FR3117290A1 - Method for generating and demodulating a signal comprising at least one chirp, computer program products and corresponding devices. - Google Patents

Abstract

Method for generating and demodulating a signal comprising at least one chirp, computer program products and corresponding devices The invention relates to a method for generating a signal comprising a first portion carrying data and a second portion carrying a word signal synchronization. The first portion includes one or more modulated chirps. The modulation corresponds to a circular permutation of the variation pattern of the instantaneous frequency of a basic chirp on the symbol time Ts. Such a method comprises: - a generation (E310) of the first portion of the signal comprising, for the generation of a given chirp among the modulated chirps, a modulation of the basic chirp according to a corresponding given modulation symbol generating the given chirp; and - a generation (E320) of the second portion of the signal comprising an amplitude modulation of the second portion of the signal, an instantaneous amplitude of the second portion of the signal being proportional to a module of the synchronization word. ABRIDGED FIGURE: Fig.3

Description

Method for generating and demodulating a signal comprising at least one chirp, computer program products and corresponding devices.

Field of invention

The field of the invention is that of data transmission via the use of a so-called “chirp” waveform.

The invention relates more particularly to a method for generating and processing such a waveform.

Such a waveform is used for the transmission of data via communication links of different kinds, e.g. acoustic, radio frequency, etc. For example, LoRa® technology dedicated to low-power transmission by objects connected via a radiofrequency link uses such a waveform. The invention thus has applications, in particular, but not exclusively, in all areas of personal and professional life in which connected objects are present. These include, for example, the fields of health, sport, domestic applications (security, household appliances, etc.), object tracking, etc.

Prior art and its drawbacks

We focus more particularly in the remainder of this document on describing an existing problem in the field of connected objects with which the inventors of this patent application have been confronted. The invention is of course not limited to this particular field of application, but is of interest for the processing of any communication signal based on the use of a so-called "chirp" waveform for the transmission of data. .

Presented as the "third revolution of the Internet", connected objects are in the process of imposing themselves in all areas of daily life and business. Most of these objects are intended to produce data thanks to their integrated sensors in order to provide value-added services for their owner.

Due to the targeted applications, these connected objects are mostly mobile. In particular, they must be able to transmit the data produced, regularly or on request, to a remote user.

To do this, long-range radio transmission of the cellular mobile radio type (2G/3G/4G, etc.) was the technology of choice. This technology made it possible to benefit from good network coverage in most countries.

However, the nomadic aspect of these objects is often accompanied by a need for energy autonomy. However, even based on one of the most energy-efficient cellular mobile radio technologies, current connected objects continue to have prohibitive consumption to allow large-scale deployment at a reasonable cost.

Faced with the problem of radio link consumption for such nomadic applications, new low-power and low-speed radio technologies dedicated specifically to "Internet of Things" networks, i.e. radio technologies for so-called LPWAN networks (for "Low-Power Wide-Area Networks" in English), are developed.

In practice, two kinds of technologies can be distinguished:

- on the one hand, there are proprietary technologies such as the technology of the Sigfox® company, or the LoRa® technology, or even the technology of the Qowisio® company. These non-standard technologies are all based on the use of the “Industrial, Scientific and Medical” frequency band, known as ISM, and on the regulations associated with its use. The advantage of these technologies is that they are already available and allow the rapid deployment of networks on the basis of limited investments. In addition, they allow the development of very energy-efficient and low-cost connected objects;

- on the other hand, there are several technologies promoted by standardization bodies. By way of example, we can cite three technologies standardized with the 3GPP (for “3rd Generation Partnership Project” in English): NB-IoT (for “Narrow Band – Internet of Things” in English), LTE MTC (for “Long Term Evolution - Machine Type Communication” in English) and EC-GSM-IoT (for “Extended Coverage – GSM – Internet of Things” in English). Such solutions are based on the use of licensed frequency bands, but can also be used on unlicensed frequency bands.

Some telecommunications operators have already taken an interest in LoRa® technology to deploy their network dedicated to connected objects. For example, patent EP 2 449 690 B1 describes an information transmission technique, on which the LoRa® technology is based.

However, the first feedback points to unsatisfactory user experiences linked to limited performance of the radio link in real conditions. In particular, the modulation used appears to be sensitive to the synchronization of the receiver. Furthermore, the "chirp" waveform itself implies an equivalence between time synchronization error (resulting for example from a shift between the ideal and effective sampling instants of the received signal) and frequency synchronization error (resulting for example a shift between the carrier frequency of the received signal and the local oscillator generating the signal used for the frequency transposition of the received signal) as explained for example in international patent application number PCT/EP2020/067276. In this way, the estimation of chirps waveform synchronization parameters can only be done jointly between the time synchronization parameters and the frequency synchronization parameters, which may require a significant computational load.

Alternatively, solutions have been considered in order to allow a decoupling between time synchronization error and frequency synchronization error when estimating the information symbols conveyed by such waveforms. For example, the aforementioned international patent application describes a technique for the differential encoding of information symbols before chirp modulation. Such a technique makes it possible to make the estimation of the symbols in question independent of frequency synchronization errors on reception. In such a situation, only the time synchronization parameters are necessary to estimate the information symbols conveyed by the chirps. The estimation of the frequency synchronization parameters thus becomes superfluous.

