

  The curves of FIG. 7 illustrate the performances of the coding technique of the invention, in the form of the bit error rate, expressed in decibels (dB), as a function of the carrier to noise ratio C / N ("Carrier To Noise Ratio ").

  Even in the simplest possible case where one can not take advantage of the time-frequency diversity of the channel, when the two paths have the same coding and are independent of each other, an improvement is observed. performs. Indeed, the difference between the bit error rate between the Offset-QPSK on the real channel (curve referenced 70) with the Offset-16QAM (curve referenced 72) is about 0.9 dB whereas the The difference between the same Offset-16QAM and the Offset-QPSK on the imaginary channel (curve referenced 71) is about 0.6 dB.

  This gain results from the fact that, although the level of interference is higher in the context of the invention which uses OFDM / OQAM symbols with complex values than in the prior art, the robustness of the QPSK constellations with respect to the constellations 16-QAM reduces the bit error rate. In fact, unlike the OFDM / OQAM modulations of the prior art for which an increase in the spectral efficiency required the use of constellations of high-quantization MAQ modulations, the use of complex-valued symbols according to FIG. The invention allows to achieve a satisfactory spectral efficiency, while using reduced number of states modulation constellations.

  These results are particularly interesting for example to transmit on both channels (real and imaginary) data streams that do not require the same error rate in reception. They are also in the case where the two channels (real and imaginary) are transmitted to two users associated with transmission channels having different signal-to-noise ratios. The relative power of the two channels can then be adjusted to account for the difference in channel quality of these two users.

  The simplified hardware architecture of the decoding device of FIG. 5 is now briefly presented with reference to FIG. 5. Such a device comprises a memory M 81, and a processing unit 80 equipped with a processor IuP, which is controlled by the computer program Pg 82.

  The processing unit 80 receives as input, via the antenna 50, a multicarrier signal OFDM / OQAM type whose symbols are, by construction, complex values.

  On reception of the multicarrier signal by the antenna 50, the microprocessor uP realizes, according to the instructions of the program Pg 52, an OFDM / OQAM demodulation, followed by an iterative processing, both described previously in detail in relation with FIGS. 6.

  The processing unit 80 outputs binary signals d; P 'and dzn' representing estimates of the source signals associated with each of the two channels, obtained at the end of the pth iteration. The value of p can be fixed a priori (for example, the process stops after 10 iterations). One can also consider stopping the iterative process when the estimated signals d; P 'and dzP' obtained at the output of the processing unit 50 are of sufficiently good quality.

  One can of course also consider issuing the estimates d; 'and dz' 'at the end of each of the iterations of treatment.

Claims (11)

  A method of decoding a received OFDM / OQAM type multicarrier signal formed of a temporal succession of symbols consisting of a set of data elements, each of said data elements modulating a carrier frequency of said signal, characterized in each of said data elements being formed by summation (44) of two real quadrature values each corresponding to a binary word of a source signal (40 ,, 402), modulated according to a predetermined modulation constellation (431, 432 ), so as to transmit information on a real channel and on an imaginary channel of said OFDM / OQAM symbols, said decoding method comprises: a phase (51) of OFDM / OQAM demodulation of said received signal delivering an equalized complex signal; a phase (52) of processing said complexed equalized signal comprising at least one iteration of the steps of: selecting one of said channels of said equalized complex signal, called first channel; processing said first channel for delivering an estimate of said source signal corresponding to said first channel, referred to as the first estimated source signal; estimating (66) an intrinsic interference generated by said first channel and affecting the other one of said channels, called second channel, delivering an estimated interfering signal; subtracting (67) said estimated interfering signal from said equalized complex signal to obtain an improved complex signal; selecting said second channel of said enhanced complex signal; processing said second channel for delivering an estimate of said source signal corresponding to said second channel, called second estimated source signal.   2. Decoding method according to claim 1. characterized in that said first channel is the channel with the highest power.   3. Decoding method according to any one of claims 1 and 2, characterized in that said processing phase also comprises steps of.   estimating (66) an intrinsic interference generated by said second channel and affecting said first channel, delivering a second estimated interfering signal; subtracting (67) said second estimated interfering signal from said equalized complex signal.   4. decoding method according to any one of claims 1 to 3, characterized in that said first and / or second source signals estimated are signals representative of at least one binary value and at least one associated confidence information .   5. Decoding method according to any one of claims 1 to 4, characterized in that, said first and second source signals having undergone on transmission a channel coding (41, 412), an interleaving (42, 422) and a real QAM (43, 432), said processing steps of said first and second channels implement, for each of said channels, sub-steps of: - real QAM demodulation (61) for converting a signal Mayor received on said first or second channel in a signal representative of at least one binary value and at least one associated confidence information, called a binary signal; deinterleaving (62) said binary signal delivering a deinterleaved binary signal; channel decoding (63), making it possible to estimate, from said de-interleaved binary signal, a signal representative of at least one binary value and at least one associated confidence information, called an improved binary signal, and a binary signal decoded.   6. decoding method according to claim 5, characterized in that said estimation steps (66) of intrinsic interference implement successive substeps of: - OFDM / OQAM modulation of an improved Maire signal obtained by interleaving and converting said enhanced bit signal, delivering a modulated complex signal; OFDM / OQAM demodulation of said modulated signal, delivering a demodulated complex signal; selecting the imaginary part of said demodulated complex signal, delivering an estimated interfering signal.   7. decoding method according to any one of claims 4 to 6, characterized in that, said first and second source signals (40 ,, 402) being correlated signals obtained from the same source data, account is taken of said confidence information associated with each of said first and second estimated source signals for determining a combination of said first and second estimated source signals for optimizing the decoding of said source data.   8. decoding method according to claim 7, characterized in that said combination implements an addition of metrics of said first and second source signals estimated.   9. Decoding method according to any one of claims 1 to 8, characterized in that one of said first and second estimated source signals is used only when the reception of said received OFDM / OQAM multicarrier signal satisfies a quality criterion. predetermined.   10. Device for decoding a received OFDM / OQAM type multicarrier signal formed of a temporal succession of symbols consisting of a set of data elements, each of said data elements modulating a carrier frequency of said signal, characterized in each of said data elements being formed by summing two real quadrature values each corresponding to a binary word of a source signal, modulated according to a predetermined modulation constellation, so as to transmit information over a real channel and an imaginary channel of said OFDM / OQAM symbols, said decoding device comprises: OFDM / OQAM demodulation means of said received signal delivering an equalized complex signal; means for processing said complexed equalized signal comprising means for: selecting one of said channels of said equalized complex signal, called first channel; processing said first channel for delivering an estimate of said source signal corresponding to said first channel, referred to as the first estimated source signal; estimating an intrinsic interference generated by said first channel and affecting the other of said channels, called second channel, delivering an estimated interfering signal; subtracting said estimated interfering signal from said equalized complex signal to obtain an improved complex signal; selecting said second channel of said enhanced complex signal; processing said second channel making it possible to deliver an estimate of said source signal corresponding to said second channel, called the second estimated source signal.   11. Computer program product downloadable from a communication network and / or stored on a computer readable medium and / or executable by a microprocessor, characterized in that it comprises program code instructions for the implementation of the method of decoding a multicarrier signal according to any one of claims 1 to 9.