Method and device for transmitting items of OFDM data

Abstract

The invention relates to a transmission method for transmitting a plurality of binary words in an OFDM symbol. The transmission method comprises, in particular, a modulation step (200), wherein a binary word to be transmitted is associated with a particular sequence Cp i having a size N obtained by cyclically shifting a root sequence, a step (201) of creating a first vector, wherein elements from at least two sequences are periodically interleaved, a step (202) of obtaining a second vector having K elements by applying an inverse discrete Fourier transform to the first vector, and a step 203) of transmitting, on K OFDM subcarriers, an OFDM symbol representative of the K elements of the second vector. The invention also relates to a method for receiving one of at least one binary word transmitted according to the transmission method, and to transmitting and receiving devices for implementing the methods.

Description

Description Title of the invention: OFDM data transmission method and device Technical field

[0001] The invention belongs to the field of telecommunications and relates in particular to a method of transmitting data over OFDM (Orthogonal Frequency Division Multiplex in English), and in particular a method advantageously combining CCSK modulations (Cyclic Code-Shift Keying in English) and OFDM. PREVIOUS ART

[0002] Low-power, long-range communication networks (LPWAN) remain a major subject of research and innovation today, largely driven by the deployment of satellite loT (Internet Of Things) networks, but also by the evolution of cellular loT markets (LTE-M & NB-IoT) and LoRa.

[0003] Such communications are generally subject to the same constraints: a need to sporadically communicate small amounts of data over long distances, a very low link budget, and limited computing capabilities. Added to this is often the constraint of a finite power source, as most of these devices operate on batteries.

[0004] Energy efficiency is therefore at the heart of the design of IoT communications technologies. One area of ​​focus is the share of communications that do not carry useful data. Indeed, to function, the physical layer traditionally requires so-called "pilot" signals to perform time and frequency synchronization operations, or even propagation channel estimation. These signals do not directly carry information, but are necessary for the proper functioning of the system. However, they represent a loss of useful energy for all systems, with particular criticality for IoT systems.

[0005] CCSK modulation allows the transmission of pilot signals without additional cost. This modulation method proposes modulating the data to be transmitted by a cyclic shift of a root sequence of complex symbols. The root sequence is such that its shifted versions are orthogonal to each other, that is to say, it offers a good autocorrelation function. Thus, each binary word to be transmitted is associated with a particular shifted version of the root sequence.

[0006] For example, a root sequence composed of the 4 complex symbols [a; b; c; d] allows 2-bit binary words to be modulated by associating the word '00' with an offset null corresponding to the sequence [a; b; c; d], the word '01' at a one-position shift corresponding to the sequence [d; a; b; c], the word '10' at a two-position shift corresponding to the sequence [c; d; a; b], and the word '11' at a 3-position shift corresponding to the sequence [b; c; d; a].

[0007] Once the sequences are received, 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], a cross-correlation calculation with the root sequence [a; b; c; d] provides the normalized result [0; 1; 0; 0]. The correlation maximum at this index thus corresponds to the binary word '01'.

[0008] A root sequence with a perfect autocorrelation function (a single correlation peak for zero offset) provides the best demodulation performance. This sequence can then also be used for time and frequency synchronization of the communication system. The use of CCSK modulation therefore allows the transmission of data, the symbols of which are also used as pilots to synchronize communications.

[0009] CCSK modulation also has the advantage of allowing "shifted demodulation". Indeed, when all the received sequences are shifted by the same value, or equivalently, if all the correlation peaks are shifted by the same value, then all the demodulated binary words will also be shifted by this value.

[0010] For example, suppose that we wish to transmit the binary words '00' and '01' over a standard single-band transmission channel using CCSK modulation based on the same coding as the previous example. The binary words are then respectively modulated by the sequences [a; b; c; d] and [d; a; b; c] and transmitted over the transmission channel by the sequence [a; b; c; d; d; a; b; c]. In case of poor time synchronization at the receiver, for example when the sequence is received with a delay of one symbol, the receiver obtains the sequence [b; c; d; d; a; b; c; 0], corresponding to the transmitted sequence shifted by one symbol to the left. The two received sequences are separated to obtain the two sequences [b; c; d; d] and [a; b; c; 0].The cross-correlation peak for the sequence [b; c; d; d] is obtained for a rotation of 3 symbols of the root sequence, i.e., with the sequence [b; c; d; a], and the cross-correlation peak for the sequence [a; b; c; 0] is obtained with the sequence [a; b; c; d], i.e., with a corresponding zero shift. Thus, the cross-correlation gives a maximum at position 3 and 0 and these indices correspond to the binary words '11' and '00'. The demodulation has therefore shifted all the binary words by the same value (-1 modulo 4).

[0011] An advantage of offset demodulation is that it allows the whole binary words of the same frame by testing the different possible offsets resulting from demodulation. By using a mechanism verifying the integrity of the frame, for example a CRC (for Cyclic Redundancy Check in English), it is possible to identify the transmitted frame.

[0012] For example, taking the demodulated binary words '11' and '00' in the example, the receiver can calculate a CRC and determine that the frame is in error. The receiver can then shift all the binary words by one position and obtain '00' and '01'. By calculating the CRC again, it validates the received frame.

[0013] Once the offset value has been identified for a first frame, it can be directly applied to successive frames, if they are received under the same conditions that led to the original offset. The corollary is that by identifying the offset value, the system can adjust certain system parameters, such as time synchronization.

[0014] These different properties make the use of CCSK modulation very attractive for IoT communications.

[0015] Many applications implement a modulation called CP-OFDM (for Cylic Prefix Orthogonal Frequency Division Multiplex in English). Such modulation uses a Cyclic Prefix (CP) consisting of a copy of the end of a block of information to be transmitted at the beginning of the frame. The prefix thus plays a buffer role in the case of multi-path transmission, which helps to avoid intersymbol interference.

