Method and device for modulating with ZADOFF-CHU sequences

Abstract

The present invention relates to a method and device for modulating using Zadoff-Chu sequences. Each K-phase symbol to be transmitted is converted into a Zadoff-Chu sequence of a predetermined length N, a predetermined root r, and a frequency offset q that depends on the K-phase symbol. Various root values ​​can be used to separate different uplink and downlink communications and synchronous or asynchronous communications. The modulation device is implemented in the frequency domain. The present invention also relates to a demodulation method and device that enable the K-phase symbols thus transmitted to be restored. The demodulation device may be implemented in the time domain or the frequency domain.

Description

[Technical field]

[0001] The present invention relates generally to the field of digital telecommunications, and more particularly to digital telecommunications using spread spectrum techniques, and is particularly applicable to wireless communication systems and the Internet of Things (IoT). [Background technology]

[0002] The emergence of long-range, low-data-rate wireless technologies adapted to autonomous, low-power objects has recently enabled the development of the Internet of Things (IoT). Wireless communication systems in IoT networks must obey several constraints.

[0003] First, the receivers of the connected objects must be highly sensitive due to the small number of base stations deployed and therefore the large size of their coverage zones.

[0004] Secondly, the transmitters / receivers of the connected objects must be particularly simple and energy efficient to enable an autonomy duration of several years.

[0005] Finally, the wireless access system must be scalable, insofar as base stations (also called IoT gateways) must be able to serve a large number of connected objects.

[0006] The Internet of Things currently encompasses a wide variety of networks.

[0007] Some of them (LTE-M, NB-IoT) use technologies from the existing cellular infrastructure, but these often prove to be too complex or too energy consuming for general use.

[0008] Others rely on specific, even proprietary technologies such as LoRa, which uses Chirped Spread Spectrum (CSS) modulation, and SigFox, which uses Ultra Narrow Band (UNB) modulation, which are energy efficient and can achieve very low reception thresholds and therefore very wide coverage.

[0009] However, these particular technologies have limitations in terms of capacity, the number of users that can be served simultaneously in the same coverage zone, due to degradation of the signal-to-noise ratio plus interference affecting the communication. In particular, the large number of unresolved collisions between CSS signals in LoRa systems is a barrier to the deployment of large-scale LPWANs based on this technology, even when different spreading factors (SF) are used, and even to the deployment of networks by different operators. A description of these limitations can be found in particular in the paper by D. Croce et al. entitled "Impact of LoRa imperfect orthogonality: analysis of link-level performance", published in IEEE Communication Letters, Vol. 22, No. 4, April 2018, pp. 796-799.

[0010] It is therefore an object of the present invention to provide a modulation method (and associated modulator) that is particularly adapted to the constraints of IoT networks and that results in greater robustness against inter-communication interference. Another object of the present invention is to provide a corresponding demodulation method (and associated demodulator). Finally, a data transmission method is also provided that makes use of said modulation method to reduce interference between communications of various users. [Prior art documents] [Non-patent literature]

[0011] [Non-Patent Document 1] IEEE Communication Letters, Vol. 22, No. 4, April 2018, pp. 796-799, “Impact of LoRa imperfect orthogonality: analysis of link-level performance,” D. Croce et al. [Non-Patent Document 2] "Polyphase codes with good correlation properties," IEEE Trans. Inform. Theory, vol. IT-18, pp. 531-532, July 1972, DC Chu [Non-Patent Document 3] IEEE Trans. on Information Theory, vol. IT-25, no. 6, Nov. 79, pp. 720-724, "Bounds on crosscorrelation and autocorrelation of sequences," DV Sarwate [Non-Patent Document 4] "Preamble sequence generation", 3GPP TS36.211, section 5.7.2 Summary of the Invention [Means for solving the problem]

[0012] The invention is defined by a method for modulating a number of K-ary symbols to be transmitted over a transmission channel, each K-ary symbol being transformed into a Zadoff-Chu sequence associated with it, said Zadoff-Chu sequence being of length N, power root r and frequency offset q, where N is log 2 where N is a positive integer such that N≧K, r is an integer such that r>1 and is prime with respect to N, and q is a relative integer, and any two K-phase symbols among the plurality of K-phase symbols are associated with Zadoff-Chu sequences having different frequency offsets, and each K-phase symbol is a combination of two or more consecutive complex elements of the Zadoff-Chu sequence associated with the K-phase symbol. KThe signal is modulated in the form of a sequence of length 100 or more.

[0013] According to a first embodiment, the sequence of successive complex elements is made up of all the elements of said Zadoff-Chu sequence. The length of the Zadoff-Chu sequence may advantageously be chosen to be equal to a power of two.

[0014] According to the second embodiment, the length of the Zadoff-Chu sequence is 2 K , the sequence of successive complex elements is then multiplied by 2 K This is obtained by shortening the

[0015] Preferably, the length of the Zadoff-Chu sequence is chosen to be greater than the difference between the maximum frequency offset for modulating the K-ary symbols and the time spreading of the transmission channel, expressed in terms of the number of transmission periods of the elements of this sequence.

