RU2803188C2 - Method and device for modulation with zadoff-chu sequences - Google Patents

Abstract

FIELD: data transmission.

SUBSTANCE: present invention relates to modulation using Zadoff-Chu sequences. The technical result is achieved by the fact that each K character to be transmitted is converted into a Zadoff-Chu sequence of a predetermined length N and with a predetermined root r, and the frequency shift q which depends on the K character. Different root values may be used to separate individual uplink and downlink transmissions and synchronous or asynchronous transmissions.

EFFECT: increase of resistance to interference during data transmission.

19 cl, 13 dwg

Description

Field of technology

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

State of the art

The emergence of ubiquitous, low-bandwidth technologies tailored for off-grid, low-power applications has recently expanded the development of the Internet of Things (IoT). IoT network wireless communication systems must obey several restrictions.

First, the receiver of the attached site must have high sensitivity due to the small number of deployed base stations and, therefore, the large size of their coverage areas.

Secondly, the transmitter/receiver of the connected object must, in particular, be simple and energy efficient to ensure autonomy for several years.

Finally, radio access systems must be scalable because the base station (also known as an IoT gateway) must be able to serve a large number of connected entities.

The Internet of Things now spans a wide variety of networks.

Some of them (LTE-M, NB-IoT) use technologies from existing cellular infrastructures. However, they often turn out to be too complex or too energy-intensive for common applications.

Others rely on specific or even proprietary technologies, such as LoRa, which uses linear spread-spectrum (CSS) frequency modulation, and SigFox, which uses ultra-narrowband (UNB) modulation. These technologies are energy efficient and can achieve very low reception thresholds and therefore very wide coverage.

However, these particular technologies have limitations in terms of capacity, i.e. the number of users that can be served simultaneously in the same coverage area due to signal-to-noise ratio degradation plus interference affecting data transmissions. In particular, the large number of unresolved collisions between CSS signals in a LoRa system, even when different spectral spreading factors (SFs) are used, is a barrier to the deployment of large LPWANs based on this technology, or even to the deployment of networks by different operators. A discussion of these limitations will be found particularly in the paper by D. Kroc et al. entitled "Impact of LoRa imperfect orthogonality: analysis of link-level performance", published in IEEE Communication Letters, Vol. 22 No. 4, April 2018. ss. 796-799.

The purpose of the present invention is therefore to provide a modulation method (and associated modulator) that, in particular, adapts to the limitations of IoT networks and provides greater robustness to data transmission interference. Another object of the present invention is to provide a corresponding demodulation method (and associated demodulator). Finally, a data transmission method is also presented that takes advantage of the present modulation method to reduce interference between data transmissions of different users.

The essence of the invention

The present invention is defined by a method for modulating a plurality of K-ary symbols to be transmitted over a transmission channel, each K-ary symbol being mapped to a Zadoff-Chu sequence associated with that symbol, said Zadoff-Chu sequence having length N, root r, and frequency shift q, where N is a positive integer, so log ₂ N ≥ K, r is an integer, so r > 1 and a prime number with respect to N, q is a relative integer, and any of two K-ary symbols of said set associated with Zadov-Chu sequences having different frequency shifts, each K-ary symbol is modulated in the form of a sequence, with a length greater than or equal to 2 ^K , of consecutive complex elements of the Zadov-Chu sequence associated with said K-ary symbol.

According to the first embodiment, the sequence of sequential complex elements consists of all elements of the mentioned Zadov-Chu sequence. The length of the Zadov-Chu sequence may advantageously be chosen to be a power of 2.

According to a second embodiment, the length of said Zadoff-Chu sequence will be chosen to be the smallest prime number greater than 2 ^K , and a sequence of consecutive complex elements is then obtained by truncating said Zadoff-Chu sequence with 2 ^K elements.

Preferably, the length of the Zadov-Chu sequence is selected to be greater than the difference between the maximum frequency offset to modulate the K-ary symbols and the transmission channel time spread, expressed in the number of transmission periods of elements of this sequence.

The real and imaginary parts of the elements of the sequence of successive complex elements of the Zadov-Chu sequence associated with the K-ary symbol respectively modulate the two quadrature carriers, and the modulated signal thus obtained is transmitted over the transmission channel.

Alternatively, a phase signal is generated in a baseband obtained as the phase of successive complex elements of the Zadoff-Chu sequence associated with a K-ary symbol, and the phase signal is converted into an RF frequency band, the RF signal thus obtained is transmitted over a transmission channel .

The invention also relates to a method for transmitting data over multiple communication channels to or from different users, wherein the data to be transmitted is converted into K-ary symbols. 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 the total length.

