FR2903833A1 - Signal receiving method for e.g. wireless telephone, involves estimating real and imaginary parts of transmission channel in neighboring region from complex values corresponding to drivers of group of region - Google Patents

Abstract

The method involves extracting complex values corresponding to drivers of a group of a neighboring region (22), after passage in a transmission channel. Real and imaginary parts of the transmission channel in the region is estimated from the complex values, by implementing a resolution of a four equation system with four unknowns for each of pair of pilots. The region is provided such that the channel is considered as constant. Independent claims are also included for the following: (1) a device for reception of a signal corresponding to a multicarrier signal emitted by an emitter via a transmission channel (2) a computer program product downloadable from a communication network and/or stored on a computer readable medium and/or executable by a processor and comprising program code instructions for implementation of a method for receiving a signal corresponding to a multicarrier signal emitted by an emitter via a transmission channel (3) a method for emitting a multicarrier signal, that is to be transmitted via a transmission channel (4) a device for emission of a multicarrier signal, that is to be transmitted via a transmission channel (5) a computer program product downloadable from a communication network and/or stored on a computer readable medium and/or executable by a processor and comprising program code instructions for implementation of a method for emitting a multicarrier signal (6) a multicarrier signal formed of temporal symbols formed of a set of data elements with real values.

Description

1 Methods of transmitting and receiving a multicarrier signal

  implement a corresponding channel estimate, devices and computer program products.  1.  FIELD OF THE DISCLOSURE The field of the invention is that of the transmission and broadcasting of digital information, in particular at high speed, over a limited frequency band.  More specifically, the invention relates to a technique for transmitting and receiving a multicarrier signal making it possible to obtain, on reception, an estimate of the transmission channel, for example in a mobile radio environment.  In particular, the technique according to the invention is well suited to the transmission of multi-carrier signals having undergone modulation of the OFDM / OQAM (Orthogonal Frequency Division Multiplexing / Offset Quadrature Amplitude Modulation) or BFDM / OQAM (Biorthogonal Frequency) type. Division Multiplexing / OQAM), for which the carriers are formatted by a prototype function.  2.  Prior art 2. 1 Multicarrier Modulations 2. 1. OFDM Modulations To date, multi-carrier modulations of OFDM (Orthogonally Frequency Division Multiplex) type are known.  Such a modulation technique provides an effective solution to the problem of broadcasting information, especially for multipath channels, wired or wireless.  As a result, the OFDM multicarrier modulation technique has been adopted in several standards and specifications for wired transmission applications, for example of the Asymmetric Digital Subscriber Line (ADSL) or the Powerline Communication (PLC) type. Online or CPL), or wireless, for example DAB (English Digital Audio Broadcasting), DVB-T (English Digital Video Broadcasting Terrestrial), or WLAN (English Wireless Local Area 2903833 2 Network).  However, the rectangular shaping of a signal made by an OFDM modulator has the disadvantage of a poor frequency localization.  Consequently, alternative solutions have been proposed, resulting in multi-carrier modulation systems in which the signal is formed by so-called prototype functions, making it possible to obtain a better frequency localization.  Indeed, all the carriers of a multicarrier modulation form a multiplex, and each of the carriers of this multiplex can be formatted using the same prototype function, denoted g (t), which characterizes the multicarrier modulation.  2. 1. OFDM / OQAM Modulations Thus, a proposed solution consists in replacing a Quadrature Amplitude Modulation (QAM) modulation, implemented on each of the carriers, by a half-time shifted modulation symbolizing the real and imaginary parts of the signals. complex symbols to be transmitted, for two successive carrier frequencies.  This alternation leads to an OFDM / OQAM type 20 multicarrier modulation.  This approach makes it possible in particular to achieve the desired orthogonality conditions with prototype filters which are not necessarily of rectangular shape.  Indeed, the offset (time offset) introduced by the OQAM modulation makes it possible to relax orthogonality constraints, or more generally biorthogonality constraints.  This modulation family thus offers a wider choice of prototype functions than the simple rectangular prototype function of an OFDM modulation.  Thus, depending on the type of transmission channel considered for a given application, such as for example the radiomobile channel or the in-line carrier channel (PLC), a choice of prototype functions suitable for the types of distortion encountered can be made. .  In particular, it is preferable to retain prototype functions having a better frequency selectivity than the cardinal sinus used in OFDM modulation, in particular in the radiomobile channel to fight against the frequency dispersion due to the Doppler effect, or in the CPL channel to better resist narrow-band jammers, and generally to more easily meet the frequency specifications of the emission masks.  The OFDM / OQAM modulation is thus an alternative to the conventional OFDM modulation, based on a judicious choice of the prototype function modulating each of the signal carriers, which must be well localized in the time / frequency space.  In particular, FIG. 1 illustrates a time / frequency representation of OFDM / OQAM modulated real-valued data elements and complex OFDM modulated complex-valued data elements without a guard interval, a complex valued symbol. OFDM / QAM or OFDM / OQAM actual values consisting of a set of data elements at a given time t.  In addition, each time / frequency slot carries a carrier frequency, called sub-carrier or directly carrier in the following description.  In this FIG. 1, the triangles at a given instant t represent the data elements with complex values of an OFDM / QAM symbol.  