There is thus a need for a technique making it possible to estimate the time synchronization parameters of a signal comprising chirps without having to estimate the frequency synchronization parameters. Furthermore, the time synchronization parameters must be estimated precisely even in the presence of frequency desynchronization.

In one embodiment of the invention, there is proposed a method for generating a signal comprising a first portion conveying data and a second portion conveying a signal synchronization word. The first portion comprises at least one chirp among M chirps, an s-th chirp among the M chirps being associated with a modulation symbol of rank s of a constellation of M symbols, s being an integer from 0 to M-1. The s-th chirp results from a modulation of a base chirp whose instantaneous frequency varies between a first instantaneous frequency and a second instantaneous frequency during a symbol time T. The modulation corresponds, for the modulation symbol of rank s, to a circular permutation of the variation pattern of the instantaneous frequency over the symbol time T, obtained by a time shift of s times an elementary time duration Tc, such that M*Tc=T. Such a process includes:

- a generation of the first portion of the signal comprising, for the generation of a given chirp among said at least one chirp, a modulation of the basic chirp as a function of a corresponding given modulation symbol generating the given chirp; And

- a generation of the second portion of the signal comprising an amplitude modulation of the second portion of the signal, an instantaneous amplitude of the second portion of the signal being proportional to a module of the synchronization word.

Thus, the invention proposes a new and inventive solution for estimating the time synchronization parameters of a signal comprising chirps, without having to estimate the frequency synchronization parameters of the signal in question.
More particularly, the modulus of the synchronization word being conveyed here by the instantaneous amplitude of the signal, the temporal synchronization can be based on reception on a detection of the modulus of the complex envelope of the received signal (the modulus in question corresponding to the instantaneous amplitude of the signal in question). Such a module is independent of the phase of the signal, and therefore of any frequency error present between the receiver and the received signal. The time synchronization parameters can thus be estimated without having to estimate the frequency synchronization parameters. Furthermore, the time synchronization parameters can be estimated precisely, even in the presence of frequency desynchronization.

In embodiments, the first portion comprises a temporal succession of chirps among the M chirps. The temporal succession of chirps includes the given chirp. Generation of the first portion includes:

- a differential encoding between, on the one hand, a modulation symbol associated with a chirp preceding the given chirp in the temporal succession of chirps and, on the other hand, a given information symbol of the constellation of M symbols, l differential encoding delivering the given modulation symbol.

Thus, the differential encoding of the information symbols before the actual modulation of the chirps makes it possible to make the estimation of the symbols independent of frequency synchronization errors at reception. The estimation of the received symbols therefore does not require the estimation by the receiver of the frequency synchronization parameters of the signal to be demodulated.

In embodiments, the differential encoding implements a modulo M addition between, on the one hand, a first operand function of the modulation symbol associated with the chirp preceding the given chirp and, on the other hand, a second operand function of the given information symbol delivering the given modulation symbol.

Thus, the implementation is simple and robust.

In embodiments, the differential encoding and modulation are implemented iteratively for a succession of information symbols delivering a sequence of chirps in the temporal succession of chirps.

Thus, all the information symbols conveyed in the chirp sequence can be estimated on reception without having to estimate the frequency synchronization parameters of the signal thus generated.

The invention also relates to a method for demodulating a signal generated according to the generation method described above (according to any one of the aforementioned embodiments). Such a demodulation method comprises:

- an estimate of time synchronization information of the signal implementing a correlation function between, on the one hand, an instantaneous amplitude of the signal and, on the other hand, the modulus of the signal synchronization word; And

- for a given portion of the first portion representative of a given chirp among said at least one chirp, the given portion being determined as a function of the time synchronization information, a demodulation of the given portion delivering an estimate of a symbol modulation of the constellation of M symbols associated with the given chirp.

Thus, the structure of the signal generated according to the technique described makes it possible to implement on reception an estimation of the time synchronization information via a correlation function between the module of the complex envelope of the signal received (the module in question corresponding to the instantaneous amplitude of the modulated signal) and the synchronization word. Such a module is independent of the phase of the signal, and therefore of any frequency error present between the receiver and the received signal. The time synchronization parameters of the signal can thus be estimated simply without having to estimate the frequency synchronization parameters.

In embodiments, the first portion comprises a temporal succession of chirps among the M chirps. The demodulation process includes:
- differential decoding between, on the one hand, the estimate of the modulation symbol associated with the given chirp and, on the other hand, an estimate of a modulation symbol previously obtained by implementing demodulation applied to another given portion of the first portion of the signal representative of a chirp preceding the given chirp in the temporal succession of chirps, the differential decoding delivering a decoded symbol. An estimate of an information symbol conveyed by the signal is a function of the decoded symbol.

Thus, the differential decoding of the modulation symbols (the modulation symbols resulting from a differential encoding of the information symbols on transmission) makes it possible to make the estimation of the received symbols independent of frequency synchronization errors. The estimation of the received symbols therefore does not require the estimation by the receiver of the frequency synchronization parameters of the signal thus generated.

In embodiments, the differential decoding implements a difference modulo M between, on the one hand, a first operand function of the estimate of the modulation symbol associated with the given chirp and, on the other hand, a second operand function of the estimate of the modulation symbol previously obtained delivering the estimate of the information symbol conveyed by the signal.