The roots used to modulate the K-ary symbols of two different users are advantageously chosen such that the absolute value of their difference is a prime number with respect to said conventional length.

The invention further relates to a modulation device for modulating K-ary symbols to be transmitted over a transmission channel, said device comprising a binary-to-decimal converter that converts K bits of each K-ary symbol into a separate integer value, and a Zadoff-Chu sequence generator having As input parameters, the length N of the sequence and the root r, so that r is an integer r, with r > 1 and prime with respect to N, as well as a frequency shift, q, the said generator generates, for each K-ary symbol, a sequence of length greater than or equal to 2 ^K , formed by complex elements of the Zadov-Chu sequence with length N, root r and frequency offset equal to the integer value provided by said binary-to-decimal converter.

The invention further relates to a method for demodulating a signal representing a Zadoff-Chu sequence modulated by a frequency shift using the modulation method as defined above. Said signal is sampled in baseband to provide a sequence of complex samples, said sequence of complex samples is correlated by cyclic correlation with a reference Zadov-Chu sequence to provide a cyclic correlation result, and a time position of the correlation peak in the absolute value of the cyclic correlation result obtained thereby is detected. way, and finally the frequency shift , corresponding to this time position, is derived from it, and said frequency offset is then converted by a decimal-binary converter to a K-ary symbol.

The invention also relates to an apparatus for demodulating a signal representing a Zadoff-Chu sequence frequency shifted modulated by a modulation method as defined above.

According to a first alternative, said signal is sampled in baseband to provide a first block of complex elements of size N, and said demodulation device comprises a DFT module configured to perform a discrete Fourier transform of size N on the first block of complex elements to provide a first block of complex elements. frequency elements, a multiplier for multiplying term-by-term complex frequency elements of the first block with complex frequency elements of the second block obtained by complex conjugation of the discrete Fourier transform for the Zadov-Chu reference sequence, an absolute value calculation module for calculating the absolute value of the results of multiplying the first and second complex frequency elements, the absolute values obtained in this way are compared in the correlation peak detector to determine the frequency position of the cyclic correlation peak, the frequency shift, , corresponding to this frequency position, is derived from it and provided to the decimal-binary converter to generate the K-ary symbol.

According to a second alternative, said signal is baseband sampled to provide a first block of complex elements of size N, and the demodulation apparatus comprises a cyclic correlator of size N for cyclically correlating said first block with a second block, wherein said second block consists of a reference sequence of Chu, an absolute value calculation module calculating the absolute value of said cyclic correlation, and a correlation peak detector determining, from this absolute value, the time position of the cyclic correlation peak and outputting a frequency shift, , corresponding to this time position, from it, wherein said frequency offset is then provided to the decimal-binary converter to generate the K-ary symbol.

Finally, the invention relates to a transmitter/receiver system comprising a modulation device and at least one demodulation device as defined above.

When the system contains a plurality of modulation devices, each modulation device of the plurality preferably corresponds to a different root value, and/or a plurality of demodulation devices according to the first or second alternative, each demodulation device of the plurality corresponds to a different root value.

Brief description of drawings

Additional features and advantages of the invention will become apparent upon reading the preferred embodiment of the invention, which is described with reference to the accompanying drawings, of which:

Fig. 1 schematically represents a modulation device according to one embodiment of the invention;

Fig. 2A and FIG. 2B represent an example of a basic Zadov-Chu sequence and an example of a modulated Zadov-Chu sequence, respectively;

Fig. 3A, FIG. 3B and FIG. 3C represent a cyclic correlation peak matrix for three examples of Zadow-Chu sequence families, respectively;

Fig. 4 schematically represents a transmission frame using a modulation method according to one embodiment of the invention;

Fig. 5 schematically represents a sequence of pilot symbols multiplied by coverage code chips;

Fig. 6 is a schematic diagram of a demodulation apparatus according to the first embodiment of the invention;

Fig. 7 is a schematic diagram of a demodulation apparatus according to a second embodiment of the invention;

Fig. 8 represents a preamble detection timing device that can be used in conjunction with a demodulation device according to one embodiment of the invention;

Fig. 9 is a schematic diagram of a first transmitter/receiver using a modulation apparatus and a demodulation apparatus according to one embodiment of the invention;

Fig. 10 schematically shows a polar modulator used in a second transmitter according to one embodiment of the invention.

Carrying out the invention

The idea behind the present invention is to perform modulation of a K-ary symbol into a signal by means of Zadoff-Chu sequence frequency shift modulation, a division between various synchronous or asynchronous, uplink or downlink data transmissions obtained by selecting different roots.