The circles and the stars at a given instant t represent the real-valued data elements of an OFDM / OQAM symbol.  For example, for two successive OFDM / OQAM symbols with real values, the circles correspond to the real part 25 and the stars to the imaginary part of a complex symbol coming from a QAM constellation that is to be transmitted using a modulation OFDM / OQAM.  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 symbol times Tu; in the context of a real type OFDM / OQAM modulation, on the other hand, the real and imaginary parts are transmitted with a temporal offset of one half symbol complex time (Tu / 2).  It can be seen in this FIG. 1 that the spectral efficiency of OFDM / OQAM 5 is identical to that of conventional OFDM without guard interval.  Indeed, by noting the spacing between two adjacent carriers of the multiplex, and the time spacing between two real-valued symbols, for the same inter-carrier spacing vo is transmitted in OFDM / OQAM, a real value per carrier every 10 time intervals' ro; - in conventional OFDM without guard interval, a complex value (i. e.  two real values) every 2 x 'ro = Tä.  In other words, the spectral efficiency of the OFDM / OQAM is (Tg + 2'ro) / 2 ro times greater than that of the classic OFDM with a guard interval of duration Tg.  2. 1. Moreover, if one chooses to have on the receiving side demodulation functions which are not necessarily the combined functions of the prototype functions used in transmission, it is possible, using the property of biorthogonality, to generalize. OFDM / OQAM to the BFDM / OQAM modulation technique.  The offset principle, related to the OQAM family, is strictly identical in the context of a BFDM / OQAM type modulation.  Therefore, Figure 1 also applies to BFDM / OQAM modulations.  More precisely, the advantage of the BFDM / OQAM type modulation is to allow, for a given length of prototype filter, a reduction in the delay brought by the transmission system.  As previously indicated, the BFDM / OQAM modulation technique, like the OFDM / OQAM, transmits actual value symbols at a rate twice that of which the OFDM transmits complex value symbols.  Consequently, these two modulations have a priori the same spectral efficiency.  More precisely, the BFDM / OQAM signal can be represented in baseband in the following form: M1 5 s (t) _ 1 am, ng (t -no) ei2ttmvote: 10mn (1), with: - am, n the actual data items to be transmitted on a carrier m at time n; M the number of carrier frequencies (necessarily even); The prototype function used by the modulator; ro the duration of a BFDM / OQAM symbol; vo inter-carrier spacing; om, n a phase term chosen to achieve a real-imaginary part-part alternation permitting orthogonality, or more generally biorthogonality.  In fact, in the biorthogonal case, the demodulation base at the reception may be different from that of the emission, and may be expressed in the following form: fm, n (t) _ f (t ù nZ0) ej2nmvot ei0m , n (2) The biorthogonality condition is then expressed in the following form: (gm, n ,. Im = J (cSm älbn n • 3) where: (. ,. ) R denotes the actual dot product and 9i {} denotes the real part.  However, a disadvantage of BFDM / OQAM (or OFDM / OQAM) type modulation techniques is that the biorthogonality (or orthogonality) condition is only realized for the actual values of symbols to be transmitted, which poses a problem. problem of estimation in reception, and in particular of estimation of the transmission channel, insofar as the received symbols are complex.  n m = 0 gm, n (t) 2903833 6 2. The transmission channel The characteristics of a transmission channel, in particular in a mobile radio environment, and the techniques for estimating such a channel are thus briefly described below.  It is recalled that the process of shaping an electrical signal from the information to be transmitted depends on the conditions under which such a signal is transmitted.  2. 2. Characteristics of the transmission channel In radiomobile environment, the transmitted wave undergoes, during its course, multiple reflections, and the receiver thus receives a sum of delayed versions of the transmitted signal.  Each of these versions is attenuated and shifted randomly.  This phenomenon, known as spread delay, generates inter-symbol interference (IES).  In particular, IES is understood to mean interference between temporal symbols and / or between carriers.  For example, in an urban-type environment, the delay spread is of the order of or less than a few microseconds.  Since the receiver (for example a mobile radiotelephone of a motorist) is supposed to be moving, the so-called Doppler effect also acts on each path, which results in a frequency offset of the received spectrum, proportional to the speed of movement of the mobile. .  The combination of these effects results in a non-stationary channel with deep fading at certain frequencies.  Such a channel is particularly qualified frequency selective channel.  For some applications, particularly interesting in the context of the present invention, the transmission band is of greater width than the channel coherence band (ie the band for which the frequency response of the channel can be considered constant over a given period).  Fading therefore appears in the band, that is to say that at a given moment, certain frequencies are strongly attenuated.  In order to combat these different phenomena (due to PIES and to the Doppler effect), it has been envisaged in the OFDM systems to add a guard interval, during which no useful information is transmitted, so that to guarantee that all information received comes from the same symbol.  In the case of coherent demodulation of the sub-carriers, the distortion provided by the channel is then corrected by estimating its value at any point in the time / frequency network.  The introduction of such a guard interval thus makes it possible to reduce the phenomena related to intersymbol interference.  However, a major disadvantage of this technique is that it is of reduced spectral efficiency, with no useful information being transmitted during the guard interval.  On the other hand, OFDM / OQAM and BFDM / OQAM modulation techniques do not require the introduction of a guard interval or a cyclic prefix, while having the same spectral efficiency as conventional OFDM modulation.  15 2. 2. 