WO2006069826A1 - Method for transmitting multicarrier signals in a planned macro-diversity multicellular network, corresponding network, signal, reception method and device - Google Patents

Abstract

The invention concerns a method for transmitting at least one multicarrier signal in a data transmission and/or broadcasting single central frequency network (SFN) said network comprising at least two geographical cells associated each with at least one transmitter, said multicarrier signal comprising at least two subcarriers each modulated by data elements during successive time slots called symbol times. The invention is characterized in that, for at least one of said cells, it consists in assigning to at least some subcarriers of said signal a cyclical time shift within the symbol time, and said cyclical time shift is specific to said cell, such that two neighbouring cells of said network are associated with two distinct cyclical time shifts.

Description

 A method of transmitting multicarrier signals in a planned macro-diversity multicell network, network, signal, method and corresponding receiving device.

FIELD OF THE INVENTION The field of the invention is that of the transmission and / or broadcasting of digital information in a cellular radio network. More specifically, the invention relates to a technique for improving the reliability of links between transmitters and receivers of a multicell radio network when such a network relies on a multicarrier modulation of the information to be transmitted. It is particularly relevant in the context of transmitting and broadcasting high bit rate digital information over a limited frequency band in a mobile radio environment.

2. Solutions of the prior art

2.1 Multicarrier Modulations and Diversity The deployment of cellular or multicellular transmission networks makes it possible to ensure continuous transmission of information over a given geographical area by artificially cutting this zone into adjacent cells, each associated with a transmitter, also called a station. basic. A mobile receiver traveling in this geographical area receives a signal transmitted by one or more of these transmitters, depending on its position relative to the cells of the network.

Multi-carrier transmissions, such as Orthogonal Frequency Division Multiplexing (OFDM), have many advantages in such mobile radio environments, since they effectively combat induced echoes. by the transmission channel. It is recalled in fact that in radiomobile environment, the transmitted wave undergoes, during its journey, multiple reflections, and the receiver thus receives a sum of delayed versions of the transmitted signal, corresponding to the multiple paths followed by the signal. Each of these versions is attenuated and shifted randomly. This phenomenon, known as spread delay, generates inter-symbol interference (IES). For example, in an urban-type environment, the spread of the delays associated with the different signal paths is of the order of or less than a few microseconds. The receiver (for example the mobile radiotelephone of a motorist) being supposed to move, the Doppler effect also acts on each path, which results in a frequency shift of the received spectrum, proportional to the speed of movement of the receiver. The combination of these effects results in a non-stationary transmission channel, with deep fading at certain frequencies (thus obtaining a frequency-selective channel), that is to say at a given moment, certain frequencies of the band are greatly attenuated.

The good resistance of multicarrier modulations such as OFDM to these different phenomena is due to their two main characteristics, namely that they exhibit a relatively long symbol time (that is to say the duration of a symbol). provide for the insertion of a guard interval, for example in the form of a cyclic prefix, between the successive symbols. These two elements combined limit interference between symbols. Although this good resistance to echoes makes the multicarrier modulations particularly suitable for multipath channels, it should however that the maximum delay between the different paths remains less than the duration of the guard interval, or cyclic prefix. Indeed, if the time difference between the different paths is greater than the length of the cyclic prefix, the interference phenomenon between symbols appears. If, on the other hand, the time difference between the different paths is negligible, there is no frequency diversity, which reduces the quality of the connection between transmitter and receiver.

For optimum use of the OFDM modulations, it is therefore appropriate that the maximum delay between the different paths is just less than the duration of the guard interval, or cyclic prefix, in order to obtain maximum diversity, and thus optimize the quality in reception.

To improve this diversity, attempts have been made to artificially create multiple paths between a transmitter and a receiver, by designing transmission systems in which each transmitter comprises several transmitting antennas, which simultaneously transmit the same multicarrier signal. In this context of so-called diversity transmission systems, it has been proposed in patent document WO 02/25857 to further increase the diversity thus achieved by creating virtual paths obtained by imposing a cyclic delay on the OFDM symbols transmitted. on each of the antennas issue of a given issuer.

It is recalled that for each OFDM symbol, or multicarrier symbol, is meant a set of symbol elements, a symbol element corresponding to a modulated subcarrier, during a symbol time, by a data element. According to this technique, the N temporal samples of a symbol element carried by one of the subcarriers of the signal are considered, and they are shifted by a predetermined number of steps, by reintroducing at the beginning of the symbol element. Samples staggered beyond the end of the symbol element. Each different version of the signal, emitted by a separate transmitting antenna, experiences a distinct cyclic delay. A guard interval between successive symbols is then introduced in the form of a cyclic prefix comprising, for each symbol element, the last samples of the symbol element after shift.

Because of this, since the offset being cyclic within a symbol time, the delay of the virtual paths can take any value, up to the total length of the OFDM symbol, and is not limited by the size of the symbol. cyclic prefix.

In reception, the different versions of the cyclically shifted signal on transmission are combined to enable the receiver to reliably reconstruct the transmitted signal, and to extract the information data therefrom.

2.2 Single core frequency networks, known as SFN The good resistance to echoes of multicarrier modulations such as OFDM also makes it possible to use them in cellular transmission networks using the same transmission frequency in all the cells, called central frequency networks. unique, or SFN ("Single Frequency Network").

Several types of multicellular networks are known, which have different frequency operating characteristics.

In a first type of cellular network, the transmitters, or base stations, use different carrier frequencies, so that the signals they emit do not interfere or little. This first type of cellular network, which is for example used in the context of the GSM standard ("Special Mobile Group"), has the disadvantage of allowing only a very small use of the available frequency spectrum.

A second type of network, called a single central frequency network, or SFN, solves this problem. In this second type of network, implemented more particularly in multicast or multicell access contexts, all the transmitters of the different cells operate in the same frequency band. It is within the framework of these SFN networks that the present invention is incorporated.

A particular use of the single central frequency networks, or SFNs, is to transmit the same signal from several transmitters or base stations of the network, for the same receiver, and to combine these signals in reception, so as to increase the quality of the received signal. The receiver thus receives a useful signal, including long echoes, from several transmitters. These long echoes are due to the differences in propagation time between the transmitters, or base stations, of the different cells of the network and the receiver. These differences can be significant, and depend on the position of the receiver in the network (depending on whether this receiver is located in the middle of two transmitters, or on the contrary close to a transmitter and away from others, etc.).

The principle of operation of the single central frequency networks of transmitting the same signal from several base stations to the same receiver, improves the reliability of the transceiver link: it is called macro-diversity .

All modulations using a long symbol time have the same property of resistance to echoes, and are therefore of particular interest for SFN networks: this is the case of multicarrier modulations, and in particular modulations MC-CDMA ("Multiple Carrier-CDMA"). "Or" Multi-Carrier CDMA "), or Code Division Multiple Access (CDMA) modulations with a large spreading factor (with or without a cyclic prefix). 3. Disadvantages of prior art

The transmission of the same useful signal by several transmitters of a single central frequency multicell network can be considered as a first step towards cellular macro-diversity, but it does not, however, make it possible to ensure sufficient reliability of the connection between transmitters and receivers in the network. If it gives fairly satisfactory results in the case of a CDMA type modulation, the transmission is not as reliable as it could be in the case of the use of an OFDM type multicarrier modulation.