First, recall that the Zadov-Chu sequence with root r, length N and frequency shift is determined by the sequence of complex elements:

where N is a positive integer, q is a relative integer, 0 < r < N and r, N are prime numbers with respect to each other. An introduction to Zadov-Chu sequences can be found in D.K. Chu, entitled “Polyphase codes with good correlation properties,” IEEE Trans. Inform. Theory, volume IT-18, pp. 531-532, July 1972

If the phases of successive elements of the Zadov-Chu sequence are taken into account, it can be seen that they include a linear component as well as a quadratic component depending on the time index k. Linear component corresponds to frequency shift , Where is the frequency with which the elements of the sequence are formed, and the quadratic component corresponds to a linear change in the instantaneous frequency with deviation by sequence length.

Zadow-Chu (ZC) sequences have some very interesting characteristics for communication systems. First, they have a constant amplitude (all elements of the ZC sequence belong to a circle of unit radius), giving rise to modulated signals with low PAPR. Second, the periodic autocorrelation of a ZC sequence is ideal in that it is zero for any non-zero shift (in other words, the periodic autocorrelation function of a ZC sequence is a Dirac comb function having period N). Finally, periodic cross-correlation of two ZC-sequences of the same odd length N and with different roots r, u, with |ru| is prime with respect to N, is a periodic function whose absolute value is limited . In addition, it was shown in the document by D.V. Sarwat, entitled "Bounds on crosscorrelation and autocorrelation of sequences", published in IEEE Trans. on Information Theory, Vol. IT-25, No. 6, Nov. 79, pp. 720-724 that this constraint on the periodic cross-correlation function is optimal for a sequence with an ideal periodic auto-correlation function. In other words, there are no better sequences than the Zadov-Chu sequences in terms of autocorrelation and cross-correlation properties.

The original method proposes to modulate the frequency shift of the Zadov-Chu sequence to transmit a word of K≤log ₂ N bits. It should be noted that the frequency offset of the ZC sequence has generally been set to zero in the prior art, especially when it is used for synchronization purposes, as is the case when generating preambles in an LTE system (see 3GPP TS 36.211, § 5.7.2 entitled "preamble sequence generation").

Fig. 1 schematically shows a demodulation apparatus according to a first embodiment of the invention.

The modulation device 100 includes a binary-to-decimal converter (BDC) 110 that converts a word of K bits to be transmitted, α ₀ , ..., α _K-1 (where α ₀ is LSB and α _K-1 is MSB) into integer value This integer value is provided as a frequency offset value to the Zadoff-Chu sequence generator 120.

The word to be transmitted may result from channel encoding of the information symbols, possibly after they have been interleaved. Alternatively, the word to be transmitted may be a pilot symbol for the transmission channel.

The word to be transmitted may also result from Gray encoding of the information symbols. Recall that, according to this encoding, two consecutive binary words create encoded words that differ in only one bit.

The generator 120 further takes as inputs the root r and the length N of the sequence to be generated. The r and N parameters may be provided by the transmission resource scheduler, as discussed later.

Complex elements of the Zadov-Chu sequence , generated by the generator 120 are then used to modulate (RF or IF) the carrier in a manner known per se.

It is important to note that in all cases the sequence of complex elements at the output of the modulator is the Zadov-Chu sequence as defined above. In particular, there is no need to perform any phase correction on the output of the oscillator 120, and the structure of the modulator is therefore very simple.

As shown above, the integers r and N are chosen to be prime to each other.

According to the first alternative, in order to comply with this constraint, the number N is chosen to be prime. In the latter case, interference between two data transmissions of the same length N, but with different roots r, u will be limited to the same value, proportional to .

It is preferred, however, for reasons of ease of implementation, that the sequence , generated by the generator, can be truncated to a length of 2 ^K' <N, while K'≥K, in particular, it can be truncated to a length of 2 ^K in order to obtain the optimal frequency.

According to a second useful alternative, for the reasons of ease of implementation stated earlier, the number N will be chosen to be a power of 2, for example N = 2 ^K , the root r is then odd, so that r and N are prime to each other.

In addition, roots of ZC sequences will be selected greater than 1 (r>1). Note that, in this case, the phase change for an unmodulated ZC sequence (i.e. with q=0) represents at least one aliasing effect between successive elements of the sequence, in other words, the instantaneous frequency has at least one break along the length of the sequence. Indeed, the phase rotation between two consecutive elements can be written as:

(3-1)

(3-2)

Thus, if r > 1, the phase rotation at the end of the sequence may exceed .

Equivalently, the instantaneous frequency can be expressed as:

(4)

and, if r > 1, the instantaneous frequency may exceed the frequency , with which the elements of the sequence are formed.

