Method and device for transmitting an OFDM data frame

Abstract

The invention relates to a method and a device for transmitting an OFDM data frame comprising q binary words having a size r modulated via a specific cyclical shift of the same root sequence having a size N=2r comprising at least one zero value. The invention makes provision for determining (402), for every i ϵ [0; N - 1], a number of occurrences of a zero value at a position i in a sequence Cp among the q sequences making up the frame, obtaining (403) an item of data relating to the quality of the channel of N OFDM subcarriers, applying (404) the same cyclical shift to the q sequences coding for the q words of the frame to be transmitted, the value of the shift being such that it maximises the number of non-zero values positioned on the subcarriers affording the best channels, and transmitting (405) the q shifted sequences in q separate OFDM symbols.

Description

Description Title of the invention: Method and device for transmitting a data frame in OFDM Technical field

[0001] The invention belongs to the field of telecommunications and relates in particular to a method for improving the link budget of an OFDM transmission. PREVIOUS ART

[0002] Low-power, long-range communication networks (LPWAN) remain a major topic of research and innovation today. These include the deployment of satellite-based Internet of Things (IoT) networks, as well as the evolution of cellular IoT markets with standards such as LTE-M, also known as eMTC (enhanced Machine Type Communication), NB-IoT (narrow-band IoT) and LoRaWAN (long-range wide-area network).

[0003] Such communications generally obey the same constraints: a need to sporadically communicate small amounts of data over long distances, or more generally with a very low link budget, with limited computing capacities. Added to this is often the constraint of a finite energy source because most of these objects operate on batteries.

[0004] OFDM (Orthogonal Frequency-Division Multiplexing) is a method of coding digital signals by orthogonal frequency division in the form of multiple subcarriers.

[0005] Fa [Fig.l] schematically represents the architecture of an OFDM modulator.

[0006] An Rb / s bit stream is modulated by a modulator 100, for example a QAM, PSK, QPSK, CCSK modulator, etc. A serial-to-parallel converter 101 forms a block of N modulated symbols that can be represented by a vector X(i). The vector X(z) is a sequence of complex values ​​in the frequency domain that is then transformed into the time domain by an Inverse Discrete Fourier Transform (IDFT) 102. A vector (n) in time is then obtained that is converted into series by a parallel / series converter 103 after adding a cyclic prefix. The sequence thus formed passes through a digital-to-analog converter 104 to construct an analog baseband signal x(t), then a modulator 106 transposes the spectrum of the baseband signal around the carrier frequency f p . So, in OFDM, the data is divided into blocks of size N symbols. Each block is called an "OFDM symbol".

[0007] Figure 2 illustrates the architecture of an OFDM demodulator. The time signal r(t) is transposed to baseband (200) and recovered by a low-frequency filter (not shown). An analog-to-digital converter (201) transforms the signal into a sequence yn). The sequence >'( / ?) is provided as input to a serial / parallel converter (202) which removes the cyclic prefix and creates vectors of samples in time y ( i ) , i E [0; / V - !]• The direct application of the Discrete Fourier Transform (203) transforms the symbols y(z),i E[0; / V - 1 ] to obtain a sequence of complex values ​​Y(i) in the frequency domain. A series-to-parallel converter (204) supplies a demodulator (205) thus allowing the recovery of the transmitted data.

[0008] In OFDM, frequency and / or time synchronization is usually achieved by pilot symbols, which are sequences of predefined values, known to the receiver. By calculating a correlation between a received sequence and the predefined sequence, the receiver is able to determine a time or frequency offset and adjust the synchronization. Pilot symbols do not carry any useful data, but they contribute to energy expenditure and limit bandwidth.

[0009] CCSK (Cyclic Code-Shift Keying) modulation can help limit the energy expenditure associated with the transmission of pilot signals. This modulation method proposes modulating the data to be transmitted by the cyclic rotation of a root sequence of complex symbols called chips. The root sequence is such that its shifted versions are orthogonal to each other, i.e. it offers a good autocorrelation function. Thus, each binary word to be transmitted is associated with a particular cyclic shift of the root sequence. Since the root sequence is known to the receiver, the transmitted sequences can be used as pilots for synchronization and channel estimation.

[0010] For example, a root sequence composed of 4 complex symbols [a; b; c; d] can modulate 2-bit binary words. The word '00' is for example associated with a zero offset corresponding to the sequence [a; b; c; d], the word '01' with a one-position offset corresponding to the sequence [d; a; b; c], the word '10' with a two-position offset corresponding to the sequence [c; d; a; b] and the word '11' with a 3-position offset corresponding to the sequence [b; c; d; a].