[0016] The real and imaginary parts of the elements of the sequence of successive complex elements of the Zadoff-Chu sequence associated with the K-ary symbol are modulated onto two orthogonal carriers, respectively, and the modulated signal so obtained is transmitted via a transmission channel.

[0017] Alternatively, a phase signal obtained as the phase of successive complex elements of a Zadoff-Chu sequence associated with a K-ary symbol is generated in baseband, the phase signal is transformed to the RF band, and the RF signal so obtained is transmitted via a transmission channel.

[0018] The present invention also relates to a method for transmitting data via multiple communications between various users, wherein the data to be transmitted are converted into K-ary symbols, the K-ary symbols of the various users are modulated by the present modulation method, and the radical roots of the Zadoff-Chu sequences used to modulate the K-ary symbols of two different users are chosen to be different and their lengths are chosen to be equal to a common length.

[0019] The radicals used to modulate the K-phase symbols of two different users are preferably chosen such that the absolute value of their difference is relatively prime to the common length.

[0020] The present invention further relates to a modulation device for modulating K-phase symbols to be transmitted via a transmission channel, said device comprising a binary-to-decimal converter for converting the K bits of each K-phase symbol into different integer values, a Zadoff-Chu sequence generator having a sequence length N, and a radical r such as r>1 and relatively prime to N, and a frequency offset q as input parameters, said generator forming, for each K-phase symbol, a sequence of length N, a radical r, and complex elements of a Zadoff-Chu sequence with a frequency offset equal to the integer value provided by said binary-to-decimal converter, 2 K sequences of the above length are generated.

[0021] The present invention further relates to a method for demodulating a signal representing a Zadoff-Chu sequence modulated with a frequency offset using the modulation method as defined above. The signal is baseband sampled to provide a sequence of complex samples, said sequence of complex samples is correlated with a reference Zadoff-Chu sequence by means of circular correlation to provide a circular correlation result, the time position of the correlation peak in the absolute value of the circular correlation result thus obtained is detected, and finally, the frequency offset

Number

[0022] The present invention also relates to a device for demodulating a signal representing a Zadoff-Chu sequence modulated with a frequency offset by means of the modulation method as defined above.

[0023] According to a first alternative, the signal is baseband sampled to obtain a first block of complex elements of size N, the demodulation device comprising a DFT module configured to perform a Discrete Fourier Transform of size N of the first block of complex elements to obtain a first block of complex frequency elements, a multiplier for term-wise multiplying the complex frequency elements of the first block by the complex frequency elements of a second block obtained by the complex conjugation of the Discrete Fourier Transform of a reference Zadoff-Chu sequence, and an absolute value calculator for calculating the absolute value of the multiplication result of the first and second complex frequency elements, the absolute values ​​so obtained being compared in a correlation peak detector to determine the frequency position of a cyclic correlation peak and a frequency offset corresponding to this frequency position.

number

[0024] According to a second alternative, the signal is baseband sampled to provide a first block of complex elements of size N, the demodulation device comprising a circular correlator of size N for circularly correlating said first block with a second block, said second block being composed of a reference Zadoff-Chu sequence, an absolute value calculator for calculating the absolute value of said circular correlation, and a frequency offset calculator for determining from said absolute value the time position of the circular correlation peak and a frequency offset corresponding to said time position.

number

[0025] The invention finally relates to a transmitter / receiver system comprising a modulation device as defined above and at least one demodulation device.

[0026] When the system comprises a plurality of modulation devices, each modulation device of said plurality of modulation devices advantageously corresponds to a different radical root value and / or the plurality of demodulation devices is according to the first or second alternative, each demodulation device of said plurality of demodulation devices corresponding to a different radical root value.

[0027] Further characteristics and advantages of the present invention will become apparent on reading the description of preferred embodiments of the invention with reference to the attached figures. [Brief description of the drawings]

[0028] [Figure 1] 1 shows a schematic representation of a modulation device according to an embodiment of the present invention; [Figure 2A] 1 is a diagram illustrating an example of a basic Zadoff-Chu sequence. [Figure 2B] FIG. 2 is a diagram illustrating an example of a modulated Zadoff-Chu sequence. [Figure 3A] FIG. 13 depicts a circulant correlation peak matrix for an example of the Zadoff-Chu sequence family. [Figure 3B] FIG. 13 depicts a circulant correlation peak matrix for an example of the Zadoff-Chu sequence family. [Figure 3C] FIG. 13 depicts a circulant correlation peak matrix for an example of the Zadoff-Chu sequence family. [Figure 4] 2A and 2B show schematic representations of transmission frames using a modulation method according to an embodiment of the present invention; [Diagram 5] FIG. 2 is a schematic representation of a sequence of pilot symbols multiplied by elements of a coverage code; [Figure 6] 1 shows a schematic representation of a demodulation device according to a first embodiment of the invention; [Figure 7] 3 shows a schematic representation of a demodulation device according to a second embodiment of the invention; [Figure 8] FIG. 2 illustrates a preamble detection and synchronization device that can be used with a demodulation device according to an embodiment of the present invention. [Figure 9] FIG. 1 schematically shows a first transmitter / receiver using a modulation device and a demodulation device according to an embodiment of the present invention. [Figure 10] FIG. 4 schematically shows a polar modulator usable in a second transmitter according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0029] The concept underlying the present invention is to perform modulation from K-phase to a signal by modulating the offset frequency of the Zadoff-Chu sequence, and separation between different synchronous communications or asynchronous communications, uplink communications or downlink communications is obtained by selecting different roots of unity.