2 Estimation of the transmission channel The distinct characteristics of the multicarrier modulations of real type on the one hand, and of complex type on the other hand, induce different treatments during the implementation of an estimation of the transmission channel.  A technique for estimating the transmission channel 20 for real type modulations, for example OFDM / OQAM or BFDM / OQAM, is described below.  Indeed, in the case of a real-type multicarrier modulation, the fact of having an orthogonality of translatées in the real sense makes the channel estimation process more difficult.  Indeed, to estimate the complex gain of the channel on a given subcarrier, it is necessary to perform the complex projection of the signal received on the subcarrier considered.  Now, the orthogonality of the translatées in the real sense and the fact that the prototypical functions, even chosen localized at best in time and in frequency, are of infinite support on at least one of the two time or frequency axes, imply that, even on a ideal channel, interference (intrinsic) between carriers is generated.  Indeed, the imaginary part of the projection of the signal received on the basis of the translatées of the prototype function is not zero.  It then appears a disruptive term which is added to the demodulated signal, and must be corrected before making the channel estimate.  It is therefore necessary to devise methods for compensating for this loss of complex orthogonality, thus overcoming at least some of the disadvantages of this prior art technique for OFDM / OQAM or BFDM / OQAM modulations.  Consider, for example, y (t) the received signal.  In particular, it is assumed that the choice of the parameters of the multicarrier modulation ensures that the channel can be considered flat on each of the sub-carriers for each OFDM / OQAM symbol.  The channel can then be modeled by a complex coefficient per subcarrier, denoted Hm, n, where m is the index of the subcarrier and n that of the OFDM / OQAM symbol.  The complex projection of the multicarrier signal at point 15 (mo, no) of the time / frequency space is then used to estimate the transmission channel Hmo, no at this location.  Thus, if we emit amo, no =, r at this location, we have: fy (t) gmo, no (t) dt (4) Hm0 no = ù Assuming that the channel is ideal (y (t) = s (t)), since the OFDM / OQAM and BFDM / OQAM modulations have only real orthogonality (equation (3)), we can not have Hmo, no = 1.  So considering a no = (s, gmo, no) c = fs (t) gao, no (t) dt, and assuming that the channel is ideal, we have: a (c) E * (5) (m, n) $ (mo, no) mo no = NIT + am, nf gm, n (t) gmo mo (t) dt Ia Io no E j9t 25 where (. ,. ) c designates the complex dot product.  Equation (5) reflects the fact that the complex projection of the perfectly transmitted signal is nevertheless tainted by an intersymbol interference (IES) intrinsic to the OFDM / OQAM or BFDM / OQAM modulations, noted by Imono In particular, the existence This intersymbol interference greatly disturbs the estimation of the transmission channel, and therefore the estimation of the symbols.  A solution to this problem has notably been proposed in the patent document WO 02/25884 published on March 28, 2002.  More specifically, the technique proposed in this document makes it possible to limit this interference, by using a specific framing of the data on transmission.  Thus, this technique associates 3 x 3 areas of the network time / frequency, said first ring, or areas of larger size, a reference data element, called pilot, and a control data.  A disadvantage of this prior art technique is to require a matrix calculation on transmission and reception, with a matrix size increasing with the size of the ring.  Another disadvantage of this technique of the prior art appears in the case of a transmission for which the time / frequency resource is distributed among several users.  In this case, the corona relationship requires that all data elements of the same corona be assigned to the same user.  This constraint notably poses problems of granularity and resource allocation, the number of pilots issued generally being between 2 and 5%.  Another existing technique used to reduce the intersymbol interference term is to emit a preamble inserted into the multicarrier signal at the beginning of the frame, a frame being formed of a set of at least one reference symbol called preamble and a set of useful symbols.  In this case, the preamble is of a minimum duration of 3'ro.  A disadvantage of this technique of the prior art is the loss of spectral efficiency related to the emission of the preamble.  There is therefore a need for a technique for obtaining a best estimate of the transmission channel, and leading to a more accurate estimation of the informative data elements carried by the multicarrier signal.  3.  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 of receiving a received signal corresponding to a multicarrier signal emitted by at least one transmitter via a transmission channel, said multicarrier signal being formed of a temporal succession of symbols consisting of a set of real-valued data elements comprising: - informative data elements, and 10 for at least some said symbols, reference data elements called pilots, whose value and the location at the time of transmission are known to at least one receiver intended to perform a reception of said multicarrier signal, each of said data elements modulating a carrier frequency of said Signal, a carrier frequency modulated by one of said data elements being called a carrier.  According to the invention, groups of at least two real-valued pilots are each located in a so-called neighborhood region in the time / frequency space, a neighborhood region being a region in which said transmission channel 20 is considered as substantially constant, said reception method comprises, for at least one of said neighborhood regions: a step of extracting at least two complex values corresponding to the pilots of the group of said neighborhood region, after passing through said transmission channel, a step of estimating the real and imaginary portions of said transmission channel in said neighborhood region from said complex values.  Thus, the invention is based on a novel and inventive approach to estimating the transmission channel in a transmission system employing a multicarrier signal carrying real-valued data elements.  In particular, such a multicarrier signal is of the OFDM / OQAM or BFDM / OQAM type.  