The cyclic shift diversity technique proposed in the patent document WO 02/25857, for its part, presents satisfactory performances in the context of multi-antenna transmission systems, but is not suitable for use in a multi-cellular network.

Indeed, the creation of virtual delays by the different base stations of a cellular network, in the form of cyclic offsets of the symbols they emit, would considerably increase the complexity of the estimation of the channel: the coherence time would be greatly reduced, because of the superposition of paths from different base stations.

In addition, it would no longer be possible for a receiver to perform a channel estimation to each base station in a dissociated manner, because of the superposition of the virtual delays introduced by the different cells of the network.

4. Objectives of the invention

The invention particularly aims to overcome these disadvantages of the prior art.

More precisely, an object of the invention is to provide a technique for transmitting digital information in a multi-cellular network of the SFN type, which makes it possible to improve the reliability of links, for systems based on a multicarrier modulation.

In particular, the invention aims to provide such a technique that is well suited to multicarrier modulations such as OFDM, MC-CDMA ("Multi Carrier CDMA" for "multi-division code division multiple access") and more particularly the MC-SS-MA ("Multi Carrier - Spectral Spreading - Multiple Access", for "Multicarrier Multiple Access by Spread Spectrum"), etc.

Another object of the invention is to propose such a transmission technique in a single central frequency network that makes it possible to implement a cellular macro-diversity of the transmitted signals.

It is another object of the invention to provide such a technique which does not significantly increase the complexity of channel estimation.

Another objective of the invention is to propose such a technique that enables a receiver to estimate the transmission channel to each of the transmitters from which it receives a signal, in a dissociated manner.

The invention also aims to provide such a technique which limits, as far as possible, the occurrence of intersymbol interference phenomena. It is another object of the invention to propose such a technique which is compatible with MIMO diversity or rate increase techniques, also known as space-time codes.

The invention also aims to provide such a technique that is simple to implement, and somewhat complex to implement on an algorithmic level.

The invention finally aims to provide such a technique that is suitable for broadcast type applications, multicast, multicell access, radiotelephony, etc.

5. Essential characteristics of the invention

These objectives, as well as others that will become apparent, are achieved by a method of transmitting at least one multicarrier signal in a single central frequency data transmission and / or broadcasting network. (SFN, "Single Frequency

Network "), said network comprises at least two geographical cells each associated with at least one transmitter, and said multicarrier signal comprises at least two subcarriers each modulated by data elements during successive time intervals called symbol times.

According to the invention, for at least one of said cells, at least some subcarriers of said signal are assigned a cyclic time shift within a symbol time, and said cyclical time shift is specific to said cell, so that that two cells neighboring said network are associated with two distinct cyclic time offsets.

Thus, the invention is based on a completely new and inventive approach to the transmission and broadcasting of high bit rate digital information in a multi-cellular network in which all the transmitters operate in the same frequency band. Indeed, the invention proposes to significantly improve the reliability of the links between transmitters and receivers of the network, by achieving a double diversity: on the one hand, the invention introduces a spatial diversity, thanks to the implementation of a single central frequency network which makes it possible to transmit the same multicarrier signal from several transmitters, or base stations of distinct cells; on the other hand, the invention introduces additional diversity by creating virtual paths of the signal, related to the application of a cyclic time shift of some, or all, subcarriers.

Such a cyclical time shift is within a symbol time, after application to the signal of an inverse Fourier transform (or IFFT (Inverse Fast Fourier Transform) in the case of an OFDM modulation). Therefore, a cyclic shift of the samples to simulate a delay is applied to the time signal from the inverse Fourier transform of an OFDM symbol (by construction, the signal is indeed composed of an integer number of periods). This may be a shift to the right of the signal, whereby the time samples of the symbol element shifted beyond the end of the symbol element are reintroduced at the beginning; it can also be a time shift to the left, whereby the time samples of the symbol element shifted before the start of the symbol element are reintroduced at the end of the symbol element. The principle of carrying out such a cyclic time shift is described in more detail below with reference to the figures, by way of example embodiment.

The invention further provides a planning of the cyclic time offsets associated with each cell of the network, which makes it possible to cause two adjacent cells of the single-carrier network to always use two distinct cyclic time offsets. The use of coordinated offsets between the different cells advantageously makes it possible to reduce the complexity of estimation of the channel by the receiver; it also allows the receiver (car radio, mobile phone, PDA ("Personal Digital Assistant"), etc.) to estimate, in a dissociated way, the transfer function of the transmission channel to each of the transmitters or base stations from which it receives a signal.

It will be noted that, by subcarrier, is meant here and throughout this document, a carrier frequency of the multicarrier signal, which can be modulated, in successive symbol times, by separate data elements. A data element is defined as a complex value element that modulates a subcarrier. In some embodiments, a data element may be a point of a QPSK, 16QAM, etc. constellation. A subcarrier modulated, during a symbol time, by a data element is called a symbol element. .

The cyclic time shift is here applied to a subcarrier (and thus generally over a plurality of successive symbol times), and not only to a symbol element.

Advantageously, said cyclic time shift specific to one of said cells is determined according to an identifier of said cell within said network. The cell indices are for example stored in correspondence with offset values in a data table accessible to network equipment. Cyclic time offsets can also be calculated from a mathematical function whose variable is the index of the cell.

Preferably, such a transmission method comprises a step of activating / deactivating said cyclical time offset assigned to the sub-carriers of said signal. Thus, the transmitters may at times operate in diversity mode, and enable the cyclic time shift of the subcarriers, and at other times, operate in conventional transmission mode, without assigning an offset to the subcarriers. At the network level, you can dynamically configure all transmitters that apply virtual delays, and those that do not apply at a given time,

Advantageously, said cyclical time shift is applied to said sub-carriers before insertion of at least one guard interval (or cyclic prefix) between two successive symbol times of said multicarrier signal. In this way, the virtual delay does not affect the cyclic prefix data.

According to a first variant embodiment, said cyclical time shift is applied to said sub-carriers of said signal by multiplication of said data elements in the frequency domain by a phase ramp. Thus (see FIG. 9), the data elements intended to modulate the subcarriers by complex exponentials are multiplied before carrying out the multicarrier modulation (IFFT), which amounts to shifting the modulated subcarriers.

According to a second variant embodiment, illustrated in FIG. 6B, such a transmission method comprises the successive steps of: distributing said subcarriers of said signal in at least two distinct sets of subcarriers; applying Partial Fourier Transforms (IFFT) to said subcarriers of each of said distinct sets; assigning said cyclical time offset to the subcarriers of one of said distinct sets; adding said subcarriers of each of said distinct sets.

In a particular embodiment of the invention, said temporal offset cyclic is assigned to all subcarriers of said signal, the entire signal then participates in a macro-diversity transmission.

According to an advantageous characteristic, said set of cells of said network is assigned a set of K distinct cyclic time offsets d _c (i) such that, for all iE [l, K],

D _c T (z) = zx -, wherein T is the duration of said symbol time, and where K is an integer. Two cells of the network K can indeed use the same cyclic shift, provided that they are not adjacent.