[0016] There is thus a need for a transmission technique that allows CCSK and CP-OFDM modulation techniques to be combined intelligently and efficiently to transmit several CCSK sequences per OFDM symbol while benefiting from the advantageous properties of CCSK modulation. Summary of the invention

[0017] For this purpose, a method is proposed for transmitting binary words of size r bits, with Q > 1, through an OFDM transmission channel, the method comprising: - A modulation step in which a binary word Pi to be transmitted, with i E [ 0; q - 1], is associated with a particular sequence Cp. of size N = 2' obtained by cyclic shift of a root sequence, - A step of creating a first vector of size K = q*N in which the N elements Cp [n] are periodically interleaved, with n E [0; N - 1] of the Cp sequences associated with the words to be transmitted, - A step of obtaining a second vector of K elements by applying an Inverse Discrete Fourier Transform (IDFT) to said first vector, - A transmission step on K OFDM subcarriers of an OFDM symbol representative of the K elements of the second vector.

[0018] The binary words to be transmitted are thus modulated by the value of a particular cyclic shift of a predetermined root sequence (for example a CCSK modulation, for Cyclic Code-Shift Keying in English), the symbols of the shifted sequences corresponding to the binary words to be transmitted being transmitted in the same OFDM symbol (for Orthogonal Frequency Division Multiplex in English), each chip being associated with a subcarrier according to a particular pattern.

[0019] Generally, the elements of CCSK sequences take their value from a finite alphabet of complex values. However, other types of alphabet, for example binary value codes, can be considered within the scope of the present invention. In a digital signal formed according to a determined law from a data element, the chips are parts emitted successively, which have characteristics distinct from the others.

[0020] The method thus allows data transmission over OFDM without the need to transmit pilot symbols. This improves efficiency compared to a transmission system using dedicated synchronization symbols that do not carry useful data.

[0021] The elements composing the CCSK sequences are further interleaved in a vector of size K = q*n according to a periodic interleaving law according to which an element Q Lj], with n G [0; N - 1] and ie [0; q - 1] occupies the rank r = i*K + n in the P _ J interleaved vector.

[0022] In other words, the successive elements of a sequence Cp. are inserted into the vector with a periodicity equal to by first placing the first element of each sequence into the vector, then taking the sequences in the same order, the second element of each sequence, and so on.

[0023] [Fig.l] shows an example of such interleaving. The data of three sequences A, B and C of size N=4 are interleaved into a vector of size K=3*N so that the elements of the same rank of each vector are successively added to the destination vector.

[0024] The interleaving of the CCSK sequences encoding the words to be transmitted allows a facilitated demodulation operation: the correlation results of all the CCSK sequences transmitted in an OFDM symbol can be obtained in a single correlation operation from a root sequence oversampled to rank K.

[0025] This particular arrangement of the chips also allows for offset demodulation at the receiver, making it possible to partially avoid time synchronization and completely avoid correction of the entire frequency offset for synchronization. frequency.

[0026] Thus, the method allows the transmission of several CCSK sequences in a single OFDM symbol and a demodulation of these sequences in a single correlation operation from an oversampled root sequence. The particular arrangement of the elements of each symbol makes it possible to preserve the properties of CCSK modulation, in particular the possibility of offset demodulation.

[0027] According to a particular embodiment, the transmission method is such that the root sequence has a perfect autocorrelation function.

[0028] The use of a root sequence with a perfect autocorrelation function, i.e., a single correlation peak for zero offset, allows its use for time and frequency synchronization of communication. In addition, the single peak indicating the offset allows optimal demodulation of the transmitted sequences.

[0029] According to a particular embodiment, the transmission method is such that the root sequence is a Zadoff-Chu sequence.

[0030] The Zadoff-Chu sequences exhibit a perfect autocorrelation function and retain a good autocorrelation property in the case of phase shift, for example in the case of phase shift caused by the passage through the Discrete Fourier Transform of T OFDM.

[0031] Furthermore, the Inverse Discrete Fourier Transform, like the Discrete Fourier Transform of a Zadoff-chu sequence, is also a CAZAC (Constant Amplitude Zero AutoCorrelation waveform). The time samples of the OFDM symbol can then be used to calculate correlation scores and improve time and frequency synchronization.

[0032] Other sequences can however be used without modifying the invention, for example Gold sequences or even m-sequences (or MES for Maximum Eighth Sequence in English) which are binary sequences presenting a very good autocorrelation and cross-correlation function (or cross-correlation in English, that is to say a correlation between two different sequences).

[0033] According to a particular embodiment, a particular distribution of the N elements composing a sequence on the K OFDM subcarriers defines a transmission group, the binary words belonging to the same particular data packet being transmitted within at least one same particular transmission group on a plurality of successive OFDM symbols.

[0034] Thus, when a particular shift affects a group, the CCSK sequences transmitted successively on this group are affected by this same shift as long as the transmission conditions remain substantially identical. It is thus possible to carry out shifted demodulation by considering each group independently.

[0035] Here, the term "data frame", "data packet" or "information frame" means a transmission unit comprising at least one binary word to be transmitted with which an integrity control mechanism is associated, for example a cyclic redundancy check (or CRC, for Cyclic Redundancy Code in English).

[0036] In other words, a set of data to which integrity control means have been associated is transmitted using one or more particular transmission groups, so that the data of the data packet can be obtained by offset demodulation applied independently to the transmission group associated with said data packet.

[0037] According to a particular embodiment, the transmission method is such that the binary words to be transmitted comprise at least a first word intended for a first user equipment and a second word intended for a second user equipment, the first and second words being respectively associated with a first and a second particular sequence respectively obtained by cyclic shifting of a first and a second root sequence of size N, the first and second root sequences being weakly correlated with each other and respectively associated with the first and second user equipment.

[0038] A multi-user downlink multiplexing is thus proposed, which ensures the confidentiality of the transmitted data. Each user can then attempt to demodulate the received signal from the root sequence assigned to them. Since the first and second sequences are weakly correlated, or even orthogonal to each other, the correlation peak determines the data intended for them.