[0030] It is first recalled that the roots of unity r, length N, and frequency offset Zadoff-Chu sequence are defined by a sequence of complex elements. [Equation] However, N is a positive integer, q is a relative integer, 0 < r < N, and r and N are relatively prime. An overview of the Zadoff-Chu sequence can be found in the paper by D.C. Chu entitled "Polyphase codes with good correlation properties", IEEE Trans. Inform. Theory, Vol. IT-18, pp. 531-532, July 1972.

[0031] When considering the phases of consecutive elements of the Zadoff-Chu sequence, it can be understood that they include a linear component as well as a quadratic component depending on the time index k. The linear component corresponds to the frequency offset [Equation] where f s is the frequency at which the elements of the sequence are generated, and the quadratic component is -rf over the length of the sequence sThis corresponds to a linear variation of the instantaneous frequency with a shift of

[0032] Zadoff-Chu (ZC) sequences have some very interesting properties for communication systems. First, they have constant amplitude (all elements of a ZC sequence belong to the unit circle), which gives rise to modulated signals with small PAPR. Second, the periodic autocorrelation of a ZC sequence is ideal in that it is zero for any non-zero offset (in other words, the periodic autocorrelation function of a ZC sequence is a Dirac comb with period N). Finally, the periodic cross-correlation of two ZC sequences of the same odd length N, and different roots r, u where |ru| is prime with respect to N, is such that its absolute value is

number

[0033] In the original method, the frequency offset of the Zadoff-Chu sequence is modulated to obtain K ≤ log 2 It is proposed to transmit a word of N bits. It should be noted that the frequency offset of a ZC sequence, especially when used for synchronization purposes, as is the case in the generation of preambles in LTE systems (see 3GPP TS 36.211, section 5.7.2, entitled "preamble sequence generation"), is generally understood in the prior art to be 0.

[0034] Figure 1 schematically shows a modulation device according to a first embodiment of the present invention.

[0035] The modulation device 100 includes a binary-to-decimal converter (BDC) 110 that converts a K-bit word α 0 ,..,α K-1 (where α 0 is the LSB and α K-1 is the MSB) to an integer value

Number

[0036] The words to be transmitted can, in some cases, be obtained from channel coding of information symbols after they are interleaved. Alternatively, the words to be transmitted can be pilot symbols for the transmission channel.

[0037] The words to be transmitted can also be obtained from Gray coding of information symbols. According to this coding, it is recalled that two consecutive binary words produce codewords that differ by only 1 bit.

[0038] The generator 120 further receives the square root r and length N of the sequence to be generated as input parameters. The parameters r and N can be provided by a transmission resource scheduler as described later.

[0039] The complex elements of the Zadoff-Chu sequence generated by the generator 120

Number

[0040] Note that in all cases, the complex element sequence at the output of the modulator is a Zadoff-Chu sequence as defined above. Specifically, no phase correction needs to be performed at the output of generator 120, and thus the structure of the modulator is particularly simple.

[0041] As understood above, the integers r and N are chosen to be relatively prime to each other.

[0042] According to a first alternative form, in order to respect this constraint, the number N is chosen to be a prime number. In the latter case, the interference between two communications with the same length N but different roots of unity r, u is

Number

[0043] However, preferably, for reasons of simplicity of implementation, the sequence generated by the generator

Number

[0044] According to a second advantageous alternative form, in order to simplify the previously shown implementation, the number N is chosen to be a power of 2, for example, N = 2^ K and the root of unity r is then odd, so that r and N are relatively prime to each other.

[0045] Furthermore, the root of unity of the ZC sequence is chosen to be greater than 1 (r>1). In this case, note that the phase fluctuation with respect to the unmodulated ZC sequence (which is the one with q = 0) presents at least one aliasing between consecutive elements of the sequence, in other words, the instantaneous frequency has at least one discontinuity over the length of the sequence. In fact, the phase rotation between two consecutive elements is

number

[0046] Therefore, for r>1, the phase rotation at the end of the sequence can exceed 2π.

[0047] Equivalently, the instantaneous frequency is

number

[0048] As an example, successive elements of an unmodulated Zadoff-Chu sequence are shown in FIG. 2A, and successive elements of the same sequence modulated by a frequency offset are shown in FIG. 2B.