Advantageously, at least one of said cells comprises at least two transmitting antennas and implements a MIMO ("multiple input multiple output") space-time coding of said multicarrier signal, and the same time cyclic shift specific to said cell is applied to at least some of said subcarriers transmitted by each of said transmitting antennas. The diversity is thus increased by joint use of the principle of virtual delay of the invention and space-time codes. The invention also relates to a single central frequency data transmission and / or broadcasting network, comprising at least two geographical cells each associated with at least one transmitter of a multicarrier signal, which comprises at least two sub-carriers modulated each by data elements during successive time intervals called symbol times. For at least some of said cells of said network, at least some subcarriers of said signal are assigned a cyclic time shift within a symbol time, said cyclical time shift being specific to said cell, and said network implements a schedule. said cyclic time offsets associated with each of said cells, so that two cells neighboring said network are associated with two distinct cyclic time offsets. The invention also relates to a multicarrier signal comprising at least two subcarriers each modulated by data elements during successive time intervals called symbol times. This multicarrier signal is transmitted by at least one transmitter of a geographical cell of a multicellular network for transmitting and / or broadcasting data at a single central frequency. At least some of said subcarriers of said signal are assigned a cyclic time shift within a symbol time, said cyclical time shift being specific to said cell and distinct from a cyclic time shift assigned to at least some subcarriers at least one multicarrier signal transmitted in at least one cell close to said network.

The invention also relates to a transmission device implementing the transmission method described above, and to a reception device which comprises: means for receiving a combined signal corresponding to the sum of at least two signals received each transmitted in a separate channel and each corresponding to the same source signal carried by a multicarrier signal, said multicarrier signals being simultaneously transmitted each in a separate cell of said network, at least some subcarriers of each of said multicarrier signals having undergone, before emission, a cyclical time shift within a symbol time, said cyclic time shift being cell-specific, so that two neighboring cells of said network are associated with two distinct cyclic time offsets; means for estimating the transfer function of each of said channels taking into account said cyclical time offsets present in said combined signal; means for reconstructing said source signal by taking into account at least two of said estimated transfer functions.

The invention finally relates to the reception method implemented in such a receiving device.

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 appended drawings, among which: FIGS. IA and IB show a block diagram of a device for transmitting a signal

OFDM implementing a scheduled cyclic time shift of the subcarriers, as well as the OFDM signal thus shifted, according to the principle of the invention; FIG. 2 illustrates in more detail the principle of cyclic time shift applied to the signal of FIG. 1B; FIG. 3 describes the principle of transmission of the same signal by three separate base stations of the same single-carrier network, each applying to the signal a distinct cyclical time shift; FIG. 4 illustrates the transmission channels of each of the base stations of FIG. 3 to a receiver, as well as the resulting channel estimated by the latter; FIG. 5 presents the principle of planning shifts in a multicell network according to the invention; FIGS. 6A and 6B illustrate the joint implementation of the invention and space-time codes of the MIMO type; Fig. 7 presents a numerical example of cyclic shift planning illustrated in Fig. 5; Figure 8 shows a block diagram of a receiver of the invention; FIG. 9 describes an example of assigning a cyclic time shift to certain subcarriers by applying a phase ramp; Figure 10 shows a block diagram of a transmission device of the invention. 7. Description of an embodiment of the invention

The general principle of the invention is based on the planning, in a multichannel network with a single central frequency (SFN), of cyclic time delays specific to the cells of the network, which are applied to some or all the subcarriers of the multicarrier signals transmitted in at least some cells, so as to implement a cellular macro-diversity, to improve the reliability of the transceiver links. The invention applies both to data transmission networks (of the radiotelephone type, video data transmission, etc.) than to broadcast data networks (broadcast or multicast). It applies in particular to the reconfigurable networks operating, at a given moment, for some SFN-type broadcast mode cells, in which all the cells transmit the same multicarrier signal, and for other cells in the data transmission mode, wherein separate cells transmit different signals to particular receivers.

It applies to any type of multicarrier modulation, and in particular to OFDM, MC-CDMA, MC-SS-MA, and other modulations. For the sake of simplicity, throughout the remainder of this document, a particular embodiment of the invention will be described in the case of OFDM modulation. Those skilled in the art will easily extend this teaching to any other type of multicarrier modulation.

The invention is particularly applicable to OFDM multicellular systems with OFDMA multiple access (for "OFDM Access"), and makes it possible to facilitate the implementation of multicast or broadcast (ie broadcast) type services in such networks. It should be remembered that, in the context of a multicast service, data that is common to at least two users is transmitted, with processing being implemented at the sender so that all the users receiving the data can receive them. correctly.

With reference to FIGS. 1A and 1B, a block diagram of a transmitter or base station according to the invention is presented. For the sake of simplification, FIGS. 1A and 1B only show the elements participating in carrying out the invention. In particular, for example, the insertion of symbols comprising reference elements (pilots) for synchronization or channel estimation has not been illustrated.

The transmitter of FIG. 1A is fed, at the input 10, with a succession of time-multiplexed data symbols which pass through a serial / parallel S / P converter 11, outputting the same data symbols in parallel. The latter undergo an OFDM modulation 12, which generates a plurality of temporal samples in parallel: each parallel channel at the output of the OFDM modulation 12 is associated with a subcarrier of the modulation, for which a plurality of temporal samples are delivered. , which can be grouped into symbol elements. These time samples feed a processing block 13 which applies a cyclic time shift, the value of which depends on an identifier 14 of the transmitter in the network. For example, the value of the cyclical time offset is related to the index of the cell of the network to which the considered transmitter belongs.

As will be seen in more detail later, the processing block 13 may impose this cyclic shift to all subcarriers of the OFDM modulation, or only to some of them. In addition, the processing block 13 may or may not be active, so that the transmitter of FIG. 1A may have two modes of operation: a first operating mode in which a cyclic shift is imposed on some or all of the sub-units. OFDM signal carriers, and a second mode of operation in which the subcarriers do not undergo any cyclic shift, when the processing block 13 is inactive. Thus, depending on the needs and configuration of the network to which the transmitter belongs, the latter may or may not operate in diversity mode.

At the output of the processing block 13, a plurality of temporal samples is obtained in parallel, possibly staggered. A guard interval is then inserted in the form of a cyclic prefix 15, followed by a parallel / serial P / S conversion 16 of the data, which generates at output 17 a sampled time signal which can be transmitted by an antenna of the transmitter, not shown in Figure IA. Fig. 1B illustrates the OFDM signal at the different processing steps imposed by the transmitter of Fig. 1A. At the output of the OFDM modulation 12, the OFDM signal comprises, for any of the subcarriers of the modulation, a succession of N temporal samples represented in the form of successive blocks numbered from 1 to N.