[0039] The invention also relates to a device for transmitting <7 binary words through an OFDM transmission channel, with q > 1, the device comprising a radiofrequency transmitter and a processor coupled to a memory in which instructions are recorded suitable for configuring the processor and the transmitter in order to implement the following steps: - A modulation step in which a binary word Pi to be transmitted, with i E [ CI, q - 1], is associated with a particular sequence Cp. of size N obtained by cyclic shift of a root sequence, - A step of creating a first vector of size K = q*N in which the N elements Cp [ ] are periodically interleaved, with n E [0; N - 1] of the sequences Cp. associated with the words to be transmitted, - A step of obtaining a second vector of K elements by applying an Inverse Discrete Fourier Transform (IDFT) to said first vector, - A transmission step on K OFDM subcarriers of an OFDM symbol representative of the K elements of the second vector.

[0040] According to another aspect, the invention relates to a method for receiving binary words, with q > 1, transmitted on an OFDM transmission channel, each binary word Pi, with ie [ 0; q - 1 ], being associated for transmission with a particular sequence Cp of size N obtained by cyclic shift of a root sequence, the method comprising the following steps: - Reception of an OFDM symbol on K = q*N subcarriers, - Obtaining a vector of K elements by applying a Discrete Fourier Transform to the received OFDM symbol, the vector comprising particular Cp sequences of size N interleaved periodically, - Calculation of a cross-correlation between the obtained vector and a root sequence of size N over-sampled to rank K by regular insertion of intermediate terms between the N elements of the root sequence, - Determination of the received binary words according to the result of the correlation.

[0041] Since CCSK sequences are periodically interleaved, an oversampled root sequence can be used to obtain the correlation results of all CCSK sequences in a single correlation operation. It is then sufficient to extract the results specific to each sequence and determine the indices of the corresponding correlation maxima.

[0042] The interlacing of the sequences also allows for offset demodulation, which partly eliminates the need for time synchronization and completely eliminates the need for correction of the entire frequency offset for frequency synchronization.

[0043] According to a particular embodiment, the reception method is such that the root sequence has a perfect autocorrelation function.

[0044] According to a particular embodiment, the reception method is such that the root sequence is a Zadoff-Chu sequence.

[0045] According to a particular embodiment, the reception method is such that the q binary words comprise at least a first word intended for a first user equipment and a second word intended for a second user equipment, the first and second words being respectively associated with a first and a second particular sequence respectively obtained by cyclic shift of a first and a second root sequence of size N, the first and second root sequences being orthogonal to each other and respectively associated with the first and second user equipment, the method comprising a step of determining a sequence intended for a particular receiver by calculating a cross-correlation between a root sequence associated with the receiver oversampled to rank K.

[0046] According to a particular embodiment, the reception method comprises a step of identifying at least one transmission group in a received OFDM symbol, a transmission group being defined by a particular distribution of the N elements of a CCSK sequence on the K OFDM subcarriers, the q CCSK sequences transmitted in an OFDM symbol modulating binary words belonging to q distinct data packets, the binary words composing a particular data packet being transmitted within a particular transmission group on a plurality of successive OFDM symbols.

[0047] It is thus proposed to consider independent serial transmission groups according to the distribution of the fragments of the sequences transmitted on the OFDM subcarriers. The CCSK sequences transmitted successively on a particular group being affected by the same offset as long as the transmission conditions remain substantially identical, it is possible to carry out offset demodulation by considering each group independently.

[0048] There is also proposed a device for receiving q binary words, with q > 1, transmitted on an OFDM transmission channel, each binary word Pi, with ie [ , q - 1], being associated for transmission with a particular sequence Cp. of size N obtained by cyclic rotation of the same root sequence, the device comprising a radiofrequency receiver and a processor coupled to a memory in which are recorded instructions adapted to configure the processor and the receiver in order to implement the following steps: - Reception of an OFDM symbol on K=q*N subcarriers, - Obtaining a vector of K elements by applying a Discrete Fourier Transform to the received OFDM symbol, the vector comprising q particular Cp sequences of size N interleaved periodically, - Oversampling of a root sequence of size N to rank K by regular insertion of intermediate terms between the N elements of the root sequence, - Determination of the received binary words by calculating a correlation between the oversampled root sequence and the obtained vector.

[0049] According to yet another aspect, the invention relates to a communication system comprising a transmitting device and a receiving device as described above.

[0050] In a particular embodiment, the different steps of the transmission method and the reception method are determined by computer program instructions.

[0051] Consequently, the invention also relates to a computer program comprising instructions adapted to the implementation of the steps of a transmission method and / or a reception method as described above, when the program is executed. by a processor.

[0052] 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.

[0053] The invention also relates to a recording medium readable by a computer on which is recorded a computer program comprising instructions for executing the steps of a transmission method and / or a reception method.

[0054] 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.

[0055] On the other hand, the information carrier may be a transmissible carrier such as an electrical or optical signal, which may be conveyed via an electrical or optical cable, by radio or by other means. The program according to the invention may in particular be downloaded from a network such as the Internet.

[0056] 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 performing the methods in question.

[0057] The various embodiments or features mentioned above may be added independently or in combination with each other, to the steps of the transmission and / or reception methods.

[0058] The devices, systems and programs have advantages similar to those of the processes to which they correspond. Brief description of the figures

[0059] Other characteristics and advantages will appear on reading a preferred embodiment described with reference to the appended drawings among which: - [Fig.l] represents a vector resulting from the periodic interlacing of 3 vectors, - [Fig.2] represents the main steps of a transmission method which can be implemented using a CCSK CP-OFDM modulator, according to a particular embodiment, - [Fig.3] illustrates the mapping of CCSK sequences to OFDM subcarriers according to a particular implementation, - [Fig.4] represents the main steps of a reception method that can be implemented in a CCSK CP-OFDM demodulator according to a mode particular achievement, - [Fig.5] shows a root sequence oversampled at rank K that can be used to calculate a correlation with received CCSK sequences, according to a particular embodiment, - [Fig.6] illustrates the phenomenon of spectrum folding, or aliasing, - [Fig.7] illustrates the effect of frequency desynchronization when CCSK sequences are sequentially positioned on OFDM subcarriers, - [Fig.8] illustrates the effect of frequency desynchronization when CCSK sequences A, B and C are periodically interleaved on OFDM subcarriers, according to a particular embodiment, - [Fig.9] illustrates the architecture of a device suitable for implementing a method of transmitting binary words according to a particular embodiment, and - [Fig.10] illustrates the architecture of a device suitable for implementing a method of receiving binary words according to a particular embodiment. Detailed description

[0060] In the following description, embodiments are described on the basis of non-limiting examples making it possible to explain the concepts on which the invention is based. In particular, although the examples and the 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 CSK or OFDM technologies referred to may be replaced by different modulation techniques based respectively on circular shifts of complex value sequences or multi-carrier transmissions without it being necessary to modify the invention.