Thus, the implementation is simple and robust.

In embodiments, the demodulation and the differential decoding are implemented iteratively for a succession of given portions of the first portion of the signal representative of a sequence of chirps in the temporal succession of chirps delivering a corresponding sequence of decoded symbols. A sequence of estimates of information symbols conveyed by the signal being a function of the sequence of decoded symbols.

Thus, all the information symbols conveyed in the sequence of chirps are estimated without having to estimate the frequency synchronization parameters of the signal thus generated.

In some embodiments, the synchronization word comprises successive terms, a value of each of the terms belonging to the group comprising a first zero value and a second non-zero predetermined value.

Thus, the waveform of the second portion of the signal thus generated has an instantaneous amplitude that is either substantially zero or substantially constant. The instantaneous amplitude of the chirps of the first portion of the signal being also constant, no major adaptation of the already existing transmitters/receivers for the processing of chirped signals is necessary in order to support the described technique. For example, saturated power amplifiers as conventionally used for efficiency reasons for amplifying chirped signals can also be used for amplifying the second portion of the signal according to the characteristics above. Such a waveform having an instantaneous amplitude that is either substantially zero or substantially constant can for example be directly generated at the level of such a power amplifier by switching it according to the switching scheme between the first and second values of the synchronization word.

In some embodiments, the synchronization word comprises N _bp =2 ⁿ −1 successive terms. An autocorrelation function of the synchronization word has a main lobe and at least one secondary lobe. A ratio between an amplitude of the main lobe and an amplitude of a secondary lobe of maximum amplitude among said at least one secondary lobe is greater than or equal to N _bp /(2 ^(n+2)/2 +1).

Thus, the estimation of the temporal synchronization information is precise through the use of synchronization words with good autocorrelation properties, such as Gold, Kasami, Zadoff-Chu sequences, etc.

In embodiments, the synchronization word belongs to a plurality of synchronization words of N _bp =2 ⁿ −1 successive terms. A cross-correlation function between, on the one hand, the synchronization word and, on the other hand, another synchronization word of the plurality of synchronization words has a main lobe and at least one secondary lobe. A ratio between an amplitude of the main lobe and an amplitude of a secondary lobe of maximum amplitude among said at least one secondary lobe is greater than or equal to N _bp /(2 ^(n+2)/2 +1).

Thus, even in the presence of collisions between signals, the receiver can synchronize itself on a given signal by searching for the synchronization word associated with the signal in question. This is possible through the use of synchronization words with good cross-correlation properties, such as Gold sequences, Kasami sequences, Zadoff-Chu sequences, etc. For example, it can be assigned one synchronization word per user, or one synchronization word per spreading factor in a use case similar to that of LoRa® technology.

The invention also relates to a computer program comprising program code instructions for the implementation of a method as described previously, according to any one of its various embodiments, when it is executed on a computer.

The invention also relates to a device for generating a signal comprising a first portion conveying data and a second portion conveying a signal synchronization word. Such a generation device comprises a reprogrammable calculation machine or a dedicated calculation machine configured to implement the steps of the generation method according to the invention (according to any one of the various aforementioned embodiments). Thus, the characteristics and advantages of this device are the same as those of the corresponding steps of the generation method described previously. Therefore, they are not further detailed.

The invention also relates to a device for demodulating a signal generated according to the generation method described above (according to any one of the aforementioned embodiments). Such a demodulation device comprises a reprogrammable calculation machine or a dedicated calculation machine configured to implement the steps of the demodulation method according to the invention (according to any one of the various aforementioned embodiments). Thus, the characteristics and advantages of this device are the same as those of the corresponding steps of the demodulation method described above. Therefore, they are not further detailed.

List of Figures

Other aims, characteristics and advantages of the invention will appear more clearly on reading the following description, given by way of a simple illustrative example, and not limiting, in relation to the figures, among which:

represents an object connected to a base station of a radio communication network according to one embodiment of the invention;

illustrates the instantaneous frequency of a basic chirp;

illustrates the basic chip modulation of the via a circular permutation of the variation pattern of its instantaneous frequency;

illustrates the instantaneous chirp frequency resulting from the modulation of the basic chirp of the via the circular permutation illustrated on the ;

represents the steps of a method for generating a signal comprising a first portion conveying data and a second portion conveying a synchronization word according to one embodiment of the invention;

represents the steps of a method for demodulating a signal generated by implementing the method of according to one embodiment of the invention;

represents an example of a device structure allowing the implementation of certain steps of the method for generating the and/or the method of demodulating the according to one embodiment of the invention.

Detailed Description of Embodiments of the Invention

The general principle of the invention is based on the use of a different waveform to convey, on the one hand, the useful data and, on the other hand, to convey the information allowing the synchronization of the receiver and the signal to be processed (i.e. the synchronization word or learning word).