As an example, successive elements of an unmodulated Zadov-Chu sequence have been presented in FIG. 2A, and frequency-shift modulated elements of the same sequence were presented in FIG. 2B.

In the illustrated case, the length of the sequence is N=17, and its root is r=4. Note that the first phase overlap occurs between the 5th and 6th elements, with phase alternations between subsequent successive elements, obviously leading to overlap, taking into account relation (3-1).

Similarly, in FIG. 2B, the same sequence, this time modulated with an offset of q=1, was examined. Note that the first phase superposition occurs here as soon as the transition between the 4th and 5th element occurs.

Thus, for the same basic ZC sequence, it is clear that different distributions of modulated sequence elements are obtained with different frequency offsets.

Fig. 3A schematically represents the cyclic correlation maxima matrix of unmodulated Zadov-Chu frequencies (q=0) with length N=521 (prime number) and corresponding roots r=2, ..., 520. In other words, for two sequences And the drawing represents on the y-axis the maximum in τ for , where τ is the shift between two sequences And , A represents a cyclic correlation of length N. In fact, it is verified that the maximum (N) is obtained on the diagonal (the maxima of the autocorrelation functions), and that the cyclic cross-correlation functions have a low absolute value (bounded ).

Fig. 3B schematically represents the cyclic correlation maxima matrix for unmodulated ZC sequences (q=0) of length N=512 and corresponding roots r=2, ..., 511. It will be noted that some of these sequences are not, strictly speaking, ZC sequences, since r and N are not prime numbers to each other.

Note that the diagonal of the matrix always consists of the maximum values (N), but that the maximums of the cyclic cross-correlation strictly depend on the pair of sequences . By appropriately selecting root values (especially values that are prime with respect to N), a subfamily is obtained whose cyclic cross-correlation functions are limited to a predetermined maximum value.

Fig. 3C schematically represents a cyclic correlation maxima matrix of unmodulated ZC sequences (q=0) of length equal to the prime number N=521, but truncated to 512 elements. It can be seen that the performance of autocorrelation and cyclic cross-correlation is only slightly degraded relative to the performance illustrated in FIG. 3A.

In general, if it is desired to select a sequence with a length equal to a power of 2, P= ^2K , a ZC sequence whose length N is equal to the smallest prime number greater than P can be selected and truncated to P elements. Thus, the performance loss will be minimal, both in terms of frequency and cross-correlation level.

Thus, for example, for a sequence of length 128, 256, 512, 1024, 2048, 4096, a ZC sequence of length 131, 257, 521, 1031, 2053, 4099 will be selected accordingly.

The length N of sequences to be generated can be selected as a function of various criteria, such as SNR (signal-to-noise ratio), BER (bit error rate), required data rate, or even transmission channel timing, as set forth later.

More precisely, the spread factor (SF) is nothing other than N/K, choosing a high value for N provides the option of reducing the BER for the same SNR value or being consistent with the BER rate for degraded SNR conditions. Likewise, a low value for N provides the opportunity to reduce the transmission bandwidth and increase the data rate.

Thus, for a given value of N, different roots are selected for data transmissions for which it is desirable to reduce interference. For example, two different roots may be assigned to two downlink data transmissions originating from the same base station and to two attached entities or two terminals located within the coverage area of that base station, regardless of whether the two data transfers are synchronous or not. Moreover, two different roots may be assigned to two different downlink data transmissions to the same receiver.

Likewise, for an uplink channel, two connected objects or two terminals that may interfere may be assigned different roots.

The assignment of sequence lengths/roots can be done dynamically by the scheduler. In some cases, dynamic scheduling may be implicit. For example, if two uplink data transmissions interfere with each other, the base station can notify the attached entities affected by this situation, and at least one of them can switch to another subsequent root in a predetermined root sequence, known as the attached the object and the base station. One skilled in the art can envision various scheduling alternatives between adjacent base stations, according to a centralized or distributed mode, without departing from the scope of the present invention.

Regardless of the embodiment, transmission of packets of K-ary symbols, namely K-bit words, is typically accomplished through transmission frames, the structure of which is illustrated in FIG. 4.

This transmission frame contains a preamble 410 and a payload 420. The preamble contains pilot symbols to enable the receiver to detect the start of the frame and synchronize.

For example, pilot symbols will be associated with a specially designated frequency offset, q ₀ , for example, q ₀ =0. In other words, in this case the preamble of the transmission frame will consist only of unmodulated ZC sequences.