In the example of FIG. 1A, the processing block 13 imposes on these N samples a shift to the right of length D, where D is an integer number of shift steps, and depends solely on the index of the cell of the network to which the issuer belongs. Thus, at the output of the processing block 13, the N samples of the signal have undergone a circular permutation, so that the first sample of the symbol now bears the ND index, and the last sample of the symbol considered now bears the NDI index. . The N temporal samples were thus shifted by D + 1 steps to the right, and the samples which, after shift, exceeded beyond the end of the symbol time (namely the ND to N index samples), were reinserted at the beginning of the symbol, so that the duration of the symbol (a symbol time) is preserved. This avoids the appearance of interference between symbols. After insertion of the cyclic prefix 15, of length P, where P represents an integer number of samples, a temporal succession of P + N samples is obtained. In the example of FIG. 1B, the last samples of the symbol after shift are inserted into the cyclic prefix, ie the time samples of index N-D-P to N-D-I, so as to introduce a redundancy. In the example of FIG. 1B, the OFDM signal has been represented for a cyclic time shift to the right and a cyclic prefix. The invention applies of course also to the case of shifts to the left, as well as cyclic postfixes.

FIG. 2 also illustrates the principle of the cyclic time shift applied, within a symbol time, to the subcarriers of an OFDM signal, but in a form of analog representation, and not in the form of successive temporal samples as in FIG. Figure IB.

By way of example, three subcarriers of an OFDM modulation, of respective frequencies f, 2f and 3f. The curves of FIG. 2 illustrate the amplitude A of these subcarriers as a function of time t. Thus, the curve referenced 20 illustrates the amplitude behavior of the subcarrier of frequency f over the duration of a symbol time T. Similarly, the curves referenced 23 and 26 respectively represent the behaviors of the frequency subcarriers 2f and 3f on a symbol time T.

These subcarriers are imposed a cyclic time shift of duration t _D. The duration of this shift is the same for each of the three subcarriers f, 2f and 3f. However, since their frequencies are different, this cyclical time shift results, for the frequency subcarrier f, by a shift of a quarter of a period, for the subcarrier of frequency 2f, by a shift of half a period , and for the frequency subcarrier 3f, by a shift of three quarters of a period.

The dotted curves referenced 21, 24 and 27 respectively represent the time evolution of the subcarriers of frequency f, 2f and 3f after cyclical time shift.

For ease of understanding, it can be considered schematically that the portion of the curve in phantom 22 _ls 25 _ls 28j of each of the three subcarriers, which exceeds beyond the end of the symbol time T after offset length t _D , is reinserted at the beginning of the symbol time in the form of a portion of the curve in phantom lines 22 ₂ , 25 ₂ , 28 ₂ . Figure IB described above thus corresponds to a temporally sampled representation of the curves of Figure 2.

Again, Fig. 2 illustrates a time shift to the right of the subcarriers, but the invention would also apply to the case of a cyclic time shift to the left of the subcarriers. The portion of the curve that would exceed beyond the beginning of the symbol time T would then be reinserted at the end of the symbol, symmetrically.

As can be seen in FIG. 2, there is talk of cyclic time shift within a symbol time of the sub-carriers of the modulation, because the start and end times of the symbols are not modified by this shift operation: for example, for the frequency subcarrier f, the solid line curve referenced 20 is replaced, after cyclical time shift t _D by a curve consisting of a phantom portion referenced 22 ₂ and a dashed portion referenced 21. The symbol conveyed by this frequency subcarrier f starts at t = 0 and ends at t = T, before as after cyclic time shift.

This cyclical time shift within a symbol time makes it possible to overcome the generation of intersymbol interference.

The principle of transmission of the same signal by three distinct base stations of the same single-carrier network, each applying to the signal a distinct cyclic time shift, is now presented in relation to FIG.

A cellular network is considered, in which three transmitters 31, 32 and 33 of three distinct cells emit, at a given moment, the same multicarrier signal 30 intended for one or more receivers (operation in individual data transmission or in broadcasting) , not shown in FIG. 3. These three emitters 31 to 33 are synchronized, so that the symbols of the multicarrier signal are emitted at the same instant in the three cells.

For the sake of simplicity, only a subcarrier 30 of the multicarrier signal emitted by the three transmitters 31 to 33 has been represented. It must of course be considered that the transmitted signal comprises a set of sub-carriers, for example a set of 1024 separate frequency subcarriers.

Each of the transmitters 31 to 33 imposes on a certain or all of the subcarriers of the transmitted signal a distinct cyclical time shift characteristic of the index of the cell to which the transmitter 31 to 33 belongs. Thus, the first transmitter 31 applies to the subcarrier 30 a zero cyclical time shift, so that the subcarrier 34j obtained after shift is identical to the initial subcarrier 30. The second transmitter 32 applies to the subcarrier 30 a cyclic time shift within a symbol time corresponding to a shift to the left of about a quarter period, which generates an offset subcarrier 34 ₂ . Finally, the cyclic time shift applied by the third transmitter 33 to the subcarrier 30 generates an offset subcarrier 34 ₃ .

Each of the transmitters then adds a cyclic prefix to the offset subcarriers thus obtained. Thus, for each of the emitters 31 to 33, cyclically shifted signals with a cyclic prefix referenced 3S ₁ to _{3 are} obtained, respectively, on which the cyclic prefix has been represented in thick lines.

The multicarrier signal, after cyclical time shift and addition of the cyclic prefix, is convoluted with the transmission channel linking the transmitter to the receiver (for example example a mobile radiotelephone or the car radio of a moving vehicle).

Thus, for the first transmitter 31, the subcarrier 35j is convoluted 36 with the transmission channel 3I ₁ between the first transmitter 31 and the receiver. Similarly, for the second and third transmitters 32 and 33, the subcarriers 35 ₂ and 35 ₃ are respectively convoluted 36 with the transmission channels 37 ₂ and 37 ₃ respectively connecting the second and third transmitters 32 and 33 to the receiver. It will be noted that the curves 3T ₁ to 37 ₃ represent the transfer functions of each of the channels as a function of the frequency.

The contributions of each of the signals emitted by each of the transmitters 31 to 33 then convoluted by the transmission channels 3T ₁ to 37 _{3 are} added 38 when traveling to the receiver, which receives a combined signal 39.

It will be appreciated that, due to the cyclic shift technique of the invention, there is no inter-symbol interference as long as the delays of the mobile radio channel paths are included in the cyclic prefix.

FIG. 4 illustrates in more detail the transmission channels of each of the transmitters 31 to 33 to the mobile receiver, as well as the overall channel estimated by the latter.

The transmission channel from the first transmitter 31 to the receiver can be considered as the combination of the virtual channel 41, associated with the cyclic time shift imposed on the subcarriers by the first transmitter 31 (in this case, a zero offset), and the 3T transmission channel ₁ to the receiver. Similarly, the transmission channel of the second transmitter 32 (respectively the third transmitter 33) to the receiver results from a combination of the virtual channel 42 (respectively 43), associated with the time offset applied by the second transmitter 32 (respectively the third transmitter 33), and the transmission channel 37 ₂ (respectively 37 ₃ ) to the receiver. The receiver estimates meanwhile a global channel 44 to which contribute the various transmission channels mentioned above. The receiver, or mobile station, can therefore estimate the contribution of each base station or transmitter 31 to 33, as long as the delays of the paths associated with each mobile radio channel are dissociated.