[0011] At the receiver, demodulation is performed by cross-correlation with the root sequence. The location of the correlation maximum indicates the information modulated via inverse mapping. For example, to demodulate the sequence [d; a; b; c], the result of cross-correlation with the root sequence [a; b; c; d] provides the normalized result [0; 1; 0; 0]. The correlation maximum at this index corresponds to the binary word '01'.

[0012] CCSK modulation can advantageously be combined with OFDM transmission so that each element composing a CCSK sequence, called a chip, is transmitted on a separate OFDM subcarrier.

[0013] In OFDM, it may happen that subcarriers are not affected in the same way by disturbances. Thus, it is customary in OFDM to prioritize transmission on subcarriers that have a better propagation channel. By selecting the "good" subcarriers in frequency, the transmitter is able to improve its link budget and therefore the system's performance. To do this, it is necessary to have an estimate of the channel for each subcarrier. For example, to estimate the channel on the subcarriers, a user equipment either uses the downlink directly (e.g., TDD system, for time-division duplex in English) or a base station transmits CSI (for Channel State Information in English).

[0014] Unfortunately, when using CCSK modulation and OFDM transmission in combination, the demodulation step requires calculating a correlation on a set of subcarriers on which the transmitted sequence fragments are distributed. Since the size of the sequences can quickly reach large values ​​(64, 128, 256, etc.), the possibilities are limited when trying to optimize the link budget by favoring subcarriers offering a good channel and avoiding modulating data on poor quality subcarriers. Moreover, if too many channels are of poor quality, the position of the correlation maximum will be incorrect, leading to the complete loss of information and therefore representing a significant waste of resources and energy.

[0015] There is thus a need for a technique to favor subcarriers offering a good transmission channel in a CCSK OFDM communication. Summary of the invention

[0016] Method for transmitting a data frame comprising q binary words of size r, each binary word p of the frame being modulated by a sequence C p of size N=2 r complex values ​​obtained by particular cyclic shift of the same root sequence of size N including at least one zero value, each sequence C p being transmitted in a separate OFDM symbol in which each complex value Cpfft], n E [O, N - 1], is positioned on a particular subcarrier, the method comprising the following steps: - Determination, for each position i E [0; N - 1], of the number of occurrences of a zero value at a position i of a sequence C p among the q sequences composing the frame, - Obtaining data relating to the quality of the channel of N OFDM subcarriers, - Application of the same cyclic shift to the q sequences coding for the q words of the frame to be transmitted, the value of the shift being such that it maximizes the number of non-mile values ​​positioned on the subcarriers offering the best channels, - Transmission of the q shifted sequences in q distinct OFDM symbols.

[0017] In this way, the method improves the link budget by concentrating the transmission of non-mile values ​​on the best quality carriers (or conversely, by concentrating the transmission of mile values ​​on low quality subcarriers). Since mile values ​​are not affected by a noisy transmission channel, such an arrangement limits transmission errors and improves transmission efficiency.

[0018] The sequences are shifted by the same amount so that they maintain the same relative shift between them, which allows the receiver to decode the received frame using a shifted demodulation technique.

[0019] A root sequence with a perfect autocorrelation function (a single correlation peak for zero offset) provides the best demodulation performance.

[0020] According to a particular embodiment, the step of applying the same cyclic shift comprises determining an offset value equal to the difference between the index of the subcarrier having the noisiest transmission channel and the position at which the number of occurrences of a zero value in the sequences making up the frame is maximum.

[0021] It is thus proposed to determine a particular cyclic shift value to be applied to all the CCSK sequences making up a frame to be transmitted in order to maximize the number of thousand values ​​to be transmitted on the lower quality subcarrier.

[0022] According to a particular embodiment, the step of applying the same cyclic shift comprises determining an offset value equal to the difference between the index of the subcarrier having the least noisy transmission channel and the position at which the number of occurrences of a non-zero value in the sequences making up the frame is maximum.

[0023] In this way, the transmission of non-mile values ​​on the best quality subcarrier is favored.