The pilot symbols will be detected by the receiver through a correlator, which performs constant correlation (or continuous correlation) on the received signal. For example, repeating a predetermined number, M, of pilot symbols may be a frame start indicator. In order to provide synchronization and detection of the start of a frame (or even a sequence of frames), these pilot symbols can be multiplied by coverage chips with good aperiodic correlation properties, such as Barker chips. Receiver synchronization by preamble detection will be described later in connection with FIG. 8. Receiver resynchronization may occur regularly during the reception time of a sequence of frames, at every frame, or generally every J frames, where J is a predetermined integer.

Fig. 5 shows a sequence of frame preamble pilot symbols, respectively, multiplied by M coverage code chips, denoted CC(1), ..., CC(M). 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), etc. until the last ZC sequence is multiplied by CC(M).

For example, Barker code elements of length M=11 are represented as:

+1,+1,+1,-1,-1,-1,+1,-1,-1,+1,-1

Upon reception, individual pilot symbols (ie, ZC sequences corresponding to these pilot symbols) are multiplied by the same coverage code elements before or after continuous correlation. When the coverage code elements perfectly match the pilot symbols, the correlation result has a maximum.

The payload of a transmission frame consists of ZC-sequences modulated by K-ary words to be transmitted. It may also include a control header and ZC sequences modulated by CRC symbols.

Although K-ary symbols are transmitted sequentially, and therefore ZC sequences modulated by these symbols also follow each other in the transmitter, the transmission channel may, if it is a multipath channel, be a source of inter-symbol interference. In other words, a copy of the ZC sequence modulated by the first symbol may be superimposed at the receiver with the ZC sequence modulated by the second symbol.

In order to reduce inter-symbol interference, a sequence of length N will be preferentially selected so that:

where Δ is the transmission channel time spread and q _max is the maximum frequency offset for modulating the K-ary symbols, both expressed in the number of ZC element generation periods. Indeed, as long as the overlap between successive symbols, due to transmission channel scatter, has a duration shorter than the duration (N - 2 ^K ) of the redundant part of the ZC sequence, the symbols in question can be reconstructed without loss of information.

Fig. 6 is a schematic diagram of a demodulation apparatus according to the first embodiment of the invention.

This first embodiment corresponds to a frequency domain implementation of a cyclic correlator.

The demodulator receives a signal corresponding to a frame of symbols as described above, ie, a series of ZC sequences obtained by modulating these symbols.

The demodulator 600 includes an FFT module 610 for performing an FFT of size N (equal to the length of the ZC sequence) on a received signal previously sampled (via two quadrature clocks) at a frequency , with which elements of ZC-sequences are formed. The result of FFT is a block of N complex values.

Detection of the start of frame and ZC sequences is accomplished through a series of pilot pulses in the preamble. It is assumed that the receiver has previously synchronized itself through preamble detection, as described later.

The demodulator includes a local ZC reference sequence oscillator, 605, identical to the oscillator present in the modulator in FIG. 3 or 4. The local oscillator is parameterized by the length N of the ZC sequence, the root r used for communication, and, if applicable, a frequency offset q ₀ specifically intended for pilot symbols. The root r used for communication may be provided by a preamble detector described later.

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

The block of N complex values thus obtained in the frequency domain is conjugated in the interface module 625 before multiplying term by term in module 620 with the block of N complex values from the DFT module 610.

The absolute values of these N complex values are calculated in module 630 and compared by correlation peak detector 640 . In other words, peak detector 640 determines the frequency offset corresponding to the largest absolute value for the complex value over the received block.

Since the cyclic correlation peak for two ZC sequences of the same length N, with the same root r and frequency shifts q, ℓ has the value , this peak discrimination is very efficient, even in the presence of truncated ZC sequences. Detector 640 therefore determines:

where σ represents the sequence of N samples received by the demodulator and ρ represents the cyclic correlation operation.

This frequency shift is eventually converted into a word of K bits, in the decimal-to-binary converter (indicated by the bit decompression block in the drawing), 650.

It should be noted that generator 605 will have the ability to generate reference sequences for all possible values of r and, if applicable, N.

The reference sequence can be any ZC sequence of the same length and with the same root as the 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 may be an unmodulated ZC sequence or a ZC sequence corresponding to the pilot symbol (and hence the offset q ₀ ).

In all cases, the set consisting of a reference sequence generator 605, a DFT module 615, and an interface module 625 can be replaced by a simple memory addressed by length N, root r, and, if applicable, a reference frequency offset. The value stored in memory at this address will simply be , Where is the value obtained through the discrete Fourier transform for , with frequency ν, and z ^* is the conjugate quantity for z. Values ,read from memory are then provided in turn to the multiplier,620.