As illustrated by FIGS. 1 to 4 above, the invention therefore consists in implementing a cyclic shift diversity technique, by applying different time offsets for different base stations of a cellular network, these shifts being able to be planned, in a preferred embodiment, during the deployment of the network. The different base stations, or transmitters, are synchronized, as in a SFN network of the prior art.

FIG. 5 illustrates the principle of the planning of the offsets at the scale of the cellular network 50. The offsets are chosen so as to maximize the diversity at any point of the network 50. For this purpose, a planning of the offsets is carried out, in ensuring that two neighboring cells of the network 50 always apply discrete cyclic time offsets to the signals, so as to maximize the diversity.

One can for example use a network planning using three distinct cyclic time shift values D1, D2 and D3, as shown in FIG. 5. Two adjacent cells of the network 50 always apply different di offsets to the multicarrier signals: for example, none of the neighboring cells of the cell 51 applies to the transmitted signals the cyclic time shift D1 assigned to the cell 51, but rather the shift D2 or D3.

In networks with many base stations, and such that a receiver is capable of receiving signal from a large number of base stations, a seven-shift schedule can be used, for example.

It is also possible to coexist within the same cellular network a pattern with three offsets with a pattern with seven offsets, for example, so as to adapt to the operating constraints of the network. Such coexistence of distinct patterns may slightly limit diversity, but this is not a problem for the overall operation of the network.

Note that the values of three or seven offsets are given here as simple numerical examples, and that the invention is in no way limited to these particular examples: any other number of distinct cyclic time offsets can be assigned to the different cells of the network. .

The choice of offset values takes advantage of the OFDM transmission properties, namely a significantly longer symbol time than the time delay of the channel, so as to guarantee a total or relative orthogonality between the different sequences transmitted. Offset values are whole numbers of samples between 0 and

OR. 0 corresponds to no shift (as previously illustrated for the transmitter 31 of Figure 3). We denote N the size of the Fourier transform, which is also the total number of subcarriers of the OFDM signal (but not all of them are necessarily modulated).

Noting K the number of offsets to consider in the planning, the offset values P (O, for i = l ... K, are given by: P (O = ρpart_tyere ((i-1) * N / K)

In general, any set of offsets such as the difference of the two-to-two offsets is multiple to one unit near all-part (NIK) is appropriate. In the case where NIK is integer, it is possible to restrict this condition: one can then say that any set of offsets such that their two by two difference is multiple of (NIK) is appropriate. If it is desired to express the cyclic time shift, not a number of samples, but length of _c (i), it can be said also that, for i = \ ... K:

D _c T (i) = ix -, wherein T represents the duration of a symbol time. d _c (i) then represents the K value of the cyclical time offsets expressed in seconds or milliseconds for example (according to the unit in which the duration of the symbol time T is expressed). In a particular embodiment of the invention, it is possible to coexist in the network transmissions using macrodiversity, as proposed by the invention (that is to say undergoing a cyclic time shift of the sub-carriers ), and others do not use it. OFDMA multiple access, or MC-SS-MA multiple access, which both use OFDM modulation, are suitable for this selective implementation. We consider a set of base stations participating in a given macro-diversity transmission, that is to say simultaneously transmitting multicarrier signals corresponding to the same source signal. If it is macro-diversity for a user, few base stations are involved, for example two, but if the terminal receives signal from several stations, it may be relevant to use more. If it is macro-diversity for a broadcast or multicast channel, it is possible that all the stations of the network are requested.

C is the set of subcarriers in the OFDM waveform, and N _{11 is} the number of useful subcarriers among these subcarrier Cs.

For each macro-diversity transmission, a subchannel (in the OFDMA sense) is defined, that is to say a subset of the sub-carriers C, denoted by C _div , which will participate in the transmission of diversity. that is to say undergo the cyclic time shift within a symbol time proposed by the invention. We have: C _div d C. This subset can possibly be dynamic in time if, for example, frequency hopping is used.

It is recalled that the frequency hopping technique consists in dynamically modifying the frequency band allocated to a given receiver. This modification is done in a pseudo-random way, to guarantee a variability of the response of the channel.)

Each base station, or transmitter, applies, for all macro-diversity transmissions in which it is involved, the same cyclic shift, that is to say that it imposes the same cyclic time shift on all sub-stations. carriers of the set C _div . For the other subcarriers of C, no offset is applied. This can be implemented in two ways presented below.

The first method consists in performing two partial IFFTs ("Fast Fourier Transform" for "fast Fourier transform"), separating the sub-carriers belonging to a diversity transmission for the considered base station from the other sub-carriers, and applying a cyclic shift at the IFFT output. Then add the outputs and insert the cyclic prefix. This is illustrated in another context in Figure 6B.

FIG. 6B illustrates the case of the use of the invention in a MIMO type space-time coding context, in which the transmitter comprises two channels and therefore two transmitting antennas 61 and 62. However, if only one of the two transmission paths 61 or 62 is considered, it can be deduced from the diagram of FIG. 6B the processing applied to the subcarriers in the case of a selective implementation of the cyclic offsets, for the more general case of transmission by a single transmitting antenna.

More specifically, first of all is duplicated 6I ₁ N ₁₁ useful subcarriers of the OFDM signal. Each set of N ₁₁ useful subcarriers thus created feeds a particular processing path.

On a first processing channel, the N ₁₁ useful subcarriers feed a block

61 ₂ of zero subcarriers of the set C _div participating in the macro-diversity. The resulting set, which comprises N ₁₁ subcarriers, of which C _div subcarriers is null, undergoes inverse fast Fourier transform, or IFFT 6I ₄ . On a second processing channel, the N ₁₁ useful subcarriers feed a block

61 ₃ of "normal" subcarriers, that is to say which do not participate in the macro-diversity, and therefore must not undergo a cyclic time shift. All resulting, which comprises N ₁₁ subcarriers, of which C _div sub-carriers non-zero (only the subcarriers of the set C _div are not set to zero), undergoes an IFFT 6I ₄ . At the output of this IFFT 6I ₄ , a cyclic time shift D _{c 6 I 5} is imposed.

The signals of these two processing channels are then added to the signals 6I ₆ , before insertion of the cyclic prefix P 6I ₇ , then analog modulation 6I ₈ and transmission of the resulting signal.

A second method for selective implementation of cyclic offsets on certain OFDM signal subcarriers is to multiply in the frequency domain the sub-carriers belonging to a diversity transmission by a phase ramp corresponding to the cyclic shift assigned to the radio station. based. The use of the same cyclic shift for all the macro-diversity transmissions of the base station (ie for all the sub-carriers of C _div ) makes it possible to limit the complexity of this method.

This second method, which is an alternative to the method presented previously with reference to FIG. 1A (cyclical time shift of the samples after the FFT), is illustrated in FIG. 9. FIG. 9 presents another example of a practical embodiment of the invention, in FIG. the case where a cyclical time shift t _D is applied to all the successive symbol elements of the subcarriers of the multicarrier signal.