It is recalled that the transmission channel is divided into cells according to the time and frequency axes.  Each cell or location of the time / frequency space is assigned a dedicated carrier.  We distribute the information to be transported on all of these carriers.  More specifically, the technique according to the invention is based on the implementation, on the receiving side, of an estimation of the transmission channel in a neighborhood region in which is located a group of at least two pilots.  In particular, a neighborhood region is a region in which the transmission channel does not vary, or little, in time and / or frequency.  Thus, this technique allows estimation of the transmission channel for the neighborhood region, without wasting the time / frequency resource, since it is not necessary to impose a constraint on the value of a data element carried by a directly adjacent carrier (located in a region called the first crown) of each of the pilots of the group.  It is therefore not necessary to impose a first-crown relationship to reduce PIES.  Indeed, the technique according to the invention makes it possible in particular, compared to the techniques of the prior art, to optimize the time / frequency resource, since it does not require the use of a guard interval, during which no useful information is transmitted, nor a specific framing of the data in transmission, while maintaining the same density of pilots.  These drivers can be boosted.  According to a particular characteristic of the invention, each of said groups consists of a pair of pilots.  According to a particular embodiment, this distribution of pilots in pairs makes it possible in particular during said estimation step to implement a resolution of a system of four equations with four unknowns for each of said pairs of pilots.  Indeed, for each pilot of each pair, during this estimation step, the complex values received at the time / frequency locations (positions) of said pilots, as well as the knowledge of the real values of said drivers before transmission, are used. by the transmission channel.  For example, said system of equations implements the following equations: ## EQU1 ## ## STR2 ## (`) = CH (r) a (r) + H (r) (j) ymi, neither ml, nor ml, nor mi, nor mi, nor with: (m0, n0) a first location of the time space / frequency and (ml, nl) a second location of the time / frequency space in said neighborhood region, -y (o) no and yöo no real values respectively equal to the real part and the imaginary part of the value complex of the received signal in (m0, n0), y (r) and y (i) real values equal respectively to the mi, ni mi and real part and to the imaginary part of the complex value of the carrier located at the location (mi) 11), Hn no the real part of the complex value of said transmission channel at the location (m0, n0) and H (r) nor the real part of the complex value of said channel l transmission at the location 20 (ml, nl), with Hm (l) nor equal to H, the transmission channel being mo, no considered substantially constant in said neighborhood region, amo and ai (no real values respectively equal to the real number and the imaginary part of the complex value of a first pilot of the group of said neighborhood region at the location (mo, n0), aml) nl and am` nl real values equal respectively to the real part and the imaginary part of the complex value of a second pilot of said group at the location (mi, nl), 5 - C a real.  In particular, said estimation step implements an intermediate calculation of a ratio between said real and imaginary parts of said complex values.  Another embodiment of the invention relates to a device for receiving a received signal corresponding to a multicarrier signal, as described previously, transmitted by at least one transmitter via a transmission channel.  According to the invention, groups of at least two real-valued pilots are each located in a so-called neighborhood region in the time / frequency space, a neighborhood region being a region in which said transmission channel is considered substantially constant, such a receiving device 15 comprises, for at least one of said neighborhood regions: means for extracting at least two complex values corresponding to the pilots of the group of said neighborhood region, after passing through said transmission channel, means for estimating the real and imaginary portions of said transmission channel in said neighborhood region from said complex values.  Such a reception device is particularly adapted to implement the reception method as described above.  For example, such a receiving device corresponds to or is included in a terminal (radiotelephone, laptop, PDA. . . ) or in a base station.  Another aspect of the invention relates to a method of transmitting a multicarrier signal, intended to be transmitted via a transmission channel, formed of a temporal succession of symbols consisting of a set of data elements with values. comprising: informative data elements, and for at least some of said symbols, reference data elements referred to as pilots whose value and location on transmission are known to at least one receiver intended to perform Receiving said multicarrier signal, each of said data elements modulating a carrier frequency of said signal, a carrier frequency modulated by one of said data elements being called a carrier.  According to the invention, such a method uses groups of at least two real value pilots, each localized in a so-called neighborhood region in the time / frequency space, a neighborhood region being a region in which said transmission channel is considered substantially constant.  More specifically, this technique relies on the implementation, on the transmission side, of groups of at least two pilots, that is to say groups of at least two reference data elements, located in a region of neighborhood, in which also informative data elements can be issued, and wherein the transmission channel is considered substantially constant.  On the reception side, it is thus possible to estimate the transmission channel at the neighborhood region.  According to a particular aspect of the invention, each of said groups consists of a pair of pilots.  In particular, the drivers constituting a pair can be divided into time and / or frequency.  