[0024] The invention also relates to a device for transmitting a data frame comprising q binary words of size r, each binary word p of the frame being modulated by a sequence C p of size N=2 r complex values ​​obtained by shifting particular cyclic sequence of the same root sequence of size N comprising at least one zero value, each sequence C p being transmitted in a distinct OFDM symbol in which each complex value Cjn], n G [O', N - 1], is positioned on a particular subcarrier, the device comprising an OFDM modulator, and a processor coupled to a memory in which instructions are recorded suitable for implementing the following steps: - Determination, for each position i G [0; N - 1], of the number of occurrences of a zero value at a position i of a sequence C p among the q sequences composing the frame, - Obtaining data relating to the quality of the channel of N OFDM subcarriers, - Application of the same cyclic shift to the q sequences coding for the q words of the frame to be transmitted, the value of the shift being such that it maximizes the number of non-mile values ​​positioned on the subcarriers offering the best channels, - Transmission of the q shifted sequences in q distinct OFDM symbols.

[0025] According to another aspect, the invention relates to a communication terminal comprising a device as described above.

[0026] According to yet another aspect, the invention relates to a communication system comprising such a communication terminal.

[0027] In a particular embodiment, the different steps of the transmission method are determined by computer program instructions.

[0028] Consequently, the invention also relates to a computer program comprising instructions adapted to the implementation of the steps of a transmission method as described above, when the program is executed by a processor.

[0029] This program may use any programming language, and may be in the form of source code, object code, or code intermediate between source code and object code, such as in a partially compiled form, or in any other desirable form.

[0030] The invention also relates to a computer-readable information medium on which is recorded a computer program comprising instructions for executing the steps of a transmission method as described above.

[0031] The information carrier may be any entity or device capable of storing the program. For example, the carrier may include a storage medium, such as a ROM, for example a CD-ROM or a microelectronic circuit ROM, flash memory, or a magnetic recording medium, such as a hard disk.

[0032] On the other hand, the information medium may be a transmissible medium such as a electrical or optical signal, which can be conveyed via an electrical or optical cable, by radio or by other means. The program according to the invention can in particular be downloaded from a network such as the Internet.

[0033] Alternatively, the information carrier may be an integrated circuit in which the program is incorporated, the circuit being adapted to perform or to be used in the performance of the method in question.

[0034] The various embodiments or features mentioned above may be added independently or in combination with each other, to the steps of the transmission method.

[0035] The devices, terminals, communication systems, programs and information media have advantages similar to those of the process to which they correspond. Brief description of the figures

[0036] Other characteristics and advantages will appear on reading a preferred embodiment described with reference to the appended drawings among which: - [Fig.l] represents the operating principle of an OFDM modulator, - [Fig.2] illustrates the operating principle of an OFDM demodulator, - [Fig.3] represents an environment suitable for implementing the transmission and reception methods according to a particular embodiment, - [Fig.4] is a flowchart representing the main stages of a transmission method according to a particular embodiment, - [Fig.5] shows 4 sequences comprising a zero value modulating 4 binary words, the fragments of which are placed opposite 4 OFDM subcarriers. - [Fig.6] shows the effect of applying an intercarrier gap shift to the sequences in [Fig.5], - [Fig.7] schematically represents an architecture of a device adapted to implement the transmission method according to a particular embodiment. Detailed description

[0037] In the following description, embodiments are described on the basis of non-limiting examples to explain the concepts on which the invention is based. In particular, although the examples and terminology used may refer to certain well-known technologies or standards, these references are not limiting and other technologies may be adapted to implement the concepts of the invention. For example, the CCSK or OFDM modulation technologies referred to may be replaced by different modulation techniques. modulation based respectively on circular shifts of complex value sequences or multi-carrier transmissions, without it being necessary to modify the invention.

[0038] [Fig.3] represents an environment 300 adapted to implement the transmission method according to a particular embodiment.

[0039] The environment 300 comprises wireless equipment 301, for example a gateway or a relay, adapted to communicate on the one hand with a device 302, and on the other hand with a server 303 of a communication network 304. Of course, this example is given for illustrative purposes and other types of equipment or other configurations of the environment can be envisaged to implement the invention. For example, the invention can be implemented between two communicating devices. The device 302 is for example a sensor, an M2M (Machine To Machine) communication device, or a communication terminal such as a tablet or a mobile phone for example. The gateway 301 can be a home gateway, such as a WiFi router, or a base station of a cellular communication network, an eNodeB, or a gNodeB.

[0040] In the example of [Fig.3], the gateway 301 communicates with the device 302 to exchange data using multi-carrier modulation, for example OFDM type modulation.