More specifically, the data is conveyed here via modulated chirps. However, as described above in relation to the prior art, such a waveform implies an equivalence between time synchronization error and frequency synchronization error. An estimation of synchronization information made from chirps thus involves jointly estimating the time synchronization error and the frequency synchronization error of the receiver. According to the proposed technique, the information allowing the synchronization of the receiver on the signal to be processed is conveyed by the instantaneous amplitude of a portion of the signal to be processed. Time synchronization can thus be based on reception on detection of the modulus of the complex envelope of the portion in question of the signal to be processed (the modulus in question corresponding to the instantaneous amplitude of the portion in question). Such a module is independent of the phase of the signal, and therefore of any frequency error present between the receiver and the signal to be processed. The time synchronization parameters can thus be estimated without having to estimate the frequency synchronization parameters. Furthermore, the time synchronization parameters can be estimated precisely, even in the presence of frequency desynchronization.

There illustrates an object 100 connected to a base station 110 of a radio communication network (for example a network of the low speed and low consumption type) according to one embodiment of the invention. More particularly, the radiocommunication network implements a so-called “chirp” waveform in order to convey the payload data and another waveform in order to convey the synchronization words as detailed below.

We now present, in relation to the , And , the modulation of a basic chirp via a circular permutation of the variation pattern of its instantaneous frequency.

Such a basic chirp is defined as the chirp from which the other chirps used for the transmission of information are obtained following the modulation process by the modulation symbols.

More specifically, the instantaneous phase (ie the phase of the complex envelope representing the considered chirp) of the basic chirp is expressed for t in the interval (Fig.2a) as with :

• T : symbol duration (also called signaling interval for example in the LoRa® standard);
• B = 2 ^SF / T : the bandwidth of the signal, with SF the spreading factor or number of bits per symbol. M=2 ^SF is thus the total number of symbols in the constellation of modulation symbols.

Based on these notations, the instantaneous frequency of the base chirp, which corresponds to the derivative of the instantaneous phase , is expressed as .

The instantaneous frequency is thus linked to the angular rotation speed in the complex plane of the vector whose coordinates are given by the in-phase and quadrature signals representing the modulating signal (ie the real and imaginary parts of the complex envelope in practice) intended to modulate the radio frequency carrier so as to transpose the basic chirp signal onto a carrier frequency.

The instantaneous frequency of the basic chirp shown on the is linear in time, ie varies linearly between a first instantaneous frequency, here −B/2, and a second instantaneous frequency, here +B/2, during the duration T of a symbol. For example, a chirp with a linear instantaneous frequency is used as a base chirp (also called a “raw” chirp) in the LoRa® standard.

In other embodiments, other types of base chirps are considered, for example base chirps whose instantaneous frequency has a negative slope, or else whose instantaneous frequency does not vary linearly in time.

back to , And , the modulation of a chirp corresponds, for a modulation symbol having a rank s in the constellation of modulation symbols, to a circular permutation of the pattern of variation of said instantaneous frequency on said symbol time T, obtained by a time shift of s times an elementary temporal duration Tc, such that M*Tc=T.

More particularly, one can note as being the instantaneous frequency of the chirp transmitted by the object 100 connected over the time interval . The instantaneous frequency of the chirp in question is obtained by time shifting a duration of and circular permutation as shown in Fig.2b and Fig.2c. Here, is an integer value between 0 and M-1 which represents the modulation symbol conveyed by the chirp transmitted by the object 100 connected on the time slot .

In this way, is expressed as the derivative of the instantaneous phase :

We thus obtain over the time interval :

And on the time interval :

Thus, if we note the complex envelope of the signal portion transmitted by the connected object 100 and conveying the useful data via modulated chirps, we can write:

Or represents the indicator function of the time interval .

Furthermore, the signal transmitted by the object 100 follows a frame structure. More specifically, the signal transmitted by the object 100 comprises a first signal portion conveying useful data (ie the signal described above) and a second signal portion conveying a synchronization word (or learning word) of duration . It is assumed in the following that the second signal portion is positioned in time upstream of the useful data. This assumption does not detract from the generality of the problem dealt with in the present application. Moreover, the nature of the second portion will be detailed further below in relation to Fig.3 and Fig.4. The complex envelope of the transmitted signal transmitted by object 100 is then written:

In this way, the signal received at the level of the base station 110 is expressed, after sampling at the frequency 1/Ts:

With :

• , Or with And represents the time desynchronization of the received signal;
• is the Doppler frequency and the phase originally related to the Doppler frequency. Without loss of generality, we assume that also contains any residual frequency offset resulting from a difference between local oscillators used to generate the transmit and receive carrier frequencies;
• h(t) is the impulse response of the propagation channel;
• is the noise seen from the receiver, assumed additive white and Gaussian.

We now present, in relation to Fig.3, the steps of a method for generating the signal , an expression of which is given by [Math.5], according to one embodiment of the invention.

During a step E310 , the first portion signal is generated. More particularly, for the generation of a given chirp (eg a chirp of index k ) among the chips that make up the first portion , a modulation of the basic chirp is performed according to a given modulation symbol associated with the given chip. is thus an integer value between 0 and M-1 which is identified with the quantity m _k mentioned above. Such modulation corresponds to the mechanism described above in relation to the , And (circular permutation of the variation pattern of the instantaneous frequency of the basic chirp on the symbol time Ts).