[0049] In the illustrated case, the length of the sequence is N = 17 and its radical root is r = 4. Given the relationship (3-1), notice that initial phase aliasing occurs between the 5th and 6th elements, and that phase rotations between successive subsequent elements lead to even more aliasing.

[0050] Similarly, in Figure 2B, the same sequence is considered, this time modulated with an offset q = 1. Note that the first phase aliasing now occurs as early as the transition between the 4th and 5th elements.

[0051] It will therefore be appreciated that for the same basic ZC sequence, a different distribution of the elements of the modulated sequence is obtained with different frequency offsets.

[0052] FIG. 3A illustrates diagrammatically the matrix of cyclic correlation maxima for unmodulated Zadoff-Chu sequences (q=0) of length N=521 (a prime number) and respective roots r=2,...,520. In other words,

number

number

number

number

number

number

number

[0053] Figure 3B shows a schematic representation of the matrix of cyclic correlation maxima for unmodulated ZC sequences (q = 0) of length N = 512 and respective roots r = 2,...,511. We note that some of these sequences are not strictly speaking ZC sequences, unless r and N are relatively prime.

[0054] The diagonal of the matrix always consists of the maximum values ​​(N), but the maximum value of the circular cross-correlation is the pair of sequences

number

number

[0055] Figure 3C illustrates diagrammatically the matrix of cyclic correlation maxima for an unmodulated ZC sequence (q = 0) of length equal to the prime number N = 521, but shortened to 512 elements. It can be seen that the autocorrelation and cyclic cross-correlation performances are only slightly degraded with respect to those shown in Figure 3A.

[0056] Generally, power of 2 P=2 K If it is desired to select a sequence of length equal to N, then a ZC sequence whose length N is equal to the smallest prime number greater than P may be selected and shortened to P elements. Thus, the performance loss is minimal in terms of both rate and level of cross-correlation.

[0057] Thus, for example, for sequences of length 128, 256, 512, 1024, 2048, 4096, ZC sequences with lengths 131, 257, 521, 1031, 2053, 4099 are chosen, respectively.

[0058] The length N of the sequence to be generated may be chosen depending on various criteria, such as the SNR (signal-to-noise ratio), the BER (bit error rate), the required communication rate, and even the time spread of the transmission channel, as will be discussed later.

[0059] More precisely, the spreading factor (SF) is just N / K, and choosing a large value of N allows the BER to be reduced for the same SNR value or to follow the BER rate for degraded SNR conditions. Similarly, a small value of N allows the transmission bandwidth to be reduced and the communication rate to be increased.

[0060] Then, for a given value of N, different radical roots are chosen for the communications whose interference is desired to be reduced. For example, two different radical roots can be assigned to two communications on a downlink channel emanating from the same base station to two connected objects or two terminals located within the coverage of this base station, regardless of whether these two communications are synchronous or not. Furthermore, two different radical roots can be assigned to two different communications on a downlink channel to one and the same receiver.

[0061] Similarly, for an uplink channel, two connected objects or two terminals that may interfere may be assigned different radical roots.

[0062] The allocation of sequence length / radical roots may be done dynamically by the scheduler. In some cases, the dynamic scheduling may be implicit. For example, if two uplink communications interfere, the base station may inform the concerned connected objects of this situation, and at least one of them may switch to another subsequent radical root in a given radical root sequence, known to both the connected object and the base station. Those skilled in the art may provide various alternative forms of scheduling between neighboring base stations according to a centralized or distributed mode without departing from the scope of the present invention.

[0063] Whatever the embodiment, the transmission of a packet of K-phase symbols, ie words of K bits, is usually carried out by means of a transmission frame whose structure is shown in FIG.

[0064] The transmission frame comprises a preamble 410 and a payload 420. The preamble contains pilot symbols to allow a receiver to detect and be synchronized to the start of the frame.

[0065] For example, pilot symbols are assigned to a dedicated frequency offset q 0 , for example, q 0is associated with =0. In other words, in this case, the preamble of the transmission frame consists only of an unmodulated ZC sequence.

[0066] Pilot symbols are detected by a receiver using a correlator that performs a correlation (i.e., continuous correlation) on the received signal. For example, a predetermined number M of repetitions of a pilot symbol can be an indicator of the start of a frame. To facilitate the synchronization and detection of the start of a frame (and further, a sequence of frames), these pilot symbols can be multiplied by elements of a coverage code with good aperiodic correlation characteristics, such as elements of a Barker code. Receiver synchronization by preamble detection will be described later with respect to FIG. 8. Receiver resynchronization can be performed periodically during the reception time of a sequence of frames in all frames, or generally, every J frames, where J is a predetermined integer.

[0067] FIG. 5 shows a series of pilot symbols of a frame preamble multiplied by M elements of a coverage code, denoted as CC(1),..., CC(M), respectively. In other words, the first ZC sequence corresponding to the first pilot symbol is multiplied by CC(1), the second ZC sequence corresponding to the second pilot symbol is multiplied by CC(2), and so on until the last ZC sequence is multiplied by CC(M).