[0061] Figure 2 illustrates the main steps of a method for transmitting at least binary words, q > 1, according to a particular embodiment of the invention.

[0062] The method comprises a first step 200 during which binary code words to be transmitted, with > 1, are coded by CCSK modulation, i.e. by the cyclic rotation of a root sequence of complex symbols.

[0063] The root sequence is preferably a sequence having a perfect autocorrelation function, i.e., a single correlation peak for zero lag. According to a particular embodiment, the root sequence is a Zadoff-Chu sequence or an m-sequence.

[0064] Each of the binary words to be transmitted, noted Pi with ie [0; q - 1], is associated with a particular CCSK sequence of size N noted Cp obtained by cyclic shift of the same root sequence of size N. The binary words are then coded by sequences of N complex symbols called “chips”.

[0065] According to a particular embodiment, the binary words to be transmitted comprise at least one binary word associated with a sequence obtained by cyclic shift of a first particular root sequence, and at least one binary word associated with a sequence obtained by cyclic shift of a second particular root sequence, the first and second sequences being of the same size and orthogonal to each other. In other words, binary words modulated from two different and orthogonal root sequences are transmitted in the same OFDM symbol. Such an arrangement makes it possible to guarantee that only a receiving device having a particular root sequence can demodulate the binary words modulated with this root sequence.Thus, an OFDM symbol can contain data intended for different devices while ensuring data confidentiality, because only a device with a particular root sequence will be able to decode the data intended for it.

[0066] In a step 201, the identically sized sequences of n snippets |Cp [ n]}' avec ie [0; q - 1] and n G [0; N - 1], are organized in a particular way in a vector in order to be advantageously positioned on OFDM subcarriers. A series-to-parallel (S / P) converter thus forms a block of N modulated symbols.

[0067] More precisely, the identically sized sequences of N symbols coding the words to be transmitted are interleaved periodically and regularly in a first vector of size K = q*N in which the elements Cp [n] of the ? sequences Cp associated with the q words to be transmitted are arranged.

[0068] It is thus proposed to intercalate the elements of the <7 sequences in a vector of size K = q*N according to a periodic interlacing law according to which an element of a sequence S with n G [0; A - 1] and ie [0; q - 1] occupies the rank r = i*K + n in the interlaced vector.

[0069] Figure 3 shows an example of such an interleaving of 3 CCSK Cp sequences Q , Cp . C p of N symbols in a vector 300 of size K=3*N for transmission on K OFDM subcarriers.

[0070] Interleaving can for example be achieved by using a matrix in which the sequences Cp. are written on successive lines, the vector being formed by concatenating the contents of the columns of the matrix.

[0071] According to a particular embodiment, the particular distribution of the N elements of a sequence on the K OFDM subcarriers defines a transmission group, the binary words composing a particular data packet being transmitted within a or more particular transmission groups on a plurality of successive OFDM symbols.

[0072] For example, with reference to [Fig.1]: - the location of elements A1, A2, A3 and A4 of sequence A defines a first transmission group, - the location of elements Bl, B2, B3 and B4 of sequence B defines a second transmission group, and - the location of elements Cl, C2, C3 and C4 of sequence C defines a third transmission group.

[0073] It is thus proposed that the binary words composing the same data packet to be transmitted are transmitted within the same group in successive OFDM symbols. In practice, the 3 transmission groups defined by the respective distribution of the chips belonging to the 3 sequences represented in [Fig.l] form 3 serial transmission channels respectively assigned to the transmission of distinct information frames.

[0074] In step 202, an Inverse Discrete Fourier Transform (IDFT) is applied to the vector created in step 201 to obtain a sample vector C[n], with ne [0; K - 1],

[0075] In step 203, the sample vector from the IDFT is serialized by a parallel-serial converter to obtain an OFDM symbol {C[ n]} transmitted on K subcarriers after adding a cyclic prefix (CP, for Cyclic Prefix).

[0076] It is noted that from a spectral point of view, the first half of the vector C[n] and the second half of the vector C[n] are inverted by T IDFT. In other words, a vector [a; b; c; d] will be transmitted in the form [c; d; a; b]. This has no particular implication in the context of the present invention because the DFT produces the inverse transformation on the receiver side. However, one can envisage an inversion step (or "swap" in English) during which the chips are reorganized so that from a spectral point of view, the position of the chips on the subcarriers reflects the organization of the vector C[n]. Of course, the reception method then also comprises a corresponding inversion step.

[0077] Figure 4 illustrates the steps of a method for receiving at least Q binary words, Q > 1, transmitted according to the transmission method described above with reference to [Fig.2], according to a particular embodiment.

[0078] In a first step 400, an OEDM symbol is received on K subcarriers and placed in a vector y[ ] of K elements by a serial-parallel converter S / P. The symbol here corresponds to an OEDM symbol transmitted according to the transmission method described above, in which binary words denoted Pi with ie [0; q - 1] and > 1, are transmitted in the form of interleaved CCSK sequences according to a law of periodic interlacing in accordance with the description previously given with reference to figures 1 and 3.