During a step E320 , the second portion signal is generated. To do this, the second portion signal is modulated in amplitude as a function of a synchronization word (or learning word). More particularly, an instantaneous amplitude (defined in the present application as equal to the modulus of the complex envelope in question) of the second portion is modulated so as to be proportional to a modulus of the synchronization word (whose complex envelope may be real or complex depending on the implementations). For example, the second portion is generated by amplitude modulation of a sinusoidal carrier as a function of the modulus of the synchronization word.

Thus, the modulus of the synchronization word being conveyed by the instantaneous amplitude of the signal, the temporal synchronization can be based on reception on a detection of the modulus of the complex envelope of the received signal. Such a module is independent of the phase of the signal, and therefore of any frequency error present between the receiver and the received signal. The time synchronization parameters can thus be estimated without having to estimate the frequency synchronization parameters. More details on synchronization as well as the nature of the synchronization word are given below in relation to the .

Returning to Fig.3, it is assumed that the first portion includes a temporal succession of several chips (ie that is strictly greater than 1). It is further assumed that the given chirp considered above (and modulated by the given modulation symbol ) is part of the temporal succession of chirps.

Thus, during a step E310a , the modulation symbol is obtained by differential encoding between, on the one hand, a modulation symbol associated with a chirp preceding the given chirp in the temporal succession of chirps and, on the other hand, an information symbol given from the constellation of M symbols.

As described for example in the aforementioned international patent application, such differential encoding of the information symbols before modulation of the chirps makes it possible to make the estimation of the symbols in question independent of frequency synchronization errors on reception. There is thus a synergistic effect between, on the one hand, such a differential encoding and, on the other hand, the nature of the second portion of the signal according to the present technique making it possible to obtain a communication link that is both robust to frequency synchronization errors and implementing simple time synchronization as detailed below in relation to the .

In embodiments, the differential encoding to obtain the modulation symbol implements a modulo M addition between, on the one hand, a first operand depending on the modulation symbol and, on the other hand, the second operand function of the information symbol given. For example, differential encoding implements the equation . During the first implementation of the differential encoding (ie for k=0), a predetermined constellation symbol is used instead of the modulation symbol .

In embodiments, the given chirp and the chirp preceding the given chirp are not adjacent in the temporal succession of chirps. In other words, the modulation symbol given is obtained by differential encoding between a modulation symbol , with p an integer greater than 1, and an information symbol given of the constellation of M symbols, for example via a sum modulo M. Thus, in the present application, the terminology “chirp preceding the given chirp in the temporal succession of chirps” covers both the case of temporally adjacent chirps and the case temporally non-adjacent chips.

In embodiments, additional differential encodings are further implemented. Each additional differential encoding is implemented between, on the one hand, a modulation symbol associated with a p-th chirp preceding the given chirp in the temporal succession of chirps, p being an integer greater than 1, and, on the other hand, an information symbol of rank k-p', p' being an integer greater than 1 different from p, in a sequence of information symbols of the constellation of M symbols. The additional differential encoding delivers a corresponding intermediate modulation symbol. The additional differential encodings implemented for K pairs ( , ) deliver K corresponding intermediate symbols. The K intermediate symbols are summed together modulo M with the symbol obtained in the aforementioned case corresponding to a single differential encoding with p'=0, in order to deliver the modulation symbol .

In embodiments, the aforementioned steps E310a and E310b (regardless of their embodiment) are implemented iteratively for a succession of information symbols in order to generate a temporal sequence of modulated chirps included in the signal to be transmitted.

In other embodiments, such differential encoding is not implemented. For example, direct chirp modulation without differential encoding is implemented with a second portion of the signal according to the present technique. Such a combination allows demodulation of the payload data in cases such as a static link in which no frequency variation takes place (eg Doppler shift, carrier frequency error between transmission and reception, etc.). Moreover, techniques other than differential encoding can be envisaged in order to obtain that the demodulation of the payload data only requires time synchronization for the demodulation. For example, the transmitted signal can be pre-distorted so as to compensate for the frequency error undergone during propagation (eg Doppler) and/or at the time of the processing implemented on reception (eg carrier frequency error between the transmission and reception).

We now present, in relation to Fig.4, the steps of a process for demodulating the signal , an expression of which is given by [Math.6], according to one embodiment of the invention.

During a step E410 , time synchronization information of the signal is estimated by implementing a correlation function between, on the one hand, an instantaneous amplitude of the signal (ie a module of the complex envelope in question: ) and, on the other hand, the module of the signal synchronization word.

The time synchronization information is for example given by the index of the sample n corresponding to the maximum, in modulus, of the correlation function in question. For example, the correlation function takes the following form:

with the total number of samples of the second signal portion carrying the synchronization word .

In the considered embodiment, the correlation function is normalized, ie varies between a minimum value and a predetermined maximum value. For example, it is decided that the time synchronization information corresponds to a signal actually detected, and therefore the time synchronization information can be used, if the module of the correlation function exceeds a predetermined threshold. The level of such a threshold is for example decided in order to obtain a given rate of missed detections (or a given probability of false alarm).

In embodiments, the correlation function is not normalized.

Whatever the embodiment considered, the structure of the signal makes it possible to estimate the time synchronization information via a correlation function implementing the modulus of the complex envelope of the received signal (the modulus in question corresponding to the instantaneous amplitude of the modulated signal) and the synchronization word. Such a module is independent of the phase of the signal, and therefore of any frequency error present between the receiver and the received signal. The time synchronization parameters of the signal can thus be estimated simply, in particular without having to estimate the frequency synchronization parameters.