[0068] For example, the elements of a Barker code of length M = 11 are +1, +1, +1, -1, -1, -1, +1, -1, -1, +1, -1 given by.

[0069] Upon reception, the individual pilot symbols (which are ZC sequences corresponding to these pilot symbols) are multiplied by the same coverage code element before or after continuous correlation. When the coverage code element is ideally aligned with the pilot symbol, the correlation result has a maximum value.

[0070] The payload of a transmission frame consists of a ZC sequence modulated by the K-ary word to be transmitted. The payload may also include a control header and a ZC sequence modulated by a CRC symbol.

[0071] Although the K-phase symbols are transmitted consecutively, and therefore the ZC sequences modulated by these symbols also follow each other at the transmitter, the transmission channel may cause intersymbol interference if it is a multipath channel. In other words, a replica of the ZC sequence modulated by the first symbol may be superimposed with the ZC sequence modulated by the second symbol at the receiver.

[0072] To reduce inter-symbol interference, N-Δ>q max (5) Advantageously, a sequence of length N is chosen such that: max is the maximum frequency offset for modulating the K-phase symbol, and both are expressed in terms of the number of element generation periods of the ZC sequence. In fact, the overlap between consecutive symbols due to the transmission channel spreading is the duration of the redundant part of the ZC sequence (N-2 K ), the symbol in question can be recovered without loss of information.

[0073] FIG. 6 illustrates a schematic representation of a demodulation device according to a first embodiment of the invention.

[0074] This first embodiment corresponds to the implementation of a cyclic correlator in the frequency domain.

[0075] The demodulator receives a signal corresponding to a frame of symbols as described above, which is a series of ZC sequences obtained by modulating these symbols.

[0076] The demodulator 600 operates at a frequency f sThe signal processing circuit 600 includes an FFT module 610 for performing an FFT of size N (equal to the length of the ZC sequence) on the received signal previously sampled (by two orthogonal clocks) at . The result of the FFT is a block of N complex values.

[0077] Detection of the start of the frame and the ZC sequence is achieved using a series of pilot symbols in the preamble. It is assumed that the receiver has previously synchronized itself by preamble detection, as will be explained later.

[0078] The demodulator includes a local generator 605 of a reference ZC sequence, identical to the generator present in the modulator of Figure 3 or Figure 4. The local generator determines the length N of the ZC sequence, the power root r used for communication, and, if applicable, the frequency offset q dedicated to the pilot symbols. 0 The root r used for communication may be provided by a preamble detector, described later.

[0079] A discrete Fourier transform (DFT) module 615 enables a DFT of size N to be performed on a block of elements of the ZC sequence provided by the local generator 605. When N is a power of 2, the DFT is performed using an FFT.

[0080] The blocks of N complex values ​​so obtained in the frequency domain are conjugated in a conjugate module 625 and then multiplied term-wise in 620 with the blocks of N complex values ​​from the DFT module 610 .

[0081] The absolute values ​​of these N complex values ​​are calculated in 630 and compared by the correlation peak detector 640. In other words, the peak detector 640 determines the frequency offset in the received block that corresponds to the complex value with the largest absolute value.

[0082] The cyclic correlation peak of two ZC sequences of the same length N, same power root r, and frequency offsets q and l has a value

number

number

[0083] This frequency offset is finally converted into a K-bit word in a decimal-to-binary converter 650 (denoted by a bit demapper in the diagram).

number

[0084] It should be noted that the generator 605 can generate reference sequences for all possible values ​​of r and N, if applicable.

[0085] The reference sequence can be any ZC sequence of the same length and of the same radical root as the ZC sequence transmitted by the transmitter, as long as the frequency offset can be determined relative to the frequency offset used to generate the reference frequency. Thus, for example, the reference sequence can be an unmodulated ZC sequence, or a pilot symbol (hence, with an offset q 0 ) may be a ZC sequence corresponding to

[0086] In all cases, the set consisting of the reference sequence generator 605, the DFT module 615, and the conjugate module 625 can be replaced by a simple memory that is addressed by the length N, the power root r, and, if applicable, the reference frequency offset. The value stored in the memory at this address is simply

number

number

number

number

[0087] FIG. 7 illustrates a schematic representation of a demodulation device according to a second embodiment of the invention.

[0088] This second embodiment corresponds to the implementation of a cyclic correlator in the time domain.

[0089] The signal received by the demodulator 700 is modulated at the frequency f s (by two quadrature clocks). The result is a block of N complex values. This block is provided to a circular correlator 710.