Thus, if the transmission channel is substantially constant in time, the pilots of the pair can be spread in time, and if the transmission channel is substantially constant in frequency, the pilots of the pair can be frequency-scaled.  If the channel is substantially constant in time and frequency, the drivers can also be spread in time and frequency.  According to a particular aspect of the invention, in a system in which at least some of said carriers carry at least two data elements intended for respective users and / or services respectively, such a transmission method implements the same carrier modulated by at least two pilots each associated with one of said users and / or services.  For example, such a system allows certain carriers to be used to carry data items for different users, using a spreading code associated with each user.  Thus, according to this particular aspect of the invention, the same carrier can be modulated by at least two drivers each associated with a distinct user.  Likewise, such a system makes it possible to use certain carriers to carry data elements intended for different services or uses, by means of a spreading code associated with each service.  Thus, according to another particular aspect of the invention, the same carrier can be modulated by at least two drivers each associated with a separate service.  Another embodiment of the invention also relates to a device for transmitting a multicarrier signal as described above.  According to the invention, such a transmission device comprises means for implementing groups of at least two real value pilots, each located in a so-called neighborhood region in the time / frequency space, a neighborhood region. being a region in which said transmission channel is considered substantially constant.  Such an emission device is particularly suitable for implementing the transmission method as described above.  In particular, it is adapted to transmit such a multicarrier signal to the reception device described above.  For example, such a transmitting device corresponds to or is included in a terminal (radiotelephone, laptop, PDA. . . ) or in a base station.  Yet another aspect of the invention relates to a computer program product downloadable from a communication network and / or recorded on a computer-readable and / or executable medium by a processor, including program code instructions for implementing the reception method described above, and / or a computer program product downloadable from a communication network and / or recorded on a computer-readable and / or executable medium by a processor, comprising code instructions program for carrying out the transmission method described above.  Finally, another aspect of the invention relates to a multicarrier signal formed of a temporal succession of symbols consisting of a set of real-valued data elements comprising: - informative data elements, and for at least some of said data elements; symbols, called reference data elements called pilots, whose value and the location at the time of transmission are known to at least one receiver intended to perform a reception of said multicarrier signal, each of said data elements modulating a carrier frequency of said signal, a carrier frequency modulated by one of said data elements being called a carrier.  According to the invention, said set of real-valued data elements comprises groups of at least two real-valued drivers, each located in a so-called neighborhood region in the time / frequency space, a neighborhood region being a region in which said transmission channel is considered substantially constant.  Such a signal may in particular represent a multicarrier signal emitted according to the transmission method described above.  It can also be received according to the reception method described above.  4.  List of Figures Other features and advantages of the invention will appear more clearly on reading the following description of a particular embodiment, given by way of a simple illustrative and nonlimiting example, and the accompanying drawings. of which: FIG. 1, already commented on in relation to the prior art, is a time / frequency representation of the 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. 2A illustrates the structure of a multicarrier signal according to one embodiment of the invention; FIGS. 2B and 2C illustrate examples of particular distribution of pilot pairs according to embodiments of the invention; FIG. 3 illustrates an exemplary positioning of the pilots of a multi-carrier signal according to one embodiment of the invention; FIG. 4 shows the main steps of the reception method according to one embodiment of the invention; FIG. 5 illustrates the comparative result of a perfect channel estimate and a channel estimate according to a particular embodiment of the invention; Figures 6A and 6B respectively show the structure of a transmitting device and a receiving device, according to a particular embodiment of the invention.  5.  DESCRIPTION OF AN EMBODIMENT OF THE INVENTION The general principle of the invention is based on taking into account at least one group of pilots in a multicarrier signal using real-valued data elements. , so as to obtain, in reception, an estimation of the transmission channel between a transmitter and a receiver.  More precisely, the invention is based, according to a particular embodiment, on the fact that each group of pilots is located in a so-called neighborhood region, in which the transmission channel is considered to be substantially constant.  Thus, the estimation of the transmission channel according to this embodiment is obtained for said neighborhood region.  A particular embodiment of the invention is described below, implemented in the context of an OFDM / OQAM type multicarrier modulation.  On the transmitting side, real-valued driver groups, i.e. reference data elements known to the receiver, located in a neighborhood region in which the transmission channel is considered invariant are inserted into the multicarrier signal. .  In particular, a group of pilots according to this embodiment of the invention consists of a pair of pilots, distributed in time and / or in frequency, depending on whether the channel is considered to be substantially constant in time and / or in frequency.  