Fig. 7 schematically shows a demodulation apparatus according to a second embodiment of the invention.

This second embodiment corresponds to a time domain implementation of a cyclic correlator.

The signal received by demodulator 700 is sampled (via two quadrature clocks) at a frequency , with which elements of ZC-sequences are formed. The result is a block of N complex values. This block is provided to the cyclic correlator, 710.

This cyclic correlator 710 further receives N consecutive elements of the ZC reference sequence generated by a local oscillator 705. This local oscillator is identical to the oscillator present in the modulator in FIG. 1. The local oscillator is parameterized by the length N of the ZC sequence, the root r used for communication, and, if applicable, a frequency offset q ₀ specifically intended for pilot symbols.

The elements of the reference ZC sequence are provided to the cyclic correlator 710. The results of the cyclic correlation (corresponding to different time domain delay times) are an absolute value calculated in module 730, and the maximum absolute value over a block is determined by a correlation peak detector 740. Peak detector 740 determines the position of the cyclic correlation peak between the reference ZC sequence and the ZC sequence received by the demodulator, and derives a corresponding time offset from it.

Given the definition of ZC sequences, it can be shown that cycling the ZC sequence by means of a time shift results in a frequency shift:

If N is an odd value:

Where is a constant phase component throughout the sequence.

Likewise, if N is an even value:

(7-2)

Where is a constant phase component throughout the sequence.

Since the detection is performed on the absolute value of the cyclic correlation result, it will be understood that the time offset, q', (expressed in the number of periods ) results in a shift in frequency q' (expressed in increments ). In other words, for the same received signal, the time offset value provided by peak detector 740 (expressed in number of periods ) will be identical to the frequency offset value provided by peak detector 640.

A decimal to binary converter 750 (called a bit decompressor in the drawing) converts this shift value into a binary number, .

It should be noted that generator 705 will have the ability to generate reference sequences for all possible values of r and, if applicable, N.

In any case, as before, the set consisting of the reference sequence generator 705 can be replaced by a simple memory addressable by length N, root r, reference time (or frequency) offset. The value stored in memory at this address will simply be . Values ,read from memory are provided to the cyclic correlator,710.

Fig. 8 schematically shows a preamble detection synchronization device that can be used in conjunction with a demodulation device according to the invention.

More precisely, this synchronization device makes it possible to provide the demodulator with the root r used for communication, on the one hand, and, on the other hand, with a time instant corresponding to the beginning of the frame, or even the payload.

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 copy of the preamble.

A copy of the preamble is generated by module 810 from ZC sequences generated by local generator 805. Local generator 805 is identical to the previously described generator 605 (or 705), and the same alternatives are also applicable. The preamble generating unit 810 concatenates the ZC sequences of the preamble pilot symbols. For example, if the preamble consists of M repeated identical pilot symbols, module 810 will repeat the ZC sequence of that pilot symbol M times. Additionally, if the preamble uses a Barker cover sequence, module 810 will multiply each ZC sequence with a corresponding code element.

Of course, generator 805 and module 810 can be implemented with a simple memory addressable at a frequency offset q ₀ corresponding to the pilot symbols, the root r and sequence length N and the number M of pilot symbol repetitions.

Correlator 820 continuously correlates a sequence of received signal samples with a sequence of preamble samples provided by module 810. In other words, in each period T _s , the correlator calculates a correlation result for the MN preamble samples with the last MN signal samples.

The absolute value of these correlation results is calculated in module 830, and correlation peak detection is performed in detector 840. In practice, the detector compares the absolute value provided by module 830 with a predetermined threshold value less than N.

The detector provides the time position of the correlation peak indicating the start of the preamble and hence the frame. The demodulator can then latch onto the start of the payload to demodulate the information symbols.

In addition, if the root r is not known to the receiver, the synchronizer can perform a sequential or parallel search on preambles formed from the root values belonging to the set of possible roots, or even a subset of the latter (for example, if the set of roots is hierarchical and the receiver has a priori knowledge of the branch tree in which the root is located). By sequential search we mean that the correlation is performed with frequency by R roots that need to be tested. By parallel search we mean that the correlation is performed on R branches in parallel, each branch being associated with one of the R roots that must be tested. In either case, the correlation results are processed by detector 840. The detector then determines the root value (providing the correlation peak) and the start of the preamble (from the position of that peak).

Fig. 9 schematically shows a first transmitter/receiver using a modulation device and a demodulation device according to one embodiment of the invention.

The transmitter/receiver 900 may be installed on a base station or attached site, for example.