[0079] The K chips {y

[0000] , y

[0001] , . .. 2], y [ K - 1]} are thus received on K OFDM subcarriers and placed in a first vector of K elements by a S / P series-parallel converter to which a Discrete Fourier Transform (DFT) is applied in step 401, after removing the CP-OFDM prefix. At Tissue of step 401, we obtain a second vector of size K denotedr in Figure 4 including the interleaved CCSK sequences.

[0080] In step 402, the interleaved CCSK sequences of the vector r are demodulated using at least one root sequence identical to that used to modulate the data during transmission. For this, a cross-correlation calculation is performed.

[0081] According to a particular embodiment, the correlation calculation is carried out using a correlation sequence obtained from a root sequence of size N from which at least one transmitted binary word has been modulated, said root sequence being oversampled to rank K.

[0082] More precisely, the root sequence is upsampled to rank K with a "zero-padding" method, i.e. without interpolation, by inserting nuisance terms between the samples so as to go from a sequence of size N to a sequence of size K. Figure 5 represents such a 500 correlation sequence obtained from a root sequence of size N upsampled to rank K using this method: zeros are regularly inserted between the samples Q[Z], with i E [0; N - 1] of the root sequence to extend it to K elements.

[0083] For each cyclic shift of the upsampled root sequence, a correlation calculation is performed with the second vector r. In other words, the demodulation is performed by cross-correlation with an oversampled root sequence, the location of the correlation maximum indicating the information modulated via inverse mapping. In this way, the correlation results of all CCSK sequences are obtained in a single cycle of correlation operations in the form of a 501 vector denoted .[ n] .

[0084] For example, let us take two CCSK sequences to be transmitted [a; b; c; d] and [c; d; a; b] corresponding respectively to the binary words '00' and '10' according to the coding scheme taken as an example in the introduction. The sequences are interleaved and transmitted according to the transmission method described above, so that the sequence of complex symbols received is [a; c; b; d; c; a; d; b].

[0085] The root sequence [a; b; c; d] is then upsampled to rank 8 to give the sequence [a; 0; b; 0; c; 0; d; 0].

[0086] A correlation score is then calculated between the received sequence and each of the 8 possible offsets of the root sequence. The correlation maxima provide the normalized results [1; 0; 0; 0] and [0; 0; 1; 0] once subsampled to rank N, corresponding to the transmitted binary words '00' and '10'.

[0087] If the interleaving of the transmitted sequences allows demodulation facilitated by the use of an over-sampled root sequence, this particular arrangement of the chips on the OFDM subcarriers presents other advantages, notably in the case of poor time or frequency synchronization.

[0088] In the case of poor frequency synchronization, the entire spectrum (and therefore the subcarriers) is shifted in one direction. By aliasing, the part of the spectrum leaving the sampling band is ultimately transposed to the opposite frequencies, so that a positive frequency offset will transpose a part of the spectrum to negative frequencies and vice versa. An illustration of the principle is provided in [Fig.6].

[0089] [Fig.6] shows a sequence [a; b; c; d] whose 4 chips are respectively transmitted on 4 OFDM subcarriers (600). When the frequency shift corresponds to an integer inter-carrier spacing (in the example of [Fig.6], +1 inter-carrier spacing), the sequence is shifted towards the top of the spectrum (601). Due to the spectrum folding phenomenon, the transmitted sequence [a; b; c; d] is finally received (602) in the form of a sequence [d; a; b; c].

[0090] It is customary in OFDM to consider the normalized value of the frequency offset f Q with respect to the inter-carrier distance A y. We thus note y = A y avec . the integer offset and f the fractional offset. The integer offset represents the frequency shift of an integer intercarrier spacing. The fractional offset represents what remains, a fraction of the intercarrier spacing, so that f [ - 0 5 0 5] -

[0091] Assuming that the fractional offset f is zero, an integer number of subcarriers corresponding to the integer offset / . therefore undergoes F aliasing. In the context of CCSK modulation, this will result in a shift of the transmitted sequence, as described with reference to [Fig.6].

[0092] A shifted demodulation then makes it possible to find a transmitted binary word.

[0093] A particular problem arises when multiple binary words are transmitted in a single OFDM symbol and the integer offset is greater than one inter-carrier gap.

[0094] [Fig.7] shows 3 CCSK sequences of 4 chips arranged sequentially in a first vector 700 to be transmitted in the same OFDM symbol. The sequences [a; b; c; d], [c; d; a; b] and [b; c; d; a] are respectively associated with the binary words '00', '10' and '11' according to the coding convention defined in the introduction.

[0095] [Fig.7] also shows a second vector 701 received with a desynchronization- frequency scaling of an integer offset equal to an inter-subcarrier distance. The chip 'a' of the sequence [b; c; d; a] located at the upper end of the vector 700 is then found at the other end of the vector 701 due to aliasing. Once separated, the shifted sequences [a; a; b; c], [d; c; d; a] and [b; b; c; d] are demodulated and allow the binary words '01', '11', and '00' to be obtained respectively, but from a non-optimal correlation score. It can be seen that the binary words corresponding to the sequences are shifted in the same way as the sequences, and that it is therefore possible to carry out shifted demodulation.

[0096] However, when a larger shift affects the vector in reception, it is no longer possible to demodulate the transmitted sequences. [Fig.7] shows a third vector 702 received with a frequency desynchronization of an integer offset equal to two inter-subcarrier distances. This time, the two chips 'd' and 'a' from the upper end of the vector 800 are displaced to the other end of the vector. Once separated, the 3 sequences resulting from this shift, [d; a; a; b], [c; d; c; d] and [a; b; b; c] can no longer be demodulated because it is no longer possible to find a maximum correlation for the transmitted sequences. Shifted demodulation is not possible.

[0097] [Fig.8] shows a first vector 800 in which 3 sequences of 4 chips are arranged according to a periodic interleaving law described previously to be transmitted in the same OFDM symbol. The sequences [a; b; c; d], [c; d; a; b] and [b; c; d; a] are respectively associated with the binary words '00', '10' and '11' according to the coding convention defined in the introduction and interleaved in the vector 800.