Unlike the method of FIG. 1A, where a cyclical time shift is applied after the OFDM (IFFT) modulation, in the variant of FIG. 9, a phase shift is applied to each data element derived from the serial / parallel conversion block S This embodiment can be chosen for its simplicity of implementation when the virtual delay has to be applied to a sub-sub-carrier sub-part (as opposed to the case where all the sub-carriers are shifted). the sub-carriers). It will be noted that, for the sake of simplicity, only seven subcarriers have been shown to undergo a cyclic time shift, the other subcarriers of the OFDM signal not being illustrated by FIG. data is a complex value element that modulates a subcarrier, and which, in some realizations, can be a point of a QPSK constellation, 16QAM, etc. The phase shift e ^τ applied to each data element depends on the frequency kf of the associated subcarrier: as illustrated in FIG. 2, the same cyclic time shift t _D applied to each subcarrier actually results in a difference of phase depending on the carrier frequency of this subcarrier. Thus, when it is desired to apply the invention to all successive subcarrier symbol elements, a phase ramp is applied to each data element prior to OFDM modulation (IFFT). In FIG. 9, the referenced data element 94, intended to modulate the frequency subcarrier /, thus undergoes a phase shift of e ^τ , and so on until the data element referenced 95 modulating the sub-frequency frequency carrier 7f, whose successive symbol elements undergo a phase shift of e ^τ .

After OFDM modulation 91, the cyclic prefix P 92 is inserted, before proceeding to a parallel conversion / P / S series 93. It will be noted that the sub-carriers which do not participate in a macro-diversity link

(ie the subcarriers not belonging to C _div ) are not very interfering on the links benefiting from the macro-diversity (ie the sub-carriers of C _div ). In fact, links benefiting from macro-diversity generally involve all the base stations, whose signals are received by the receiving terminal with a high power (and which would have been interfering if they had not participated in the macro-diversity). diversity, that is to say if they had not undergone a temporal cyclic shift). The technique of the invention is therefore flexible with respect to how to organize the OFDMA subchannels.

Note also that, on a given subchannel, it is possible to modulate QAM (Quadrature Amplitude Modulation) symbols, or to use the MC-SS-MA technique, without this modify the way in which the invention applies.

FIGS. 6A and 6B show the particular implementation of the invention in the context of a MIMO-type space-time coding ("Multiple Input Multiple Output", for "multiple multiple input multiple outputs" ). Indeed, the invention can be used in conjunction with MIMO diversity or rate increase techniques, also called space-time codes. In the context of the invention, the efficiency of these techniques is improved because the diversity is increased. The implementation of the invention in this context is simple because the invention modifies only the properties of the transmission channel, increasing the diversity. The invention can therefore be implemented in a slightly complex manner on the algorithmic level in the MIMO context.

FIG. 6A recalls the structure of a transmitter of the prior art, comprising two transmission antennas 61 and 62, and implementing a MIMO technique, for example according to the principle of the Alamouti scheme (presented for example in "A Simple Transmitter Diversity Scheme for Wireless Communications", IEEE JSAC, October 1998, p. 1457-1458 by SM Alamouti). Those skilled in the art will easily extend the example of the emitter of FIGS. 6A and 6B to a transmitter comprising more than two transmitting antennas.

Consider a set 60 data set referenced 6O ₁ , 6O ₂ to 60 _m , which each feed a coder / modulator 63 ₁ to 63 _m . The parallel outputs of these encoders / modulators 63 j to 63 _m in turn supply an OFDMA or MC-SS-MA multiplexing block 64. The multiplexed signal outputted from this block 64 undergoes a space-time encoding 65, which is performed in parallel on each subcarrier of the OFDM modulation. The space-time coding 65 generates two separate signals but linked by the space-time coding matrix, which each feed one of the two transmission channels 61 and 62. On each of these channels 61, 62, the signal undergoes a signal. inverse fast Fourier transformation IFFT 6I ₄ , 62 ₄ . It is then added a cyclic prefix P 6I ₇ , 62 ,, before performing an analog modulation 6I ₈ , 62 ₈ , and transmit the resulting signal on one of the transmitting antennas 61, 62.

Figure 6B illustrates the implementation of the invention in a transmitter, or a base station using a space-time code. Such an emitter according to the invention differs from that of FIG. 6A in that at the output of the space-time coding block 65, and on each of the two transmission paths 61, 62, one duplicates 6I ₁ , 62j the set of N _u subcarriers useful. As previously described for the first transmission channel 61, the following successive processes are carried out on each of the two transmission paths 61, 62: for a first set of N _u useful subcarriers, 6I ₂ is set to zero ,

62 ₂ the sub-carriers that participate in the macro-diversity and must therefore undergo a cyclical time shift; for a second set of N _u useful subcarriers, is set to zero 6I ₃ ,

62 ₃ sub-carriers that do not participate in the macro-diversity and therefore must not undergo a cyclical time shift.

After zeroing, each of the subcarrier sets of each of the channels undergoes an IFFT 6I ₄ , 62 ₄ . Then, only the second sets of sub-carriers of each of the two channels 61, 62 (ie those which crossed the block referenced 6I ₃ , 62 ₃ of zero of the subcarriers which do not participate in the macro-diversity), undergo a cyclic time shift D _c 6I ₅ , 62 ₅ .

On each of the transmission channels 61, 62, then is added 6I ₆ 62 ₆ the first and second sets of sub-carriers, prior to insertion of the cyclic prefix P 6I _7, 62 _7, then analog modulation 6I _8, 62 ₈ and emission the resulting signal.

The implementation of the invention in a base station using a MIMO coding technique makes it possible to transmit, on each of the transmitting antennas, a set consisting of a symbol and the same symbol shifted cyclically in time. The same cyclical time shift is applied to each of the transmission channels of the considered base station. The signals emitted by each of the antennas are linked to each other by the space-time coding matrix 65.

It should be noted that in FIGS. 6A and 6B, the insertion of symbols comprising reference elements for synchronization or channel estimation has not been represented. This operation is conventional for the skilled person, which will easily deduce its implementation in the context of the invention.

It will also be noted that, as previously described, it is easy to deduce from the diagram of FIG. 6B the structure of a transmission device of the invention that would not implement a space-time coding simply by deleting one of the two transmission channels, for example the second transmission channel 62 (which amounts to eliminating all the successive blocks referenced 62 _!, 62 ₂ , 62 ₃ , 62 ₄ , 62 ₅ , 62 ₆ , 62 ₇ and 62 ₈ ).

Finally, in connection with FIG. 7, a numerical example of implementation of cyclic time offsets in a multicellular single-carrier network is presented.

An OFDM system comprising 736 modulated subcarriers is considered, with a Fourier transform size of 1024 points. The guard interval has 128 samples.

The modulated subcarriers (or useful subcarriers) are numbered from 0 to 735, and the relation between the modulated subcarrier number i, and the index ffi_index (j) corresponding to the index of this subcarrier in the vector that undergoes the FFT (or more precisely the IFFT) is the following:

On considère la voie descendante, de la station de base vers un récepteur mobile. La trame est constituée de "modulation units", définies comme un ensemble de 32 sous- porteuses adjacentes utilisées pendant 10 temps symboles consécutifs. Le premier temps symbole d'une telle "modulation unit" est constitué par un préambule, et les neuf temps symboles suivants comportent des données utiles pour un utilisateur, un groupe "multicast", ou pour tous les utilisateurs (dans le cas d'une diffusion de type "broadcast").