More precisely, as illustrated with reference to FIG. 2A, a multicarrier signal formed of a temporal succession of symbols 211, 212, is considered. . . , 21N, consisting of a set of real-valued data items comprising: informative data items, corresponding to blank blanks; and for at least some of said symbols, reference data elements called pilots, corresponding to the white circles annotated with the letter P, the value and the location of which on transmission are known to at least one receiver intended to perform a reception of the multicarrier signal.  More precisely, the pilots are inserted, in pairs, in the multicarrier signal so as to allow an estimate of the transmission channel, in reception, for the region in which said pilots are inserted in transmission.  For example, if we consider that the channel is substantially constant in time on a neighboring region 22, delimited by dashed lines, the pilots P1 and P2 of the pair 221, delimited in solid lines in FIG. 2A, are added in the multi-carrier signal on two consecutive real time symbols of the region 22.  Depending on the temporal variations of the transmission channel, the drivers can be inserted on two non-consecutive real symbols, for example the pilots P3 and P4 of the pair 231, a neighborhood region 23 considered for the estimation of the channel being delimited in FIG. dashed lines.  According to an alternative embodiment in which the transmission channel does not vary or is little in frequency, the pilots P5 and P6 of the pair 241 (delimited in solid lines in FIG. 2A) are added in the multicarrier signal on two consecutive frequencies of a neighborhood region 24 (delimited in dashed lines).  Similarly, according to the frequency variations of the transmission channel, the drivers can be inserted on two non-consecutive frequencies.  According to another embodiment of the invention, the pilots P7 and P8 of the pair 251 are added to the multicarrier signal on two consecutive real symbols 15 and on two consecutive frequencies.  In this case, the channel is considered substantially constant in time and frequency, in a neighborhood region 25 (defined in dashed lines), considered for the estimation of the channel.  Two examples of alternative embodiments are illustrated in FIGS. 2B and 2C.  In Figure 2B, the actual pilot pairs are inserted on the same symbol to form a preamble.  The pairs of drivers (P2m-1, P2m) make it possible to estimate the channel at time t.  In FIG. 2C, the real pilot pairs are inserted on a dedicated carrier, on consecutive symbols in time.  The pairs of pilots (P2m-1, P2m) make it possible to estimate the channel at the frequency m.  According to another variant embodiment, illustrated in FIG. 3, certain carriers of the multicarrier signal carry at least two data elements intended respectively for different users and / or services.  The transmission method of the invention, according to this variant embodiment, inserts on the same carrier, represented in this figure as a time / frequency block in which the channel is assumed to be substantially constant, two pilots P9 and P10, transmitted at the same time as data, and intended for two users and / or two separate services.  The axis called "code" makes it possible to associate with each data element (informative or reference) a code known to the receiver and used to distinguish the users and / or services.  On the receiving side, as illustrated in connection with FIG. 4, the complex values corresponding to the pilots of a group in a neighborhood region are extracted from the received signal y (t).  For example, a group comprising a pair of pilots is considered.  From these complex values, we obtain a system of four equations with four unknowns, for each pair of pilots.  An intermediate calculation 32 of the ratio between the real and imaginary parts of the channel is first carried out, taking into account the real values of the two pilots of the pair and the complex values received on their positions and extracted at step 31.  The system of equations is then solved in step 33 to obtain the estimation of the transmission channel for the neighborhood region of the pilot pair under consideration.  An exemplary implementation of the reception technique according to this particular embodiment of the invention, in the context of a BFDM / OQAM type modulation, is described below.  More precisely, the received signal y (t) can be written in the form: M-1 y (t) = Hmcnam, nSm, n (t) + b (t) (6), nm = 0 with Hm ~ n the complex coefficients representative of the transmission channel at each time / frequency location, with m the frequency index and n the time index, and b (t) the noise component.  As already specified for the OFDM / OQAM modulations, the transmitted signal and the transmission channel being modeled in baseband by complex numbers, the coefficient has to be estimated in reception for each mo, no. 2903833 21 element (mo, no ) of the time / frequency network is also a complex number.  Assuming also that the channel is approximately constant over a given region of the time / frequency space, due to the biorthogonality of the pair of functions (f, g) described in relation to equation (3), the received signal on the carrier mo at time no is estimated by: (c) ym0, n0 _ \ S'fmo, no C (`) = H (a - + H (`) ymo, no mo, no mono mo, no am, nf gm, n (t) fmo, no (t) dt _ (m, n) # (mo, no) + D, no, no + no Cmo, no (7) In this expression, the term C , no, äo is related to the interference created in the region where the transmission channel is assumed to be constant, and the term Dm (), no is associated with the interference created in the regions where the transmission channel is not more supposed constant.  In the remainder of the description, the noise component b is omitted, and the term D ,,, 4 no is neglected so as to simplify the equations.  It is assumed here that the channel is substantially constant in time and / or frequency, that is to say on (Sn + 1) zo and / or on (Sm + 1) vo, with Sn and Sm integers.  It is thus considered that the received signal, at any location (m, n) of the time / frequency network, can be interpreted as the result of the product of a complex channel by a complex coefficient, that is to say: ym (n = Hmcnamcn = H, (nn (a (nr) n + ja, (n), n), where a (; n and a, (n, `), n are real values (the exponent ( r) indicating the real part of a complex value, and the exponent (i) the imaginary part).  We consider here a particular variant of the mode of