It includes a modulation device 910, according to an embodiment, as described in connection with FIG. 1. The modulation device converts the K-ary symbols to be transmitted into ZC sequences, the K-ary symbol is converted into a frequency shift of the ZC sequence. Complex elements of the ZC sequence are provided by the modulation device with a frequency . The real part of these complex elements is provided to the in-phase channel and the imaginary part to the 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 sine wave oscillator, 950. The RF signal output from the quadrature modulator is amplified in a power amplifier, 960, and then transmitted by antennas, 980, via duplexer, 970.

Upon reception, the antenna signal received through duplexer 970 is amplified by a low noise amplifier (LNA), 965, and then quadrature demodulated by a quadrature mixer 945. The I and Q signals at the output of the mixer are amplified by amplifiers 935, filtered by low-pass filters 925, and then sampled by frequency . The complex values thus obtained are provided to the demodulation apparatus 915 according to the first embodiment, as described in connection with FIG. 6, or according to a second embodiment as described in connection with FIG. 7.

The quadrature modulator 940 in the direct conversion transmitter/receiver of FIG. 9 uses the real and imaginary parts of the elements of the ZC sequences to be transmitted. Instead of a quadrature modulator, a polar modulator can be used, as described later in this document.

Fig. 10 schematically shows a polar modulator used in the application of a second transmitter according to one embodiment of the invention.

In general, in a polar modulator, complex symbols to be transmitted are converted in a polar baseband converter, 1020, into an envelope signal (the absolute value of the complex elements) and a phase signal (the instantaneous phase of the complex elements) representing the baseband signal:

(8)

Where A(t) is the envelope signal, and is a phase signal. The phase signal is then converted to RF frequency band by a mixer 1050 before being amplified by a power amplifier 1060. The envelope signal is delayed in the module 1025 to compensate for differences in propagation time that may exist between the amplitude branch and the phase branch, and modulates the amplitude of the RF signal, through the envelope modulator 1040, adjusting the gain of the power amplifier. The RF signal thus modulated is then transmitted via antenna 1070.

In the case where, as in the present invention, there is a question of transmitting complex ZC sequence elements that are generated by the modulator 1010, it is clear that the generation of an envelope signal will be unnecessary since the absolute value of these elements is constant and equal to 1. Delay 1025 and envelope modulator 1040 can simply be removed and the power amplifier gain is set to the specified value.

One skilled in the art will appreciate that, symmetrically, a polar demodulator can be implemented on the receive side.

RF transmitter/receiver architectures other than those illustrated in FIGS. 9 and 10 may be further discussed without departing from the scope of the present invention.

Of course, different modulation/demodulation elementary devices may be provided in the same transmitter/receiver to separate data transmissions to/from different users. As previously stated, these modulation/demodulation devices will then use different root values. Advantageously, for a ZC sequence of length N, the root values r _p , p=1,...P will be chosen such that |r _p -r _p' | is a prime number with respect to N, □p, p'.

Claims (19)