The downlink is considered from the base station to a mobile receiver. The frame consists of "modulation units", defined as a set of 32 adjacent subcarriers used for 10 consecutive symbol times. The first symbol time of such a "modulation unit" is constituted by a preamble, and the following nine symbol times comprise data useful for a user, a "multicast" group, or for all the users (in the case of a broadcasting type broadcast).

23 "modulation units" can be transmitted in parallel. The first has subcarriers from 0 to 31, the second from 32 to 63, ..., the 23 ^lth the subcarriers from 714 to 735. One frame contains one hundred OFDM symbols, which corresponds to the duration of ten

"modulation units"; a frame therefore comprises 230 "modulation units".

The allocations use a frequency hopping, that is, a user does not use the same "modulation unit" twice in succession to average intercell interference. The system is deployed in a multicellular context with a single central frequency, that is to say that uses a single frequency, or narrow frequency band (the spectral reuse factor is therefore equal to 1). It is considered that the system is synchronous, that is, the OFDM symbols of all the cells are temporally aligned, so that the bulk of the received energy corresponds to paths of delay less than the duration of the cyclic prefix.

A schedule of cyclic time offsets in the network is adopted with a repeating pattern of three. Cyclic offsets are taken as 0, 1024/3 = 341, and 1024 * 2/3 = 682. The multi-cellular deployment is as shown in FIG. 7: the network 50 comprises a first subset of geographical cells that do not apply a cyclic time shift (DC = O), a second subset of cells that apply a cyclic time shift of DC = 341 samples, and a third subset of cells that apply a cyclic time shift of DC = 682 samples. Two neighboring cells of the network 50 always belong to two distinct subsets, i.e. two neighboring cells always apply to the transmitted signals different cyclic time offsets.

We then create a group of "multicast" on several cells of the network. This group is defined by a sequence of "modulation units" (a sequence of frequency hops), identical for all base stations participating in the "multicast" broadcast. This sequence is for example: {0,10,12,5,3,2,0,0,0,0,0}, which means the use of the modulation unit No. 0, then No. 10, then N ° 12, then n ° 5, then N ° 3, then N ° 2, then silence during the second half of frame (for example for a transmission "multicast" requiring half of the flow).

All base stations broadcasting this "multicast" use these "modulations units" in each frame, and each apply to these "modulations units" the cyclic shift that was assigned to them during network planning.

Figure 8 schematically illustrates the structure of a receiver of the invention. For the sake of simplification, FIG. 8 only shows the receiver elements necessary for understanding the invention.

Such a receiver comprises a reception antenna 70, on which arrives a combined signal, which corresponds to the sums of the contributions of the signals emitted by several emitters of distinct cells of the network, operating in macro-diversity. In the example of FIG. 7, it is considered that the combined signal incident on the reception antenna 70 comes from two transmitters 71 and 72, which have both transmitted a multicarrier signal, corresponding to the same source signal (this is ie conveying the same useful data), but which has undergone, for at least some of its subcarriers, a different cyclical time shift characteristic of the cell to which the transmitter 71, 72 belongs.

The signal received on the antenna 70 undergoes a demodulation 73. The receiver also comprises means 74 and 75 for estimating each of the transmission channels to each of the transmitters 71 and 72. These estimation means 74, 75 make it possible to to dissociate the channel towards each of the transmitters, taking into account the cyclic time offsets present in the combined signal, as previously illustrated in FIG. 4.

The channel estimation is conventionally done by means of pilot symbol elements, which correspond to the modulation of subcarriers by reference elements, called pilots, whose transmission value is known to the receiver (as opposed to symbol elements which correspond to the modulation of subcarriers by informative data elements, whose value at the time of transmission is not known a priori of the receiver). These pilot symbol elements can be grouped at the beginning of the frame by example, in the form of reference symbols. They can also be distributed, in a predetermined pattern, throughout the time-frequency plane: it is called distributed pilots.

After estimation of the different transmission channels, the receiver can reconstruct the source signal S from the transfer functions of the estimated channels.

More precisely, the receiver of FIG. 8 operates according to a source discrimination technique, according to which it estimates separately the different channels between the emission points 71, 72, and the reception point. Thus, the channel between the first transmission point 71 and the reception point is estimated by the first referenced estimation module 74 and the channel between the second transmission point 72 and the reception point is estimated by the second module referenced channel estimate 75. The estimation of the channels is then carried out according to the following steps:

1. In the demodulation module 73, an OFDM symbol is received, the cyclic prefix is deleted and the symbol is FFT-fed to obtain the data elements modulating each subcarrier of the signal.

2. The estimation modules 74 and 75 select the reference elements (pilots) and replace all the other data elements with zeros. The data elements thus obtained are filtered by an IFFT to obtain a new OFDM symbol (described in time). Each channel estimation module 74 or 75 selects the time samples of the obtained OFDM symbol corresponding to the range of presumed delays for a given emission point 71, 72. These delay ranges are obtained by, on the one hand, the cyclic delays applied by the emission points 71 and 72 and, secondly, a maximum delay hypothesis of the multipath, which maximum delay is conventionally considered to be less than the length of the guard interval (or cyclic prefix). The other time samples are replaced by zeros. The first channel estimation module 74 thus obtains a set of temporal samples A and the second channel estimation module 75 obtains a set of temporal samples B. 3. The first channel estimation module 74 filters the sample set A by an FFT, and thus obtains an estimate of the reference elements (pilots) emitted by the first transmission point 71. Similarly, the second channel estimation module 75 filters the set of samples B by an FFT, and thus obtains an estimate of the reference elements (pilots) emitted by the second transmission point 72. 4. The channel estimates between the first transmission point 71 and the point are available on some sub-carriers, at some given symbol times. They are interpolated by the first channel estimation module 74 in frequency and / or time to obtain an estimate of the channel on each subcarrier at each symbol time. Thus estimates of the channel between the first transmission point 71 and the reception point are available for each symbol element. The second channel estimation module 75 similarly obtains estimates of the channel between the second transmission point 72 and the reception point, which are available for each symbol element. In a second mode of operation, a receiver of the invention can operate according to a conventional OFDM reception technique, but by exploiting the diversity introduced by the transmitters of the invention. Such an operating mode applies when the symbol elements conveying useful information are shifted in the same way as the pilot symbol elements, i.e. the virtual delays applied by each transmission point to the elements. Symbol symbols are also applied to symbol elements conveying useful information. Indeed, the diversity consists in exploiting, in reception, the combination of signals received from different emission points, corresponding to the same source signal affected by different cyclic time offsets, characteristic of the emission point. In the case where the informative or useful symbol elements do not undergo the same cyclic time shift as the pilot symbol elements, the information symbol elements from the different transmitters do not combine in the same way as the drivers, and the Therefore, the channel estimation performed from the combined drivers can not be exploited to determine the informative data elements. In reception, the following steps are then implemented:

1. An OFDM symbol is received, the cyclic prefix is removed, and the symbol is FFTed to obtain the data elements modulating each subcarrier.