[0041] To transmit a data frame to the gateway 301 under the best conditions, the device 302 implements the transmission method according to a particular embodiment. In this way, the subcarriers having a quality channel are favored in the transmission and the efficiency of the connection is improved.

[0042] Here, the term "frame" or "packet" of data is understood to mean a transmission unit comprising one or more binary words with which an integrity control mechanism can be associated, for example a cyclic redundancy check (or CRC, for Cyclic Redundancy Code in English). Such an integrity control mechanism is known per se and makes it possible to detect transmission errors. For example, a data frame prepared by the device 302 may comprise one or more binary words constituting useful data, and one or more binary words, or even a part of a binary word, comprising data for the integrity control of the frame.

[0043] [Fig.4] is a flowchart illustrating the main steps of a particular implementation of the transmission method which is the subject of this description.

[0044] In a first step 400, the device 302 prepares a first data frame to be transmitted to the gateway 301. The frame comprises q binary words of size r bits to be transmitted. Preferably, the frame comprises data enabling an integrity check allowing the receiver to verify the integrity of the received bits, for example a checksum or a CRC.

[0045] In step 401, the binary words composing the prepared frame are modulated. For this, each binary word is associated with a particular sequence of complex values ​​obtained by cyclic shifting of a root sequence of size N=2 r , the root sequence comprising at least one zero value. For example, the device 302 may use CCS K type modulation.

[0046] A root sequence containing at least one zero value can be obtained in several ways. For example, one can use an oversampled Zadoff Chu type sequence, i.e. one in which one inserts thousand values ​​between the elements of the sequence.

[0047] [Fig.5] shows an example of modulation of a data frame comprising 4 binary words of 2 bits, “01”, “11”, “00” and “11”.

[0048] Each binary word is modulated by a particular shift of a root sequence [a, 0, c, d] with a zero value at index 1. Thus, the binary word "00" is associated with a zero shift of the root sequence, that is, with the root sequence itself. The word "01" is associated with a shift of one position, that is, [0, c, d, a], the binary word "10" is associated with the sequence [cd, a, 0] and the binary word "11" with the sequence [d, a, 0, c], that is, with a cyclic shift of 3 positions.

[0049] The frame is thus modulated by the C sequences o = [0, C, d, a], Cj = [d, a, 0, c], C2= [a, 0, c, d], and C3= [d, a, 0, c].

[0050] The root sequence [a, 0, C, d] used is such that it exhibits a good autocorrelation function, or a perfect autocorrelation function.

[0051] At the end of step 401, we obtain q vectors of complex values ​​corresponding to the q sequences C p of size N = r used to modulate the binary words of the frame. Each sequence thus obtained is intended to be transmitted in an OFDM symbol in which each element of a sequence is associated with a particular OFDM subcarrier. [Fig.5] thus shows 4 subcarriers noted SCO to SC3 corresponding to the 4 complex values ​​of the sequences to be transmitted.

[0052] In a step 402, for each index i, i E [0; N - 1], the number of occurrences of a zero value at index i in the set of q sequences that make up the frame is counted in order to determine the positions at which the thousand values ​​are most frequent in the frame. In the example of [Fig.5], one zero value is thus counted at indices 0 and 1, two thousand values ​​at index 2 and no zero value at index 3.

[0053] At step 403, the device obtains a channel estimate for each subcarrier. For example, the device may directly determine such estimates. if it uses a TDD (Time Division duplex) transmission system, the upstream and downstream data using the same frequencies, or use CSI (Channel State Indicator) indicators transmitted by the remote device. Channel estimation allows the device to obtain an indicator relating to the actual quality of the transmission on the different subcarriers.

[0054] In [Fig.5], the transmission quality for each of the subcarriers SCO to SC3 is represented by a gray level. Thus, the subcarriers SCO and SC2 are of good quality, while the subcarrier SC3 has a degraded quality and the subcarrier SCI has a very degraded quality. We can see in [Fig.5] that the values ​​positioned at index 3 in the sequences to be transmitted are positioned on the OFDM subcarrier SC3 whose quality is degraded. We can also see that the complex values ​​located at index 2 include two thousand values ​​and are positioned on the subcarrier SC2 offering a good quality channel.

[0055] The method comprises a step 404 during which the quality of the subcarriers is matched with the number of thousand values ​​counted in the sequences making up the frame to determine a cyclic shift to be applied to the sequences to optimize the positioning of the complex values ​​on the subcarriers.