Moreover, as described above in relation to Fig.3, the instantaneous amplitude (equal to the modulus of the complex envelope in question) of the second portion signal is proportional to the modulus of the synchronization word. Thus, thanks to this characteristic, the aforementioned correlation function implements an autocorrelation function of the synchronization word. In this way, in certain embodiments, the estimation of the time synchronization information is precise via the use of synchronization words having good autocorrelation properties, such as for example the sequences of Gold, of Kasami, of Zadoff-Chu, etc. For example, for Gold sequences comprising N _bp = 2 ⁿ − 1 successive terms, the ratio between the amplitude of the main lobe and the amplitude of the secondary lobes can only take the three values N _bp , N _bp / (2 ^(n+1)/2 + 1) and N _bp / (2 ^(n+1)/2 − 1) if n is odd, and N _bp , N _bp / (2 ^(n+2)/2 + 1) and N _bp / (2 ^(n+2)/2 − 1) if n is even.

In this way, in certain embodiments, the synchronization word comprises N _bp =2 ⁿ −1 successive terms. An autocorrelation function of the synchronization word has a main lobe and at least one secondary lobe. A ratio between an amplitude of said main lobe and an amplitude of a secondary lobe of maximum amplitude among the secondary lobe(s) being greater than or equal to N _bp /(2 ^(n+2)/2 +1).

Moreover, the aforementioned sequences also exhibit good cross-correlation properties. For example, reconsidering Gold sequences comprising N _bp = 2 ⁿ − 1 successive terms, the ratio between the amplitude of the main lobe and the amplitude of the secondary lobes of cross-correlation functions between specifically chosen sequences can also take only the three values N _bp , N _bp / (2 ^(n+1)/2 + 1) and N _bp / (2 ^(n+1)/2 − 1) if n is odd, and N _bp , N _bp / (2 ^(n+2)/2 + 1) and N _bp / (2 ^(n+2)/2 − 1) if n is even.

Thus, in certain embodiments, the synchronization word belongs to a plurality of synchronization words of N _bp =2 ⁿ −1 successive terms. A cross-correlation function between, on the one hand, the synchronization word and, on the other hand, another synchronization word of the plurality of synchronization words has a main lobe and at least one secondary lobe. A ratio between an amplitude of the main lobe and an amplitude of a maximum amplitude sidelobe among the sidelobe(s) is greater than or equal to N _bp /(2 ^(n+2)/2 +1).

Thus, even in the presence of collisions between signals, the receiver can synchronize itself on a given signal by searching for the synchronization word associated with the signal in question. For example, a different synchronization word can be allocated, among a given plurality of synchronization words, per user. Alternatively, a different synchronization word can be assigned per spreading factor, e.g. in a use case similar to that of LoRa® technology.

In certain embodiments, the value of the successive terms of the synchronization word belongs to the group comprising a first zero value and a second non-zero predetermined value.

Thus, the waveform of the second portion of the signal thus generated has an instantaneous amplitude that is either substantially zero or substantially constant. Since the instantaneous amplitude of the chirps of the first portion of the signal is also constant, no major adaptation of the already existing transmitters/receivers for the processing of chirp signals is necessary in order to support the technique described. For example, saturated power amplifiers as conventionally used for efficiency reasons for amplifying chirped signals can also be used for amplifying the second portion of the signal according to the characteristics above. Such a waveform having an instantaneous amplitude that is either substantially zero or substantially constant can for example be directly generated at the level of such a power amplifier by switching it according to the switching scheme between the first and second values of the synchronization word.

Returning to FIG. 4, during a step E420, a given portion of the first portion representative of a given chirp among the chirps conveyed by the first portion in question is determined according to the time synchronization information determined during the implementation of step E410. For example, the portion given is a fraction of the first portion over a time window with a duration equal to the duration of a chirp. The time window in question is selected around the correlation peak which was determined during the implementation of step E410.

Returning to step E420, a demodulation of the given portion delivers an estimate a modulation symbol associated with the given chirp (eg a chirp of index k ). For example, the modulation symbol associated with the given chirp is estimated according to the principles set out in the patent document EP 2 449 690 B1.

Returning to Fig.4, it is assumed in this embodiment that the first portion includes a temporal succession of chips (ie that is strictly greater than 1). It is further assumed that the given chirp considered above is part of the temporal succession of chirps. Thus, during a step E430 , an estimate of an information symbol conveyed by the first portion is obtained by differential decoding between, on the one hand, the estimate of the modulation symbol associated with the given chirp and, on the other hand, an estimate of a modulation symbol previously obtained by an implementation of step E420 applied to another portion of the first portion representative of a chirp preceding the given chirp in the temporal succession of chirps.

In embodiments, the differential decoding implements a difference modulo M between, on the one hand, a first operand depending on the estimate of the modulation symbol associated with the given chirp and, on the other hand, a second operand depending on the estimate of the previously obtained modulation symbol. For example, differential decoding implements the equation . During the first implementation of the differential decoding (ie for k=0), a predetermined constellation symbol is used instead of the estimated .