2. The reference elements (pilots) are estimated. Each estimated reference element allows estimation of the overall channel (sum of the channels from each emission point) on a sub-carrier, at a given symbol time. 3. Available channel estimates on some subcarriers, at some given symbol times are interpolated in frequency and / or time in order to obtain an estimate of the channel on each subcarrier at each symbol time. 4. Data elements conveying useful information are estimated using channel estimates provided by previous steps 1-3. This reception is a "classic" OFDM reception. However, the signal received benefits from increased diversity thanks to the transmission method described in this document. The invention, by introducing cyclic time offsets of certain subcarriers artificially modifies the properties of the transmission channel. The receiver of Figure 8 must be aware of these changes to, for example, perform the channel estimation in good conditions.

Figure 10 schematically illustrates a transmission device of the invention. Only the elements involved in the implementation of the invention have been shown in FIG. 10, which is therefore not intended to exhaustively describe all the elements of an OFDM transmitter. Such an emitter comprises a processing block 101 and at least one transmitting antenna 102 (several in the case of MIMO). The data to be transmitted 100 undergo a S / P 103 serial / parallel conversion, then an OFDM modulation 104 (IFFT). The transmitter comprises means 105 for allocating a cyclic time shift D _c to some (or all) sub-carriers from the OFDM modulation 104. after offset D _c

105, a cyclic prefix P 106 is inserted, before performing a parallel / serial conversion

107, to obtain the OFDM signal to be transmitted on the antenna 102.

Such a transmitter can implement either the variant embodiment of FIG. 6B (described in the particular case of MIMO) or the variant of FIG. 9 (phase ramp offset), so that the cyclic time shift can be performed, either in time on the subcarriers after OFDM and IFFT modulation, or in frequency, by multiplication of the data elements by a phase ramp, before OFDM modulation and IFFT.

Claims

A method for transmitting at least one multicarrier signal in a single central frequency data transmission and / or broadcasting network, said network comprising at least two geographical cells each associated with at least one transmitter, said multicarrier signal comprising at least two subcarriers each modulated by data elements during successive time intervals called symbol times, characterized in that, for at least one of said cells, at least some subcarriers of said signal are assigned an offset cyclic time within a symbol time, and in that said cyclical time shift is specific to said cell, so that two neighboring cells of said network are associated with two distinct cyclic time offsets. 2. Transmission method according to claim 1, characterized in that said cyclical time shift specific to one of said cells is determined according to an identifier of said cell within said network. 3. Transmission method according to any one of claims 1 and 2, characterized in that it comprises a step of activating / deactivating said cyclical time offset assigned to the sub-carriers of said signal. 4. Transmission method according to any one of claims 1 to 3, characterized in that said cyclical time shift is applied to said subcarriers of said signal by multiplying said data elements in the frequency domain by a phase ramp. 5. Transmission method according to any one of claims 1 to 3, characterized in that it comprises the successive steps of: distribution of said sub-carriers of said signal in at least two distinct sets of subcarriers; applying partial Fourier transformations to said subcarriers of each of said distinct sets; assigning said cyclical time offset to the subcarriers of one of said distinct sets; adding said subcarriers of each of said distinct sets. 6. Transmission method according to any one of claims 1 to 5, characterized in that said cyclical time shift is assigned to all subcarriers of said signal. 7. Transmission method according to any one of claims 1 to 6, characterized in that assigns to said cells of said network a set of K cyclic time offsets T distinct d _c (i) such that, for all iE [l, K], d _c (i) = ix -, where T is the duration of said time K symbol, and where K is integer. 8. Transmission method according to any one of claims 1 to 7, characterized in that at least one of said cells comprises at least two transmitting antennas and implements a space-time coding MIMO type (" Multiple Input Multiple Output "," multiple multiple input multiple ") of said multicarrier signal, and in that the same time cyclic shift specific to said cell is applied to at least some of said subcarriers transmitted by each of said transmitting antennas. A single central frequency data transmission and / or broadcasting network comprising at least two geographical cells each associated with at least one transmitter of a multicarrier signal, said multicarrier signal comprising at least two subcarriers modulated each by data elements during successive time intervals called symbol times, characterized in that, for at least some of said cells of said network, at least some subcarriers of said signal are assigned a cyclic time shift within a time symbol, said cyclic time shift being specific to said cell, and in that said network implements a planning of said cyclic time offsets associated with each of said cells, so that two neighboring cells of said network are associated with two cyclic time offsets. distinct. Multicarrier signal comprising at least two sub-carriers each modulated by data elements during successive time intervals called symbol times, said multicarrier signal being emitted by at least one transmitter of a geographical cell of a multicellular network for transmitting and / or broadcasting data at a single central frequency, characterized in that at least some of said sub-carriers of said signal are assigned a cyclical time shift within a symbol time, said cyclical time shift being specific to said cell and distinct from a cyclic time shift assigned to at least some subcarriers of at least one multicarrier signal transmitted in at least one a cell neighboring said network. 11. Device for transmitting at least one multicarrier signal, associated with a geographical cell of a multicellular network for transmitting and / or broadcasting data at a single central frequency, said multicarrier signal comprising at least two subcarriers modulated each by data elements during successive time intervals called symbol times, characterized in that it comprises means for assigning a cyclic time shift within a symbol time to at least certain subcarriers of said signal, said cyclical time shift being specific to said cell, so that two neighboring cells of said network are associated with two distinct cyclic time offsets. A device for receiving multicarrier signals from a single central frequency data transmission and / or broadcasting network, said network comprising at least two geographical cells each associated with at least one transmitter, each of said multicarrier signals comprising at least two subcarriers each modulated by data elements during successive time intervals called symbol times, characterized in that said receiving device comprises: means for receiving a combined signal corresponding to the sum of at least two received signals each transmitted in a separate channel and each corresponding to the same source signal carried by a multicarrier signal, said multicarrier signals being simultaneously transmitted simultaneously in a cell distinct from said network, at least some subcarriers of each of said multicarrier signals having undergone, before emission, a cyclical time shift within a t symbol, said cyclic time shift being cell-specific, so that two neighboring cells of said network are associated with two distinct cyclic time offsets; means for estimating the transfer function of each of said channels taking into account said cyclical time offsets present in said combined signal; means for reconstructing said source signal by taking into account at least two of said estimated transfer functions. A method for receiving multicarrier signals from a single central frequency data transmission and / or broadcasting network, said network comprising at least two geographical cells each associated with at least one transmitter, each of said multicarrier signals comprising at least two subcarriers each modulated by data elements during successive time intervals called symbol times, characterized in that said receiving method comprises: a step of receiving a combined signal corresponding to the sum of at least two received signals each transmitted in a separate channel and each corresponding to the same source signal carried by a multicarrier signal, said multicarrier signals being simultaneously transmitted simultaneously in a cell distinct from said network, at least some subcarriers of each of said multicarrier signals having undergone, before transmission, a cyclical time shift within a time symbol, said cyclic time shift being cell-specific, so that two neighboring cells of said network are associated with two distinct cyclic time offsets; a step of estimating the transfer function of each of said channels taking into account said cyclical time offsets present in said combined signal; a step of reconstructing said source signal by taking into account at least two of said estimated transfer functions.