[0056] In practice, a particular cyclic shift is determined to be applied to the sequences that make up the frame, which makes it possible to maximize the number of non-zero values ​​transmitted on good quality subcarriers and / or which maximizes the number of zero values ​​transmitted on low quality subcarriers. For this, in a particular implementation, an index i E [0; N - 1] is determined such that the number of occurrences of a zero value at a position i of a sequence C pamong the q sequences composing the frame is maximal, and an index j corresponding to the position of the subcarrier offering the lowest quality channel, and a cyclic shift of value d = i- j is applied to the q sequences coding for the q words of the frame to be transmitted. The applied shift cyclically rotates the elements of the sequences in order to move the positions where the thousand values ​​are the most frequent opposite the subcarrier of lower quality.

[0057] Figure 6 shows the 4 sequences of Figure 5 before (600) and after (601) applying a cyclic shift of one position (602). We note that the thousand values ​​are now more numerous at index 1 and positioned on the SCI subcarrier whose quality is the lowest. Furthermore, no zero element is positioned on the SC2 subcarrier whose quality is satisfactory. To determine the value of the shift applied, we determine the absolute value of the difference between the position containing the most thousand values ​​and the position of the lowest quality subcarrier. In other words, the shift d applied corresponds in this example to the difference between the position i at which the thousand values ​​are the most frequent and the index j of the subcarrier of lower quality, that is d = i- j.

[0058] Alternatively or in combination, an offset can be determined in the same way from the difference between the position containing the most non-mile values ​​and the position of the highest quality subcarrier so as to favor the transmission of non-mile values ​​on good quality subcarriers.

[0059] Of course, the root sequence used for CCSK modulation can have a plurality of thousand values. Generally, the offset is determined to globally optimize the positioning of the thousand values ​​on the lowest quality subcarriers and thus optimize the final link budget.

[0060] In a particular implementation, the expected link budget is evaluated for each sequence and for each possible shift and a score is calculated from a weighted average of these link budgets. The different calculated scores thus make it possible to determine the best shift to apply.

[0061] The shift applied to [Fig. 6] results in a modification of the binary frame after demodulation at the receiver. Indeed, since the binary words are modulated by a particular shift, a modification of this shift modifies the binary word during demodulation. Thus, the demodulation of the shifted sequences such as in [Fig. 6] gives the binary words “10”, “00”, “01”, and “00”.

[0062] To recover the initial frame, the receiver can implement offset demodulation.

[0063] To do this, the receiver reconstructs the transmitted frame by demodulating the successively received sequences in the OFDM symbols from the root sequence used for modulation. This root sequence is known to the receiver, for example because it is defined by a standard, or because it was negotiated during an initial configuration step. The receiver can then check the integrity of the reconstructed frame from a checksum or a CRC associated with the frame. If the integrity check shows that the frame is erroneous, the receiver can apply a cyclic shift to all the sequences in the frame and perform a new demodulation before checking its integrity again. The operation is thus repeated by the receiver until a positive integrity check allows it to determine the shift applied to the transmission.

[0064] The method finally comprises a step 405 during which the OFDM symbols carrying the shifted sequences are transmitted to the gateway 301 in a conventional manner.

[0065] [Fig.7] illustrates the architecture of a device 700 adapted to implement the method of transmitting a data frame according to a particular embodiment of the invention.

[0066] The device 700 comprises a data processing module comprising a storage space 701, for example a memory (MEM), a processing unit 702, equipped for example with a microprocessor (PROC), and controlled by a computer program (PGR) 703 whose instructions are configured to implement the transmission method as described previously in relation to [Fig.4].

[0067] At initialization, the code instructions of the computer program 703 are for example loaded into the memory 701 before being executed by the processor of the processing unit 702. The microprocessor of the processing unit 702 implements, according to the instructions of the computer program 703, the steps of the transmission method described above with reference to [Fig.4].

[0068] For this, in addition to the memory 701 and the processor 702, the device comprises communication means 704, for example a 3G, 4G, 5G, WiFi or other network interface, comprising an OFDM transducer adapted to transmit and receive signals on a plurality of orthogonal carriers. The communication means 704 are for example configured by computer program instructions to allow the transmission of a sequence of complex values ​​by distributing these complex values ​​on distinct sub-carriers of an OFDM symbol.

[0069] The device 700 further comprises a CCSK modulator 705. The modulator 705 is for example configured by code instructions adapted to associate with each binary word p of size r bits of a frame of q binary words, a sequence C p particular including N = '2 r complex values, obtained by cyclic shift of a root sequence comprising at least one zero value and exhibiting a good autocorrelation function, or even a perfect autocorrelation function.