In the aforementioned embodiments in relation to Fig.3 in which the modulation symbol is obtained by differential encoding between a modulation symbol , with p an integer greater than 1, and an information symbol given from the constellation of M symbols, a differential decoding between the estimated and an estimate of the modulation symbol conveyed by the p-th chirp preceding the given chirp in the temporal succession of chirps, ie , is implemented to deliver the estimate of the information symbol, for example via a difference modulo M. In these embodiments, the rank kp (ie relative to the given chirp) of the chirp preceding the given chirp in the temporal succession of chirps is identical for the implementation of the differential decoding and differential encoding as described above in connection with the .

Similarly, in the aforementioned modes in relation to FIG. 3 in which additional differential encodings are also implemented, corresponding additional differential decodings are also implemented between, on the one hand, an estimated of the modulation symbol associated with a p-th chirp preceding the given chirp in the temporal succession of chirps, p being an integer greater than 1, and, on the other hand, an estimated of the modulation symbol associated with a p'-th chirp preceding the given chirp in the temporal succession of chirps, p' being an integer greater than 1 different from p. The additional differential decoding in question delivers a corresponding decoded symbol. More precisely, the indices kp and k-p' of the components of each pair of estimates on which a differential decoding is applied correspond to the indices of a corresponding pair ( , ) for which a differential encoding was implemented during the generation of the temporal succession of chirps. Such differential decoding implemented for K pairs ( , ) delivers K corresponding decoded symbols. The K decoded symbols in question are summed together modulo M with the decoded symbol obtained in the aforementioned case corresponding to a single differential decoding with p'=0, in order to deliver the estimate of the information symbol.

In embodiments, the aforementioned steps E420 and E430 (regardless of their embodiment) are implemented iteratively for a succession of portions of the first portion representative of a sequence of chirps in the temporal succession of chirps in order to extract a sequence of information symbols conveyed by the signal.

In the embodiments discussed above in connection with the in which no differential encoding is implemented during the implementation of step E310, no differential decoding is implemented in the present demodulation method. In other words, step E430 is not implemented in some embodiments.

We now present, in relation to the an example of device structure 500 making it possible to implement certain steps of the method for generating the and/or the method of demodulating the according to one embodiment of the invention.

The device 500 comprises a random access memory 503 (for example a RAM memory), a processing unit 502 equipped for example with a processor, and controlled by a computer program stored in a read only memory 501 (for example a ROM memory or a hard disc). On initialization, the code instructions of the computer program are for example loaded into the RAM 503 before being executed by the processor of the processing unit 502.

This illustrates only one particular way, among several possible, of making the device 500 so that it performs certain steps of the generation of the and/or the method of demodulating the (according to any of the embodiments and/or variants described above in relation to the And ). Indeed, these steps can be carried out either on a reprogrammable calculation machine (a PC computer, a DSP processor or a microcontroller) executing a program comprising a sequence of instructions, or on a dedicated calculation machine (for example a set of logic gates like an FPGA or an ASIC, or any other hardware module).

In the case where the device 500 is produced with a reprogrammable calculating machine, the corresponding program (that is to say the sequence of instructions) could be stored in a removable storage medium (such as for example a CD- ROM, a DVD-ROM, a USB key) or not, this storage medium being partially or totally readable by a computer or a processor.

In some embodiments, device 500 is included in base station 110.

In some embodiments, device 500 is included in object 100.

In some embodiments, the device 500 is included in radio network monitoring equipment.

Claims (10)