[0098] [Fig.8] also shows a second vector 801 received with a frequency desynchronization of an integer offset equal to an inter-subcarrier gap. The chip 'a' located at the upper end of the vector 800 is then found at the other end of the vector 801 due to aliasing.

[0099] [Fig.8] finally shows a third vector 802 received with a frequency desynchronization of an integer offset equal to two inter-subcarrier gaps. The two chips 'b' and 'a' from the upper end of vector 800 are found displaced to the other end of vector 802.

[0100] In [Fig.8], 3 transmission groups are shown, denoted A, B and C. These groups are defined to correspond to the locations of the interleaved CCSK sequences in the transmitted vector denoted 800 in the figure. The groups thus correspond to fixed locations in the vectors 800, 801 and 802.

[0101] In vector 800, group A comprises the sequence [b; c; d; a], group B the sequence [c; d; a; d] and group C the sequence [b; c; d; a].

[0102] In vector 801, group A comprises the sequence [c; d; a; b], group B the sequence [a; b; c; d] and group C the sequence [a; b; c; d].

[0103] In vector 802, group A comprises the sequence [a; b; c; d], group B the sequence [a; b; c; d] and group C the sequence [b; c; d; a].

[0104] It can be seen that, unlike the example described with reference to [Fig.7], the particular arrangement of the sequences in the initially transmitted vector always makes it possible to determine a correlation peak in the sequences received within each group, whatever the shift experienced in reception.

[0105] We also note that for a shift according to a frequency offset equal to one (1) difference between subcarriers, all the subcarriers are shifted by one position. The position of the groups does not change, it is the sequences which move between the groups. Thus, group B recovers the sequence transmitted in group C. Group A recovers the sequence transmitted in group B. Group C recovers the sequence transmitted in group A, cyclically shifted by one position.

[0106] In the case of a shift due to a frequency offset equal to 2 inter-subcarrier gaps, the same logic applies as can be observed on vector 802. All the subcarriers are thus shifted by an additional position and the sequence recovered by group C undergoes a cyclic rotation due to aliasing.

[0107] Because some sequences transmitted in the OFDM symbol undergo cyclic rotation while others do not, it is not possible to perform offset demodulation on the entire OFDM symbol. This would require all sequences to be shifted in the same way. On the other hand, all sequences transmitted in the same group in successive OFDM symbols undergo the same cyclic shift: it is therefore possible to perform offset demodulation by considering each group independently.

[0108] Thus, the demodulation step comprises a sub-step of identifying at least one transmission group in a received OFDM symbol, a transmission group being defined by a particular distribution of the N elements of a CCSK sequence on the K OFDM sub-carriers, the binary words composing a particular data packet being transmitted within one or more particular transmission groups on a plurality of successive OFDM symbols.

[0109] In practice, the data belonging to the same information frame are transmitted in series on one or more of the same transmission groups by a plurality of CCSK sequences transmitted successively by a plurality of successive OFDM symbols.

[0110] Since the sequences transmitted successively on a particular transmission group are affected by the same cyclic shift due to poor frequency synchronization, it is then possible to carry out a shifted demodulation of the data received on this group, the shift being determined for example by verifying the integrity of the received frame.

[0111] It is thus possible to find all the binary words of the same frame by testing the different possible shifts resulting from the demodulation. By using a mechanism verifying the integrity of the frame, for example using a cyclic redundancy check (CRC), it is possible to identify the transmitted frame. More precisely, it is proposed to verify the integrity of the frame for each cyclic shift, so that a positive integrity check makes it possible to know the shift undergone by the OFDM symbol and find the transmitted sequences.

[0112] In a step 403, the binary words corresponding to the correlation peaks determined in step 402 are reordered and transmitted to the application layers. Since the correlation scores for each sequence are also interleaved, as can be seen in [Fig.5], it is necessary to deinterleave them by separating the correlation scores of each sequence. The maximum correlation peak is then obtained for each sequence by calculating a maximum argument (argmax). The position of the correlation peak obtained allows demodulation by determining the binary word that corresponds to this maximum.

[0113] The principle of offset demodulation takes advantage of the fact that when all received sequences are cyclically shifted by the same value, or equivalently, if all correlation peaks are shifted by the same value, then all demodulated binary words will also be shifted by this value. The proposed interleaving and data transmission by group allows this advantage to be maintained in the presence of poor frequency synchronization even when several CCSK sequences are transmitted in the same OFDM symbol.

[0114] Figure 9 illustrates the architecture of a device 900 adapted to implement the method of transmitting binary words according to a particular embodiment of the invention.

[0115] The device 900 comprises a data processing module comprising a storage space 901, for example a memory (MEM), a processing unit 902, equipped for example with a microprocessor (PROC), and controlled by a computer program (PGR) 903 whose instructions are configured to implement the transmission method as described previously in relation to [Fig.2].

[0116] At initialization, the code instructions of the computer program 903 are for example loaded into the memory 901 before being executed by the processor of the processing unit 902. The microprocessor of the processing unit 902 implements, according to the instructions of the computer program 903, the steps of the method for transmitting binary words, q > 1, on an OFDM radiofrequency transmission channel, and in particular the modulation step in which a binary word P i to be transmitted, with i G [ 0; q - 1 ], is associated with a particular sequence Cp of size N obtained by cyclic shift of a root sequence, the step of creating a first vector of size K =q*N in which the N elements Cp n ] are periodically interleaved, with n E [0; N - l] of the ? sequences Cp associated with the Q words to be transmitted, the step of obtaining a second vector of K elements by applying an Inverse Discrete Fourier Transform (IDFT) to said first vector, and the step of transmitting by a radiofrequency transmitter each element of the second vector on a particular OFDM subcarrier.

[0117] For this, the device 900 comprises a CCSK modulation unit 904 configured to modulate Q binary words in the form of sequences of N chips. For this, the modulation unit 904 can be implemented by computer program instructions configured to associate a binary word to be transmitted of size p bits with a particular sequence of chips of size N = 2P chips obtained by cyclic rotation of a particular root sequence of size N, so that a given binary word is always associated with a version of the root sequence obtained by the application of a particular cyclic shift applied to said root sequence. According to a particular embodiment, the modulation unit 904 uses a root sequence having a perfect autocorrelation function, for example, a Zadoff-Chu sequence, or an M-sequence.