1. A method for modulating a plurality of K-ary symbols to be transmitted over a transmission channel, characterized in that each K-ary symbol is converted into a Zadoff-Chu sequence associated with that symbol, said Zadoff-Chu sequence having length N, root r and a frequency shift q, where N is a positive integer such that log ₂ N ≥ K, r is an integer such that r > 1 and is a prime number with respect to N, q is a relative integer, and that any two K- ary symbols from said set are associated with Zadov-Chu sequences having different frequency shifts, each K-ary symbol is modulated in the form of a sequence with a length greater than or equal to 2 ^K consecutive complex elements of the Zadov-Chu sequence associated with said K-ary symbol. 2. The modulation method according to claim 1, characterized in that the sequence of successive complex elements consists of all the elements of the mentioned Zadov-Chu sequence. 3. The modulation method according to claim 2, characterized in that the length of the Zadov-Chu sequence is equal to the power of 2. 4. The modulation method according to claim 1, characterized in that the length of said Zadov-Chu sequence is selected equal to the smallest prime number greater than 2 ^K , and that a sequence of consecutive complex elements is obtained by truncating said Zadov-Chu sequence to 2 ^K elements. 5. The modulation method according to any one of claims 1 to 4, characterized in that the length of the Zadov-Chu sequence is selected greater than the difference between the maximum frequency shift to modulate K-ary symbols and the time spread of the transmission channel, expressed in the number of transmission periods of elements this sequence. 6. The modulation method according to one of claims 1 to 5, characterized in that the real part and the imaginary part of the elements of the sequence of successive complex elements of the Zadov-Chu sequence associated with the K-ary symbol respectively modulate two quadrature carriers, and that the modulated signal, thus obtained is transmitted over the transmission channel. 7. The modulation method according to any one of claims 1 to 5, characterized in that the phase signal is generated in a baseband obtained as the phase of successive complex elements of the Zadov-Chu sequence associated with the K-ary symbol, and that the phase signal is converted to RF -frequency band, the RF signal thus obtained is transmitted through the transmission channel. 8. A method for transmitting data over multiple communication lines to or from different users, characterized in that the data to be transmitted is converted into K-ary symbols, the K-ary symbols of the different users are modulated by the modulation method of one of the preceding paragraphs, roots 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 the total length. 9. The data transmission method of claim 8, characterized in that the roots used to modulate the K-ary symbols of two different users are selected such that the absolute value of their difference is a prime number with respect to said conventional length. 10. A modulation device for modulating a plurality of K-ary symbols to be transmitted over a transmission channel, characterized in that it comprises a binary-to-decimal converter (110) converting the K bits of each K-ary symbol into a separate integer value, and a generator (120 ) of the Zadov-Chu sequence, having as input parameters the length N of the sequence and the root r, such as an integer r, r > 1 and being prime with respect to N, as well as the frequency shift q, q is a relative integer, and the generator generates, for each K-ary symbol, a sequence having a length greater than or equal to 2 ^K , formed by complex elements of the Zadov-Chu sequence with length N, root r and a frequency offset equal to the integer value provided by the binary-to-decimal converter, wherein the generator Zadov-Chu sequences are configured in such a way that any two K-ary symbols of the said set are associated with Zadov-Chu sequences having different frequency shifts. 11. A method for demodulating a signal representing a Zadov-Chu sequence modulated by a frequency shift by a modulation method according to one of claims 1 to 7, characterized in that said signal is sampled in a baseband to provide a sequence of complex samples that said sequence of complex samples is correlated by cyclic correlation (710) with the Zadow-Chu reference sequence to provide a cyclic correlation result, and that the temporal position of the correlation peak in the absolute value of the cyclic correlation result thus obtained is detected (740), and that a frequency frequency is derived therefrom shift, corresponding to this time position, said frequency offset is then converted by a decimal-to-binary converter (750) to a K-ary symbol. 12. An apparatus for demodulating a signal representing a Zadov-Chu sequence modulated by a frequency shift by the modulation method according to one of claims 1 to 7, said signal being sampled in a baseband to provide a first block of complex elements of size N, characterized in that contains a DFT module (610) configured to perform a discrete Fourier transform of size N on the first block of complex elements to provide a first block of complex frequency elements, a multiplier (620) for multiplying the complex frequency elements of the first block term by term by the complex frequency elements of the second block, obtained by complex conjugation of the discrete Fourier transform of the Zadov-Chu reference sequence, an absolute value calculation module (630) for calculating the absolute value of the results of multiplication of the first and second complex frequency elements, the absolute values thus obtained are compared in the correlation peak detector (640) to determine frequency position of the cyclic correlation peak, and that the frequency offset is output, corresponding to this frequency position, said frequency offset is supplied to a decimal-to-binary converter (650) to generate a K-ary symbol. 13. An apparatus for demodulating a signal representing a Zadov-Chu sequence modulated by a frequency shift by the modulation method according to one of claims 1 to 7, said signal being sampled in the baseband to provide a first block of complex elements of size N, characterized in that contains a cyclic correlator (710) of size N for cyclically correlating said first block with a second block, said second block consisting of a Zadov-Chu reference sequence, an absolute value calculation module (730) calculating the absolute value of said cyclic correlation, and a peak detector (740) correlation, which determines, from this absolute value, the time position of the cyclic correlation peak and derives the frequency shift from it corresponding to said time position, said frequency shift is supplied to a decimal-to-binary converter to generate a K-ary symbol. 14. A transceiver, characterized in that it contains at least one modulation device according to claim 10 and at least one demodulation device according to claim 12. 15. The transceiver according to claim 14, characterized in that said at least one modulation device is a plurality of modulation devices according to claim 10, each modulation device from said plurality corresponding to a different root value. 16. The transceiver according to claim 14, characterized in that said at least one demodulation device is a plurality of demodulation devices according to claim 12, each demodulation device from said plurality corresponding to a different root value. 17. A transceiver, characterized in that it contains at least one modulation device according to claim 10 and at least one demodulation device according to claim 13. 18. The transceiver according to claim 17, characterized in that said at least one modulation device is a plurality of modulation devices according to claim 10, each modulation device from said plurality corresponding to a different root value. 19. The transceiver according to claim 17, characterized in that said at least one demodulation device is a plurality of demodulation devices according to claim 13, each demodulation device from said plurality corresponding to a different root value.

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