  embodiment, in which the channel is substantially constant in time, over a period 2 'ro. In this example, pilots a, no, no, and a, no, äo + l are inserted in the signal at the locations (mo, no) and (mo, no +1), and form a pair used for estimating the channel in a neighborhood region where the channel is considered substantially constant in time. The real and imaginary parts of the received signal y (c,), are written in the following way, by putting H, (n = Hm (n + jHm ') n. The channel being assumed constant on 2 -ro we have H, (c), no = H, (äo) ,, b + 1. By placing H, (ä '), ~ = CH (r, ~, C being a real, we obtain the system of four equations with four unknowns: (r) ù H (r) a (r) ù CH (r) a (') Ymo, nb, no mono mono mo, no (') = CH ~ 'r) a ( ## EQU1 ## , no + l mo, no mono *, (`) = CH (r) a (r) + H (r) a (')` Ymo, no + 1 mo, no mo, no + 1 mono mo, no + 1 from which we can deduce: a (r) (r) a (r) (r) = CH (r) a (r) a (m`o) no + a (mor), no a (') mono + I mn (mo, no Ymo, no +1 mo, n (mo, n + 1 mono + 1) a (r) (') = H (r) / a (r) a (') ù a (r) a ('mono + lYmo, n mo, noY, no + 1 ù mo, nol mono + l mono mono mono + 1) Making the ratio of the two equations: (r) (r) (r) (r) C arno, no + lYmo, no ù am0, noYmo, no + 1 (r) (i) (r) (i) amo, noYmo, np + 1 ù anb, no + lymo, no and considering that the equation (8) can be written in the following way: (r) = H (r) a (r) ù CH (r) a (') Ymn m, nm, nm, nm, n (i) (r) (r) (r) (i) YmnùCHmnnn + Hmnnnn we obtain: (r) = H (r) a (r) ùCH (a (') Ymo, no mono mono mono mo, no C (') = C2H ( r) a (r) + CH (r) a (') Ymo, no ma, no mono mono mo, no By summing these two equations, we have: Hm (o no a, () äro no (1 + C2) = yni no + CY, (, 'b) hence: y (r) + Cy (n H (r) = m, n mo, no and H (') = CH (r) mono a ( r) (1 + CZ) mo, mono mono This gives the estimation of the channel in a neighborhood region, where the channel is considered substantially constant, and in which a pair of = H (r) a (r) ## EQU1 ## Pilots, nm, nm, n (8) (9) were inserted according to the embodiment described above. More generally, the two real drivers can be inserted in the signal at any position (m1, n1) and (m2, n2) with (m2 = m1 + 8m, n2 = n1 + 6n), the only constraint being that the channel remains substantially invariant on a number of frequencies corresponding to 5m + 1 and on a number of real symbols equivalent to Sn + 1. The resolution method as described above applies to this general case. In the particular case illustrated in FIG. 3, already described above, the resolution method is the same, with a pilot value received at a location (m, n) that can be written in the following way: N-1 am n =: am, n, u with am nu = Cu (m, n) dm, n (11) u = 0 where Cu (m, n) denotes the spreading code, making it possible to recognize the user and / or the associated service, and dm n a driver value.

With reference to FIG. 5, a bit error rate performance curve is presented, taking into account a perfect estimate of the transmission channel 51 and a channel estimation 52 according to a particular embodiment of FIG. the invention. The parameters taken into consideration for estimating a radio-mobile type channel, called SCM ("Spatial Channel Mode" in English), according to this example, are the following: central frequency: 2000 MHz, moving speed of the mobile: 50 km / h, number of paths: 6 25 power profile (in dB): -3.0, 0.0, -2.0, -6.0, -8.0, -10.0, delay profile (ns): 0, 195.315, 488.28125, 976.5625, 2246.09375 , 4882.8125. The prototype function used is the IOTA function, as described for example in patent document FR 2733869, of length 4 v0, and each carrier is modulated by a phase modulation. Curve 52 represents the received bit error rate, taking into account an estimation of the channel according to a particular embodiment of the invention, in which it is assumed that the channel is substantially constant over two consecutive frequencies and in which the pairs of pilots were distributed over the entire frame. The other curve 51 represents the bit error rate on reception, taking into account a perfect knowledge of the transmit transmission channel. The simplified structures of a transmitting and receiving device according to the particular embodiment described above are now presented in connection with FIGS. 6A and 6B. As illustrated in FIG. 6A, such a transmission device comprises a memory 61, a processing unit 62, equipped for example with a microprocessor P, and driven by the computer program 63, implementing the transmission method according to the invention.

At initialization, the code instructions of the computer program 63 are for example loaded into a RAM before being executed by the processor of the processing unit 62. The processing unit 62 receives as input data to be transmitted, in the form of informative data elements. The microprocessor of the processing unit 62 implements the steps of the transmission method described above, so as to construct a multicarrier signal comprising groups of at least two real value pilots, each located in a region called neighborhood in the time / frequency space, a neighborhood region being a region in which the transmission channel is considered substantially constant.

For this purpose, the transmission device comprises means for implementing groups of at least two real-valued pilots. These means are controlled by the microprocessor of the processing unit 62. The processing unit 62 outputs the multicarrier signal mentioned above. As illustrated in FIG. 6B, a reception device comprises a memory 64, a processing unit 65, equipped for example with a microprocessor P, and controlled by the computer program 66, implementing the reception method according to the invention. At initialization, the code instructions of the computer program 66 are for example loaded into a RAM before being executed by the processor of the processing unit 65. The processing unit 65 receives as input a multicarrier signal received y (t). The microprocessor of the processing unit 65 implements the steps of the reception method described above, according to the instructions of the computer program 66, for estimating the transmission channel and decoding the received data. For this, the receiving device comprises means for extracting at least two complex values corresponding to the pilots of the group of said neighborhood region, after passing through said transmission channel, and means for estimating the real and imaginary said transmission channel in said neighborhood region from said complex values. These means are controlled by the microprocessor of the processing unit 65.