[0118] The device 900 also comprises a parallelization unit 905 configured to position in a vector the K = q*N chips resulting from the modulation of the <7 binary words by the CCSK modulation unit 904. In a particular embodiment, the parallelization unit 905 is implemented by program instructions adapted to periodically interleave the sequences produced by the modulation unit. For this, the parallelization unit can arrange each of the sequences of N chips on different lines of a matrix of N columns and ? lines, so that the chips of the same index are aligned vertically, and concatenate in an output vector the elements of the matrix according to a column reading, the content of the columns of index 0 to Nl being added sequentially in said vector. The unit 905 is thus configured to carry out a mapping (or a distribution) of the K chips on K subcarriers.

[0119] According to a particular embodiment, the particular distribution of the N chips of a particular CCSK sequence on the K subcarriers defines a transmission group, the binary words to be transmitted belonging to Q distinct data packets, the binary words composing a particular data packet being transmitted within at least one particular transmission group on a plurality of successive OFDM symbols.

[0120] The device 900 also comprises a unit for calculating an inverse discrete Fourier transform IDFT 906. The IDFT calculation unit 906 is for example implemented by computer program instructions configured to take as a parameter the vector generated by the parallelization module 905 and generate in return a vector of the same size comprising the samples calculated by a discrete inverse Fourier transform IDFT. The instructions implement for example an IFFT (Inverse Fast Fourier Transform) algorithm. Thus, the vector generated by the parallelization unit, considered as a complex sequence in the frequency domain, is transformed into the time domain by the IDFT calculation unit.

[0121] The device 900 further comprises a serialization unit 907 adapted to generate an OFDM symbol capable of being transmitted to a receiver by radio frequency communication means 908. The serialization unit 907 is for example implemented by computer program instructions configured to convert the output of the inverse Fourier transformer 906 from a parallel data signal to a serial data signal. In a particular embodiment, the serialization unit may further be configured to add a cyclic prefix (CP) to the OFDM symbol. The sequence thus formed passes through a digital-to-analog converter to construct a baseband analog signal capable of being transmitted by the RF communication means.

[0122] The radiofrequency communication means 908 correspond for example to a WiFi®, 3G, 4G, 5G interface, or any other transmission means suitable for transmitting a signal on a plurality of subcarriers.

[0123] According to a particular embodiment, the transmission device 900 is integrated into a user equipment (UE), a connected object (loT), such as a sensor or a radio unit of a base station.

[0124] Figure 10 illustrates the architecture of a device 1000 adapted to implement the method of receiving binary words described above with reference to [Fig.4], according to a particular embodiment.

[0125] The device 1000 comprises a data processing module comprising a storage space 1001, for example a memory (MEM), a processing unit 1002, equipped for example with a microprocessor (PROC), and controlled by the computer program (PGR) 1003 whose instructions are configured to implement the reception method as described previously in relation to [Fig.4].

[0126] At initialization, the code instructions of the computer program 1003 are for example loaded into the memory 1001 before being executed by the processor of the processing unit 1002. The microprocessor of the processing unit 1002 implements, according to the instructions of the computer program 1003, the steps of the method for receiving <7 binary words, with ? > 1, transmitted on an OFDM transmission channel, each binary word Pj, with ie [0; q - 1], being associated for transmission with a particular sequence Cp. of size N obtained by cyclic rotation of the same root sequence, and in particular the steps of receiving an OFDM symbol on K = q*N subcarriers, obtaining a vector of K elements by applying a Discrete Fourier Transform to the received OFDM symbol, the vector comprising q particular Cp sequences of size N periodically interleaved, oversampling a root sequence of size N to rank K by regularly inserting nuisance intermediate terms between the N elements of the root sequence, and determining the received binary words by calculating a correlation between the oversampled root sequence and the obtained vector.

[0127] For this, in addition to the processing unit 1002 and the memory 1001, the receiving device 1000 comprises radiofrequency communication means 1004, for example a WiFi®, 3G, 4G, 5G receiver, or any other receiver suitable for receiving an OFDM signal. The communication means 1004 are configured to transpose the analog signal received by the RF receiver into baseband and extract the cyclic prefix. An analog-to-digital converter makes it possible to obtain a sequence of samples.

[0128] The device 1000 further comprises a parallelization module 1005 adapted to convert the sequence of samples received by the radio frequency receiver 1004 into a parallel data signal. The module 1005 is for example implemented by program instructions adapted to generate a vector comprising K chips received on K OFDM subcarriers by the RF receiver 1004.

[0129] The device 1000 also comprises a module 1006 for calculating a discrete Fourier transform (DFT). The module 1006 can be implemented by computer program instructions adapted to calculate a DFT from the vector of K samples generated by the parallelization module 1005 and produce a second vector of size K comprising q interleaved CCSK sequences whose chips are transformed into the frequency domain.

[0130] The device 1000 also comprises a demodulation unit 1007 adapted to demodulate the received CCSK sequences by calculating a cross-correlation with a particular root sequence. The module 1007 can be implemented by computer program instructions configured to obtain a root sequence of size N used to modulate the binary words during transmission (the root sequence is for example preconfigured, or transmitted prior to transmission), and oversample this root sequence to rank K.More specifically, the program instructions are configured to upsample the root sequence using a zero-padding technique so that it reaches a size K by inserting nuisance terms between the samples forming the initial sequence and performing a correlation calculation between the vector produced by the DFT module 1006 and each particular cyclic shift of the upsampled root to identify a binary word. transmitted according to the position of a correlation maximum.

[0131] In a particular embodiment, the demodulation module 1007 is configured to identify at least one transmission group in the vector generated by the DFT calculation module, a transmission group being defined by a particular distribution of the N chips of a CCSK sequence in the vector, the q CCSK sequences modulating binary words belonging to q distinct data packets, the binary words composing a particular data packet being transmitted within a particular transmission group on a plurality of successive OFDM symbols. In a particular embodiment, the associations between the data packets and the transmission groups are transmitted beforehand by any suitable means.