[0090] The cyclic correlator 710 additionally receives N consecutive elements of a reference ZC sequence generated by a local generator 705. This local generator is identical to the generator present in the modulator of Fig. 1. It determines the length N of the ZC sequence, the power root r used for communication and, if applicable, a frequency offset q dedicated to the pilot symbols. 0 It is parameterized by

[0091] The elements of the ZC reference sequence are provided to a cyclic correlator 710. The cyclic correlation results (corresponding to different delay times in the time domain) are absolute values ​​calculated in 730 and the maximum absolute value in the block is determined by a correlation peak detector 740. The peak detector 740 determines the positions of the cyclic correlation peaks between the ZC reference sequence and the ZC sequence received by the demodulator and estimates the corresponding time offsets therefrom.

[0092] Given the definition of a ZC sequence, it can be shown that a cyclic permutation of a ZC sequence with a time offset results in a frequency offset.

[0093] If N is an odd number,

number

number

[0094] Similarly, if N is even,

number

number

[0095] Since detection is performed on the absolute value of the circular correlation result, (period T s =1 / f s The time offset q' (expressed as the number of

number

[0096] A decimal-to-binary converter 750 (referred to in the figure as a bit demapper) converts this offset value into a binary number

number

[0097] It should be noted that generator 705 can generate reference sequences for all possible values ​​of r and N, if applicable.

[0098] In either case, as before, the set consisting of the reference sequence generator 705 can be replaced by a simple memory addressed by the length N, the power root r, and the reference time (or frequency) offset. The value stored in the memory at this address is simply

number

number

[0099] FIG. 8 represents diagrammatically a preamble detection synchronization device which may be used together with a demodulation device according to the invention.

[0100] More precisely, this synchronization device makes it possible to provide the demodulator, on the one hand, with the root r used for the communication and, on the other hand, with the instant corresponding to the start of the frame and even of the payload.

[0101] The continuous correlator 820 receives at a first input a stream of (complex) samples of the received signal and at a second input a sequence of samples corresponding to a replica of the preamble.

[0102] A copy of the preamble is formed by module 810 from the ZC sequence generated by local generator 805. Local generator 805 is identical to generator 605 (or 705) described previously, and the same alternatives apply. Preamble formation module 810 concatenates the ZC sequences of the pilot symbols of the preamble. For example, if the preamble consists of M repeated identical pilot symbols, module 810 repeats the ZC sequence of the pilot symbols M times. Furthermore, if the preamble uses a Barker cover sequence, module 810 multiplies each ZC sequence with the corresponding code element.

[0103] Naturally, the generator 805 and the module 810 also determine the frequency offset q corresponding to the pilot symbol. 0 , the power root r, and the length of the sequence N, as well as the number of repetitions M of the pilot symbols.

[0104] The correlator 820 performs successive correlations of the sequence of samples of the received signal with the sequence of samples of the preamble provided by the module 810. In other words, for each period T s At , the correlator calculates the correlation result of the MN preamble samples with the last MN signal samples.

[0105] The absolute values ​​of these correlation results are calculated in 830 and correlation peak detection is performed in detector 840. In practice, the detector compares the absolute values ​​provided by 830 with a predefined threshold smaller than N.

[0106] The detector provides the time location of the correlation peak, which indicates the start of the preamble and therefore the frame. The demodulator can then acquire and track the start of the payload in order to demodulate the information symbols.

[0107] Furthermore, if the radical root r is unknown to the receiver, the synchronization device can perform a sequential or parallel search for preambles generated from radical root values ​​that belong to the set of possible radical roots, or even a subset of the latter (e.g., if the set of radical roots is hierarchical and the receiver has prior knowledge of the tree branch in which the radical root is located). A sequential search will yield a rate Rf for the R radical roots to be tested. s It is intended that the correlation is performed with . By parallel search, the correlation is performed in parallel for R branches, each branch being associated with one of the R radical roots to be tested. In either case, the correlation result is processed by a detector 840. The detector then determines both the radical root value (which provides the correlation peak) and the start of the preamble (from the position of this peak).

[0108] FIG. 9 illustrates a schematic representation of a first transmitter / receiver using a modulating device and a demodulating device according to an embodiment of the present invention.

[0109] The transmitter / receiver 900 may be adapted for example to a base station or a connected object.

[0110] The transmitter / receiver 900 comprises a modulation device 910 according to an embodiment as described with respect to Fig. 1. The modulation device converts the K-phase symbols to be transmitted into a ZC sequence, the K-phase symbols being converted to a frequency offset of the ZC sequence. The complex elements of the ZC sequence are modulated by the modulation device at a rate f s The real parts of these complex elements are provided to an in-phase channel and the imaginary parts to a quadrature channel, each channel typically comprising a low-pass filter 920 and a baseband amplifier 930. The I and Q signals of these channels are respectively mixed with two quadrature carrier signals in a quadrature modulator 940. The carrier signals are generated by a frequency stabilized sinusoidal oscillator 950. The RF signal at the output of the quadrature modulator is amplified in a power amplifier 960 and then transmitted by an antenna 980 via a duplexer 970.