[0070] The device 700 also comprises an analysis module 706. The analysis module 706 is for example implemented by code instructions configured to determine, among the q sequences composing the frame, a frequency of occurrence of a zero value at a location i E [G, N - 1] of a sequence C pTo do this, the module can place in a column in a matrix the q sequences of N elements making up the frame, and count the number of thousand values ​​line by line.

[0071] The device 700 also comprises a channel estimation module 707. The estimation module 707 is configured by code instructions adapted to obtain an estimation of the downlink channel of a plurality of OFDM subcarriers. The module can obtain such data from CSI messages transmitted by another device, or by estimating the uplink channel if the transmission uses a TDD technology for example. In practice, the estimation module is configured to associate a quality level with each OFDM subcarrier.

[0072] The device finally includes a cyclic shift module 708. The shift module can be implemented by code instructions configured to de- complete a cyclic shift to be applied to the q sequences of the frame so as to maximize the number of thousand values ​​positioned on low-quality subcarriers and / or so as to maximize the number of non-thousand values ​​positioned on good-quality subcarriers. In a particular embodiment, the shift d is determined by the absolute value of the difference between the index of the position comprising the most thousand values ​​and the index of the lowest-quality subcarrier. To apply the shift of a position, the module 708 causes, for example, aliasing (or spectrum overlap) by a frequency shift of an inter-carrier gap.

[0073] The communication module 704 is further configured to generate and transmit q OFDM symbols from the q shifted sequences.

[0074] According to a particular embodiment, the device 700 is included in a communication terminal, a connected object or sensor, a base station or any type of communicating equipment implementing multi-carrier transmission.

Claims

Claims

1. Method for transmitting a data frame comprising q binary words of size r, each binary word p of the frame being modulated (401) by a sequence C p of size N=2 r complex values ​​obtained by particular cyclic shift of the same root sequence of size N including at least one zero value, each sequence C p being transmitted in a distinct OFDM symbol in which each complex value C ^n], ne [O, N - 1], is positioned on a particular subcarrier, the method comprising the following steps: - Determination (402), for each i G [O, N - 1], of a number of occurrences of a zero value at a position i of a sequence C p among the q sequences composing the frame, - Obtaining (403) data relating to the quality of the channel of N OFDM sub-carriers, - Application (404) of the same cyclic shift to the q sequences coding for the q words of the frame to be transmitted, the value of the shift being such that it maximizes the number of non-mile values ​​positioned on the subcarriers offering the best channels, - Transmission (405) of the q shifted sequences in q distinct OFDM symbols.

2. Method according to claim 1 in which the step of applying the same cyclic shift comprises determining an offset value equal to the difference between the index of the subcarrier having the noisiest transmission channel and the position at which the number of occurrences of a zero value in the sequences making up the frame is maximum.

3. Method according to claim 1 in which the step of applying the same cyclic shift comprises determining an offset value equal to the difference between the index of the subcarrier having the least noisy transmission channel and the position at which the number of occurrences of a non-zero value in the sequences making up the frame is maximum.

4. Device for transmitting a data frame comprising q binary words of size r, each binary word p of the frame being modulated by a C sequence p of size N=2 r complex values ​​obtained by particular cyclic shift of the same root sequence of size N including at least one zero value, each sequence C p being transmitted in a separate OFDM symbol in which each complex value n G [ , N - 1], is positioned on a particular subcarrier, the device comprising an OFDM modulator (704), and a processor (702) coupled to a memory (701) in which instructions (703) are recorded adapted to implement the following steps: - Determination, for each i G [0; N - 1], of a number of occurrences of a zero value at a position i of a sequence C p among the q sequences composing the frame, - Obtaining data relating to the quality of the channel of N OFDM subcarriers, - Application of the same cyclic shift to the q sequences coding for the q words of the frame to be transmitted, the value of the shift being such that it maximizes the number of non-mile values ​​positioned on the subcarriers offering the best channels, - Transmission of the q shifted sequences in q distinct OFDM symbols.

5. A communication terminal comprising a transmission device according to claim 4.

6. Computer program comprising instructions adapted to implement the steps of a transmission method according to any one of claims 1 to 3, when the program is executed by a processor.

7. A computer-readable information medium on which is recorded a computer program comprising instructions for carrying out the steps of a transmission method according to any one of claims 1 to 3.

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