[0132] The device 1000 finally comprises a module 1008 for obtaining the transmitted binary words. The module 1008 is for example implemented by code instructions configured to deinterleave the correlation scores obtained by the module 1007 for each sequence, to calculate a maximum argument (argmax) making it possible to obtain the maximum correlation peak for each sequence, and to determine the binary word which corresponds to this maximum.

[0133] According to a particular embodiment, the receiving device is integrated into user equipment (UE), such as a mobile telephone, a connected object or a base station.

Claims

Claims

1. A method of transmitting 7 binary words, with q > 1, through an OFDM transmission channel, the method comprising: - A modulation step (200) in which a binary word Pi to be transmitted with ie [ 0; q - 1 ] is associated with a particular sequence Cp of size N obtained by cyclic shift of a root sequence, - A step (201) of creating a first vector of size K = q*N in which the N elements C [n] are periodically interlaced, with n E [0; TV - 1] of the ? sequences Cp. associated with the words to be transmitted, - A step (202) of obtaining a second vector of K elements by applying an Inverse Discrete Fourier Transform to said first vector, - A step (203) of transmission on K OFDM sub-carriers of an OFDM symbol representative of the K elements of the second vector.

2. The method of claim 1 wherein the root sequence has a perfect autocorrelation function.

3. Method according to any one of the preceding claims in which a particular distribution of the N elements composing a sequence on the K OFDM subcarriers defines a transmission group, the binary words belonging to the same particular data packet being transmitted within at least one same particular transmission group on a plurality of successive OFDM symbols.

4. Method according to any one of the preceding claims in which the binary words to be transmitted comprise at least a first word intended for a first user equipment and a second word intended for a second user equipment, the first and second words being respectively associated with a first and a second particular sequence respectively obtained by cyclic shifting of a first and a second root sequence of size TV, the first and second root sequences being weakly correlated with each other and respectively associated with the first and second user equipment.

5. Device for transmitting binary words through an OFDM transmission channel, with q > 1, the device comprising a radiofrequency transmitter (908) and a processor (902) coupled to a memory (901) in which are recorded instructions (903) adapted to configure the processor (902) and the transmitter (908) in order to implement the following steps: - A modulation step in which a binary word Pi, with i E [ 0; r / - 1 ] to be transmitted is associated with a particular sequence Cp. of size N obtained by cyclic shift of a root sequence, - A step of creating a first vector of size K = q*N in which elements Cp [n] are periodically interleaved, with n E [0; N - l] of the q sequences Cp. associated with the ? words to be transmitted, - A step of obtaining a second vector of K elements by applying an Inverse Discrete Fourier Transform to said first vector, - A transmission step on K OFDM subcarriers of an OFDM symbol representative of the K elements of the second vector.

6. Method for receiving < binary words, with q > 1, transmitted on an OFDM transmission channel, each binary word Pi, with i E [ 0; q - 1 ], being associated for transmission with a particular sequence Cp. of size N obtained by cyclic shift of a root sequence, the method comprising the following steps: - Reception (400) of an OFDM symbol on K = q*N subcarriers, - Obtaining (401) a vector of K elements by applying a Discrete Fourier Transform to the received OFDM symbol, the vector comprising? particular Cp sequences of size N interleaved periodically, - Calculation (402) of a cross-correlation between the obtained vector and a root sequence of size N over-sampled to rank K by regular insertion of intermediate terms between the N elements of the root sequence, Determination (403) of the binary words received according to the result of the correlation.

7. The method of claim 6 wherein the root sequence has a perfect autocorrelation function

8. A reception method according to any one of claims 6 to 7 such that it comprises a step of identifying at least a first and a second transmission group in a received OFDM symbol, a transmission group being defined by a particular distribution of the N elements of a sequence Cp. on the K OFDM subcarriers, the sequences defining ? transmission groups on a succession of OFDM symbols, the first and second transmission groups being respectively associated with a first and a second data packet on a plurality of successive OFDM symbols.

9. Reception method according to any one of claims 6 to 8 in which the Q binary words comprise at least a first word intended for a first user equipment and a second word intended for a second user equipment, the first and second words being respectively associated with a first and a second particular sequence respectively obtained by cyclic shift of a first and a second root sequence of size N, the first and second root sequences being weakly correlated with each other and respectively associated with the first and second user equipment, the method comprising a step of determining a sequence intended for a particular receiver by calculating a cross-correlation between a root sequence associated with said receiver oversampled to rank K.

10. Device for receiving binary words, with q > 1, transmitted on an OFDM transmission channel, each binary word Pi, with i E [Q, q - 1], being associated for transmission with a particular sequence Cp of size N obtained by cyclic rotation of the same root sequence, the device comprising a radiofrequency receiver (1004) and a processor (1002) coupled to a memory (1001) in which instructions (1003) are recorded, adapted to configure the processor (1002) and the receiver (1004) in order to implement the following steps: - Reception of an OFDM symbol on K = q*N subcarriers, - Obtaining a vector of K elements by applying a Discrete Fourier Transform to the received OFDM symbol, the vector comprising particular Cp sequences of size N interleaved periodically, - Calculation of a cross-correlation between the obtained vector and a root sequence of size N over-sampled to rank K by regular insertion of intermediate terms between the N elements of the root sequence, and - Determination of the received binary words by calculating a correlation between the oversampled root sequence and the obtained vector.

11. A communication system comprising a transmitting device according to claim 5 and a receiving device according to claim 10.

12. Computer program product comprising instructions adapted to the implementation of the steps of a transmission method according to any one of claims 1 to 4 and / or of a reception method according to any one of claims 6 to 9, when the program is executed by a processor.

13. A computer-readable recording medium on which is recorded a computer program comprising instructions for executing a transmission method according to any one of claims 1 to 4 and / or a reception method according to any one of claims 6 to 9.

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