[0111] On receive, the antenna signal received via duplexer 970 is amplified by low noise amplifier (LNA) 965 and then quadrature demodulated by quadrature mixer 945. The I and Q signals at the output of the mixer are amplified by amplifier 935, filtered by low pass filter 925 and then mixed at a rate f s The complex samples so obtained are provided to a demodulation device 915 according to a first embodiment as described with reference to FIG. 6 or according to a second embodiment as described with reference to FIG.

[0112] The quadrature modulator 940 in the direct conversion transmitter / receiver of Figure 9 uses the real and imaginary parts of the elements of the ZC sequence to be transmitted. A polar modulator rather than a quadrature modulator may be used as described below.

[0113] FIG. 10 illustrates a schematic representation of a polar modulator usable within a second transmitter according to an embodiment of the present invention.

[0114] Generally, in a polar modulator, a complex symbol to be transmitted is converted in a polar baseband converter 1020 into an envelope signal (absolute value of the complex elements) and a phase signal (instantaneous phase of the complex elements) representing the baseband signal. s(t)=A(t)cos(2πf 0 t+φ(t)) (8)

[0115] where A(t) is the envelope signal, cos(2πf 0 t+φ(t)) is the phase signal. The phase signal is then converted to the RF band by a mixer 1050 and amplified by a power amplifier 1060. The envelope signal is delayed in 1025 to compensate for differences in propagation time that may exist between the amplitude and phase branches, and the amplitude of the RF signal is modulated using an envelope modulator 1040 by controlling the gain of the power amplifier. The so modulated RF signal is then transmitted by an antenna 1070.

[0116] In the case of the problem of transmitting the complex elements of a ZC sequence as generated by modulator 1010, as in the present invention, it is understood that the generation of an envelope signal is unnecessary as long as the absolute values ​​of these elements are constant and equal to 1. The delay 1025 and envelope modulator 1040 may simply be removed and the gain of the power amplifier is set to a setpoint value.

[0117] Symmetrically, those skilled in the art will appreciate that a polar demodulator may be implemented at the receiver.

[0118] RF transmitter / receiver architectures other than those shown in Figures 9 and 10 may further be contemplated without departing from the scope of the present invention.

[0119] Of course, different basic modulation / demodulation devices may be provided in the same transmitter / receiver in order to separate the communications between different users. As previously indicated, these modulation / demodulation devices then use different radical root values. Advantageously, for a ZC sequence length equal to N, |r p -r p' |(∀p, p') is prime to N, so that the root value r p (p=1,..,P) is chosen. [Explanation of symbols]

[0120] 100 Modulation Devices 110 Binary to Decimal Converter 120 Zadoff-Chu sequence generator 410 Preamble 420 Payload 600 Demodulator 605 Local Generator 605 Reference sequence generator 610 FFT module, DFT module 615 Discrete Fourier Transform Module 620 Multiplier 625 Conjugate Module 630 Absolute Value Calculator 640 Correlation Peak Detector 650 Decimal to Binary Converter 700 Demodulator 705 Local Generator 710 Circular Correlator 730 Absolute Value Calculator 740 Correlation Peak Detector 750 Decimal to Binary Converter 805 Local Generator 810 Preamble Forming Module 820 Continuous Correlator 840 Detector 900 Transmitter / Receiver 910 Modulation Devices 915 Demodulation Device 920, 925 Low Pass Filter 930 Baseband Amplifier 935 Amplifier 940 Quadrature Modulator 945 Quadrature Mixer 950 Sine Wave Oscillator 960 Power Amplifier 965 Low Noise Amplifier 970 Duplexer 980 Antenna 1010 Modulator 1020 Polar Baseband Converter 1025 Delay 1040 Envelope Modulator 1050 Mixer 1060 Power Amplifier 1070 Antenna

Claims

1. 1. A method for modulating a plurality of K-bit words to be transmitted over a transmission channel, comprising the steps of: converting each K-bit word into a Zadoff-Chu sequence associated with said K-bit word, said Zadoff-Chu sequence being of length N, power root r, and frequency offset q, where N is log 2 wherein N≧K, r is an integer such that r>1 and is prime with respect to N, and q is an integer; and wherein any two K-bit words among the plurality of K-bit words are associated with Zadoff-Chu sequences having different frequency offsets, and each K-bit word is a combination of two or more of consecutive complex elements of the Zadoff-Chu sequences associated with the K-bit word. K A modulation method according to claim 1, characterized in that the signal is modulated in the form of a sequence of the above length.

2. 2. A modulation method according to claim 1, characterized in that said sequence of successive complex elements consists of all complex elements of said Zadoff-Chu sequence.

3. 3. A modulation method according to claim 2, characterized in that the length of the Zadoff-Chu sequence is equal to a power of two.

4. The length of the Zadoff-Chu sequence is 2 K and said sequence of successive complex elements is chosen to be equal to the smallest prime number greater than 2 K 2. A method of modulation according to claim 1, characterized in that the modulation is obtained by shortening the signal to a factor of 1.