Method for generating a signal comprising a first portion conveying data and a second portion conveying a synchronization word of said signal, said first portion comprising at least one chirp among M chirps, an s-th chirp among said M chirps being associated with a modulation symbol of rank s of a constellation of M symbols, s being an integer from 0 to M-1, said s-th chirp resulting from a modulation of a base chirp whose instantaneous frequency varies between a first instantaneous frequency and a second instantaneous frequency during a symbol time T, said modulation corresponding, for said modulation symbol of rank s, to a circular permutation of the variation pattern of said instantaneous frequency over said symbol time T, obtained by a time shift of s times an elementary time duration Tc, such that M*Tc=T, said method comprising:
- a generation (E310) of said first portion of said signal comprising, for the generation of a given chirp among said at least one chirp, a modulation (E310b) of said basic chirp as a function of a corresponding given modulation symbol generating said given chip,
characterized in that it comprises:
- a generation (E320) of said second portion of said signal comprising an amplitude modulation (E320a) of said second portion of said signal, an instantaneous amplitude of said second portion of said signal being proportional to a module of said synchronization word.
Generation method according to claim 1 wherein, said first portion comprising a temporal succession of chirps among said M chirps, said temporal succession of chirps comprising said given chirp, said generation of said first portion comprises:
- a differential encoding (E310a) between, on the one hand, a modulation symbol associated with a chirp preceding said given chirp in said temporal succession of chirps and, on the other hand, a given information symbol of said constellation of M symbols, said differential encoding delivering said given modulation symbol.
Method of demodulating a signal comprising:
- a first portion of said signal comprising at least one chirp among M chirps, an s-th chirp among said M chirps being associated with a modulation symbol of rank s of said constellation of M symbols, s being an integer from 0 to M- 1, said s-th chirp resulting from a modulation of a base chirp whose instantaneous frequency varies between a first instantaneous frequency and a second instantaneous frequency during a symbol time T, said corresponding modulation, for said rank modulation symbol s, to a circular permutation of the pattern of variation of said instantaneous frequency over said symbol time T, obtained by a time shift of s times an elementary time duration Tc, such that M*Tc=T; And
- a second portion of said signal, an instantaneous amplitude of said second portion being proportional to a module of a synchronization word of said signal,
characterized in that it comprises:
- an estimate (E410) of time synchronization information of said signal implementing a correlation function between, on the one hand, an instantaneous amplitude of said signal and, on the other hand, of said modulus of said synchronization word of said signal; And
- for a given portion of said first portion representative of a given chirp among said at least one chirp, said given portion being determined as a function of said time synchronization information, a demodulation (E420) of said given portion delivering an estimate of a modulation symbol of said constellation of M symbols associated with said given chirp.
Demodulation method according to claim 3, wherein said first portion comprises a temporal succession of chirps among said M chirps, said method comprising:
- a differential decoding (E430) between, on the one hand, said estimate of said modulation symbol associated with said given chirp and, on the other hand, an estimate of a modulation symbol previously obtained by implementing said demodulation applied to another given portion of said first portion of said signal representative of a chirp preceding said given chirp in said temporal succession of chirps, said differential decoding delivering a decoded symbol, an estimate of an information symbol conveyed by said signal being a function of said decoded symbol.
Generation method according to Claim 1 or 2, or demodulation method according to Claim 3 or 4, in which the said synchronization word comprises successive terms, a value of each of the said terms belonging to the group comprising a first zero value and a second value predetermined non-zero.
Generation method according to any one of claims 1, 2 or 5, or demodulation method according to any one of claims 3, 4 or 5, in which said synchronization word comprises N _bp = 2 ⁿ − 1 successive terms, an autocorrelation function of said synchronization word having a main lobe and at least one secondary lobe, a ratio between an amplitude of said main lobe and an amplitude of a secondary lobe of maximum amplitude among said at least one secondary lobe being greater than or equal to N _bp / (2 ^(n+2)/2 + 1).
Generation method according to Claim 6, or demodulation method according to Claim 6, in which the said synchronization word belongs to a plurality of synchronization words of N _bp = 2 ⁿ − 1 successive terms, a cross-correlation function between, d on the one hand, said synchronization word and, on the other hand, another synchronization word of said plurality of synchronization words having a main lobe and at least one secondary lobe, a ratio between an amplitude of said main lobe and an amplitude of a secondary lobe of maximum amplitude among said at least one secondary lobe being greater than or equal to N _bp /(2 ^(n+2)/2 +1).
Computer program product comprising program code instructions for implementing the method according to any one of claims 1 to 7, when said program is executed on a computer.
Device (500) for generating a signal of a signal comprising a first portion conveying data and a second portion conveying a synchronization word of said signal, said first portion comprising at least one chirp among M chirps, an s-th chirp among said M chirps being associated with a modulation symbol of rank s of a constellation of M symbols, s being an integer from 0 to M-1, said s-th chirp resulting from a modulation of a base chirp whose an instantaneous frequency varies between a first instantaneous frequency and a second instantaneous frequency during a symbol time T, said modulation corresponding, for said modulation symbol of rank s, to a circular permutation of the variation pattern of said instantaneous frequency over said symbol time T , obtained by a time shift of s times an elementary time duration Tc, such that M*Tc=T, said device comprising a reprogrammable computing machine (502) or a dedicated computing machine ie configured to perform:
- a generation of said first portion of said signal comprising, for the generation of a given chirp among said at least one chirp, a modulation of said basic chirp as a function of a corresponding given modulation symbol generating said given chirp,
characterized in that said reprogrammable computing machine (502) or said dedicated computing machine is configured to perform:
- a generation of said second portion of said signal comprising an amplitude modulation of said second portion of said signal, an instantaneous amplitude of said second portion of said signal being proportional to a module of said synchronization word.
Device (500) for demodulating a signal from a signal comprising:
- a first portion of said signal comprising at least one chirp among M chirps, an s-th chirp among said M chirps being associated with a modulation symbol of rank s of said constellation of M symbols, s being an integer from 0 to M- 1, said s-th chirp resulting from a modulation of a base chirp whose instantaneous frequency varies between a first instantaneous frequency and a second instantaneous frequency during a symbol time T, said corresponding modulation, for said rank modulation symbol s, to a circular permutation of the pattern of variation of said instantaneous frequency over said symbol time T, obtained by a time shift of s times an elementary time duration Tc, such that M*Tc=T; And
- a second portion of said signal, an instantaneous amplitude of said second portion being proportional to a module of a synchronization word of said signal,
characterized in that it comprises a reprogrammable computing machine (502) or a dedicated computing machine configured to perform:
- an estimate of time synchronization information of said signal implementing a correlation function between, on the one hand, an instantaneous amplitude of said signal and, on the other hand, of said modulus of said synchronization word of said signal; And
- for a given portion of said first portion representative of a given chirp among said at least one chirp, said given portion being determined as a function of said time synchronization information, a demodulation of said given portion delivering an estimate of a symbol of modulation of said constellation of M symbols associated with said given chirp.