Claims (12)

  A method of receiving a received signal corresponding to a multicarrier signal transmitted by at least one transmitter via a transmission channel, said multicarrier signal being formed of a temporal succession of symbols consisting of a set of data elements to actual values comprising: - informative data elements, and - for at least some of said symbols, reference data elements called pilots, whose value and location at the time of transmission are known to at least one receiver intended for performing a reception of said multicarrier signal, each of said data elements modulating a carrier frequency of said signal, a carrier frequency modulated by one of said data elements being called a carrier, characterized in that groups (221) of at least two pilots to actual value (P1, P2) being each located in a so-called neighborhood region (22) in the time / frequency space, a region of vo isinage being a region in which said transmission channel is considered substantially constant, said receiving method comprises, for at least one of said neighborhood regions (22): a step of extracting (31) at least two corresponding complex values to the pilots of the group of said neighborhood region, after passing through said transmission channel; - a step of estimating (33) the real and imaginary portions of said transmission channel in said neighborhood region from said complex values.   2. Reception method according to claim 1, characterized in that each of said groups consists of a pair of pilots.   3. Reception method according to claim 2, characterized in that said estimating step implements a resolution of a system of four equations with four unknowns for each of said pairs of pilots. 2903833 27   4. Reception method according to claim 3, characterized in that said system implements the following equations: with: (r) Ymo, n0 (i) Ymo, no (r) Y, n1, n1 (i) Yml, ## EQU1 ## ## STR2 ## ## STR1 ## ## STR2 ## ## STR1 ## ## STR1 ## r) a (`) ml, ml, ml, ml, nl (mo, n0) a first location of the time / frequency space and (ml, nl) a second location of the time / frequency space in said neighborhood region, Y and y (mo i) no real values respectively equal to the real part and the imaginary part of the complex value of the received signal in (m0, n0), - y (r) and y (`) real values equal respectively to the mi, ni ml, nl real part and to the imaginary part of the complex value of the carrier located at the location (mi, nl), - Hj () nro n0 the real part the complex value of said transmission channel at the location ( m0, n0) and H (ml) nor the real part of the complex value of said transmission channel at the location (ml, nl), with H (r) and not equal to Hr (nro), the transmission channel being, , n is regarded as substantially constant in said neighborhood region, 20 - a (nro) and a ((n0 real values equal respectively to the real part and to the imaginary part of the complex value of a first pilot of the group of said region of neighborhood at the location (m0, n0), a (r) and a (`) real values equal to the ml, nl ml, and real part, respectively, and to the imaginary part of the complex value of a The second pilot of said group at the location (mi, nj), - C a real.   5. Reception method according to any one of claims 1 to 4, characterized in that said estimating step implements an intermediate calculation of a ratio (32) between said real and imaginary parts of said complex values.   6. Device for receiving a received signal corresponding to a multicarrier signal transmitted by at least one transmitter via a transmission channel, said multicarrier signal being formed of a temporal succession of symbols consisting of a set of data elements real-valued system comprising: informative data elements, and for at least some of said symbols, reference data elements, called pilots whose value and the location on transmission are known to at least one receiver intended to perform A reception of said multicarrier signal, each of said data elements modulating a carrier frequency of said signal, a carrier frequency modulated by one of said data elements being called a carrier, characterized in that groups (221) of at least two pilots to real value 20 (P1, P2) being each located in a so-called neighborhood region (22) in the time / frequency space, a region a neighborhood in which said transmission channel is considered substantially constant, said receiving device comprises, for at least one of said neighborhood regions: - extraction means (31) of at least two corresponding complex values the pilots of the group of said neighborhood region, after passing through said transmission channel, means for estimating (33) the real and imaginary portions of said transmission channel in said neighborhood region from said complex values. 2903833 29   7. Computer program product downloadable from a communication network and / or recorded on a computer readable medium and / or executable by a processor, characterized in that it comprises program code instructions for the implementation of the receiving method according to at least one of claims 1 to 5.   A method of transmitting a multicarrier signal for transmission via a transmission channel, formed of a temporal succession of symbols consisting of a set of real-valued data elements comprising: data elements , and for at least some of said symbols, reference data items called pilots whose value and location on transmission are known to at least one receiver for performing a reception of said multicarrier signal, each of said elements of data modulating a carrier frequency of said signal, a carrier frequency modulated by one of said data elements being called a carrier, characterized in that said transmission method uses groups (221) of at least two real-valued pilots ( P1, P2), each located in a so-called neighborhood region (22) in the time / frequency space, a neighborhood region 20 being a region in the which said transmission channel is considered substantially constant.   9. Transmission method according to claim 8, characterized in that each of said groups consists of a pair of pilots.   A multicarrier signal transmission apparatus for transmission via a transmission channel, formed of a temporal succession of symbols consisting of a set of real-valued data elements comprising: data elements informative, and for at least some of said symbols, reference data items called pilots, the value and location of which is known to be transmitted from at least one receiver for receiving a multicarrier signal, each of said data elements modulating a carrier frequency of said signal, a carrier frequency modulated by one of said data elements being called carrier, characterized in that said transmitting device comprises means for implementing groups (221) of at least two real-valued pilots (P1, P2), each located in a so-called neighborhood region (22) in the time / frequency space, a region of neighborhood being a region in which said transmission channel is considered substantially constant. 10   11. Computer program product downloadable from a communication network and / or recorded on a computer readable medium and / or executable by a processor, characterized in that it comprises program code instructions for the implementation of the emission process according to at least one of claims 8 and 9.   12. Multicarrier signal formed of a temporal succession of symbols consisting of a set of real-valued data elements comprising: informative data elements, and for at least some of said symbols, reference data elements called pilots whose value and the location at transmission are known to at least one receiver for receiving said multicarrier signal, each of said data elements modulating a carrier frequency of said signal, a carrier frequency modulated by one of said elements wherein said set of real-valued data elements comprises groups (221) of at least two real-valued drivers (P1, P2), each located in a so-called neighborhood region. (22) in the time / frequency space, a neighborhood region being a region in which said transmission channel is considered as substantially constant.