5. 5. Modulation method according to claim 1, characterized in that the length of the Zadoff-Chu sequence is chosen to be greater than the difference between the maximum frequency offset for modulating the K-bit word, expressed in the number of transmission periods of a complex element of the sequence, and the time spreading of the transmission channel.

6. 6. The modulation method according to claim 1, characterized in that the real and imaginary parts of the complex elements of the sequence of successive complex elements of a Zadoff-Chu sequence associated to a K-bit word are modulated onto two orthogonal carriers, respectively, and the modulated signal thus obtained is transmitted via the transmission channel.

7. 6. A modulation method according to claim 1, characterized in that phase signals obtained as the phases of successive complex elements of the Zadoff-Chu sequence associated to a K-bit word are generated in baseband, said phase signals are transformed into an RF band and the RF signal so obtained is transmitted via the transmission channel.

8. 8. A method for transmitting data via multiple communications between various users, characterized in that the data to be transmitted are converted into K-bit words, the K-bit words of the various users are modulated by a modulation method according to one of claims 1 to 7, the radical roots of the Zadoff-Chu sequences used to modulate the K-bit words of two different users are chosen to be different and the length of the Zadoff-Chu sequences is chosen to be equal to a common length.

9. 9. Data transmission method according to claim 8, characterized in that the radical roots used to modulate the K-bit words of two different users are chosen such that the absolute value of their difference is prime with respect to the common length.

10. A modulation device for modulating a number of K-bit words to be transmitted over a transmission channel, said modulation device comprising: a binary-to-decimal converter (110) for converting the K-bit words into different integer values; and a Zadoff-Chu sequence generator (120) having as input parameters a sequence length N and a radical root r, such as an integer r>1 and prime with respect to N, and a frequency offset q, where q is an integer, said Zadoff-Chu sequence generator being formed for each K-bit word by a complex element of a Zadoff-Chu sequence of length N, radical root r, and frequency offset equal to said integer value provided by said binary-to-decimal converter. K 1. A modulation device comprising: a modulation circuit for generating a sequence having a length of at least one K-bit word, the Zadoff-Chu sequence generator being configured such that any two K-bit words of the plurality of K-bit words are associated with Zadoff-Chu sequences having different frequency offsets.

11. A method for demodulating a signal representative of a Zadoff-Chu sequence modulated with a frequency offset by a modulation method according to one of claims 1 to 7, comprising the steps of: baseband sampling said signal to obtain a sequence of complex samples; correlating said sequence of complex samples with a reference Zadoff-Chu sequence by a cyclic correlation (710) to obtain a cyclic correlation result; detecting (740) a time position of a correlation peak in the absolute value of the cyclic correlation result thus obtained; and detecting a frequency offset corresponding to this time position. [0010] is estimated therefrom, and said frequency offset is then converted into a K-bit word by a decimal-to-binary converter (750).

12. 8. A device for demodulating a signal representative of a Zadoff-Chu sequence modulated with a frequency offset by a modulation method according to claim 1, wherein the signal is baseband sampled to obtain a first block of complex elements of size N, the device comprising: a DFT module (610) configured to perform a Discrete Fourier Transform of size N of the first block of complex elements to obtain a first block of complex frequency elements; a multiplier (620) for term-wise multiplying the complex frequency elements of the first block of complex frequency elements with the complex frequency elements of a second block obtained by complex conjugation of a Discrete Fourier Transform of a reference Zadoff-Chu sequence; and an absolute value calculator (630) for calculating the absolute value of the multiplication result of the complex frequency elements of the first block of complex frequency elements with the complex frequency elements of the second block of complex frequency elements, the absolute values ​​so obtained being compared in a correlation peak detector (640) to determine a frequency position of a cyclic correlation peak and the frequency offset corresponding to that frequency position. [0025] and wherein the frequency offset is estimated and provided to a decimal-to-binary converter (650) to generate a K-bit word.

13. 8. A device for demodulating a signal representative of a Zadoff-Chu sequence modulated with a frequency offset by a modulation method according to one of claims 1 to 7, said signal being baseband sampled to provide a first block of complex elements of size N, said device comprising: a circular correlator (710) of size N for circularly correlating said first block with a second block, said second block consisting of a reference Zadoff-Chu sequence; an absolute value calculator (730) for calculating the absolute value of the circular correlation; and a frequency offset calculator (740) for determining the time position of a circular correlation peak from said absolute value and for calculating a frequency offset corresponding to said time position. [0030] and a correlation peak detector (740) for estimating therefrom a frequency offset, wherein the frequency offset is provided to a decimal-to-binary converter to generate a K-bit word.

14. Transmitter / receiver, characterized in that it comprises at least one modulation device according to claim 10 and at least one demodulation device according to one of claims 12 or 13.

15. The transmitter / receiver according to claim 14, characterized in that it comprises a plurality of modulation devices according to claim 10, each modulation device of the plurality of modulation devices corresponding to a different radical root value, and / or a plurality of demodulation devices according to one of claims 12 or 13, each demodulation device of the plurality of demodulation devices corresponding to a different radical root value.

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