METHOD FOR GENERATING A MULTI-BEARER REFLECTOMETRY SIGNAL THAT HAS THE FORM OF A CHIRP SIGNAL

Description

[0001] The invention relates to the field of wired diagnostic systems based on the principle of reflectometry for identifying and characterizing electrical faults on cables or, more generally, transmission lines. More specifically, the invention relates to the field of multi-carrier OMTDR (Orthogonal Multi-Tone Time Domain Reflectometry), which uses multi-carrier signals generated using OFDM (Orthogonal Frequency Division Multiplexing) modulation.

[0002] The invention aims to propose a method for generating an OMTDR reflectometry signal having a "chirp" signal form, such a signal exhibiting advantageous properties in terms of peak power to average power ratio and in terms of compressibility, this signal having a parsimonious character.

[0003] Cables are ubiquitous in all electrical systems, for power supply or data transmission. These cables are subject to stress and can fail. It is therefore essential to be able to analyze their condition and provide information on detecting faults affecting them, including the existence of faults, their location, and their type. Fault analysis helps with cable maintenance. Standard reflectometry methods allow for this type of analysis.

[0004] Reflectometry methods use a principle similar to that of radar: an electrical signal, the probe signal or reference signal, is injected at one or more points in the cable being tested. The signal propagates through the cable or cable network and reflects some of its energy back when it encounters an impedance discontinuity. An impedance discontinuity can result, for example, from a branch connection, the end of the cable, a fault, or more generally, a disruption in the signal propagation conditions within the cable. It results from a fault that locally alters the characteristic impedance of the cable, causing a discontinuity in its linear parameters.

[0005] Analyzing the signals returned to the injection point allows us to deduce information about the presence and location of these discontinuities, and therefore any potential defects. An analysis in the time domain or frequency domain is usually performed. These methods are designated by the acronyms TDR (Time Domain Reflectometry) and FDR (Frequency Domain Reflectometry).

[0006] Multi-carrier reflectometry (MCR) uses multi-carrier signals. Its advantage lies in the great flexibility with which the spectrum of the emitted signal can be modulated, thus allowing adaptation to constraints specific to online diagnostics. For example, if transmission is prohibited on a frequency band located in the middle of the test signal spectrum, it is entirely possible to cancel the signal energy on that frequency band. Multi-carrier time-domain reflectometry (MCTDR) is also known, as described in the referenced document [1].

[0007] Orthogonal Multi-tone Time Domain Reflectometry (OMTDR), based on the OFDM principle, uses a multi-carrier signal (as with MCTDR), but adds modulation, such as PSK phase modulation or QAM phase and amplitude modulation, to the transmitted signal. Thus, OMTDR signals allow both the diagnosis of faults in a cable and the transmission of information via the modulated signal.

[0008] The invention falls within the scope of wire diagnostic methods using reflectometry and is applicable to all types of electrical cables, particularly power transmission or communication cables, in fixed or mobile installations. The cables in question may be coaxial, twin-wire, parallel-line, twisted-pair, or other, provided that it is possible to inject a reflectometry signal at a point along the cable and measure its reflection at the same point or at another point.

[0009] A primary challenge in a wired diagnostic system is the attenuation experienced by the signal injected into the cable being analyzed as it propagates along the cable until it encounters a fault that causes a reflection. The reflection peak is typically determined by cross-correlation between the injected and reflected signals. When the cable is long relative to the signal's wavelength, the signal undergoes attenuation during both propagation and backpropagation, with the attenuation being a function of the distance traveled. This attenuation presents a significant drawback during the analysis of reflected signals using time-domain reflectometry (TDR), which aims to identify an amplitude peak in the cross-correlation between the emitted and reflected signals. Indeed, the greater the attenuation, the more difficult it becomes to detect the signature of a fault in the reflected signal measurement.This is all the more true when the defect in question is a non-obvious defect, that is to say, one which corresponds to a small variation in impedance, that is to say, a superficial defect.

[0010] To limit signal attenuation during propagation in a cable, it is therefore desirable to use an amplifier to amplify the signal before injection, in order to compensate for the effects of attenuation.

[0011] However, signal amplifiers exhibit non-linear behavior, leading to saturation at high values ​​of the signal being amplified. This non-linear behavior is even more pronounced when the signal has a high peak-to-average power ratio (PAPR). This is the case for multi-carrier signals such as OFDM (Orthogonal Frequency Division Multiplexing) signals, or signals specifically designed for reflectometry, such as MCTDR (Multi-Carrier Time Domain Reflectometry) or OMTDR (Orthogonal Multi-tone Time Domain Reflectometry) signals, which are based on the OFDM principle.

[0012] This non-linear behavior degrades the signal-to-noise ratio of the measurements taken, which has a detrimental effect on the accuracy of fault detection, or even a failure to detect faults in some cases, particularly for the case of non-clear faults.

[0013] Therefore, there is a need to reduce or limit the peak power of multi-carrier signals intended for use by a reflectometry system.

[0014] A second problem to solve concerns the broadening of the signal's spectral bandwidth, which is directly related to the spatial resolution with which a defect can be detected. In other words, the higher the signal's frequency bandwidth, the more precisely a defect, even a very small one, can be located. However, a reflectometry system is limited by the sampling frequency of the analog-to-digital converter. Indeed, the higher this frequency, the more expensive the converter is to implement, the more energy it consumes, and the higher its noise level. Furthermore, it is theoretically impossible to properly sample a signal with a frequency higher than the sampling frequency (Nyquist theorem).

[0015] One solution to overcome this problem and to use wideband signals while maintaining an analog-to-digital converter with a limited sampling rate is to use sparse signals. These sparse signals are inherently compressible within a specific domain defined by a database, such as the time, frequency, or time-frequency domain, and allow for compressed sampling at a sampling rate lower than the signal's maximum frequency. However, OMTDR signals are, by nature, non-sparse and therefore not compressible.

[0016] One objective of the invention is to solve the aforementioned problems in order to design OMTDR reflectometry signals that are parsimonious and have a reduced crest factor.

[0017] The Applicant's patent application FR3097971 describes a method for generating an OMTDR signal with a reduced peak-to-average power ratio compared to a conventional OMTDR signal. The method is based on subcarrier reservation. It has the following drawbacks: it requires adapting the transmission chain by adding further processing to modify the signal structure, thus complicating the method. Furthermore, the resulting signal is not parsimonious and therefore not compressible. It does not allow operation with an analog-to-digital converter with a low sampling rate.

[0018] Publication [1] describes an MCTDR reflectometry method comprising a compressed acquisition system that reduces the sampling frequency of the analog-to-digital converter to a frequency lower than the maximum signal frequency. However, the described method is not compatible with an OMTDR signal.

[0019] The publication "Using quadrature modulation for precise fault location over wired communication channels" by Christophe Layer et al, 2017 15th IEEE International new circuits and systems conference describes another known reflectometry method.

[0020] The invention proposes a new method for generating an optimized OMTDR signal in which the phases of the modulated symbols are determined according to a particular distribution law which has the effect of modifying the shape of the signal so that it is of the type of a "chirp" signal.

[0021] A chirp signal is a sinusoidal signal with a variable frequency. In other words, it is a frequency-modulated pseudo-periodic signal whose frequency evolves over time according to a predetermined evolution law, such as a linear or logarithmic law. By construction, a chirp signal has a sparse structure in the time-frequency plane and also exhibits a low crest factor.

[0022] Thus, the invention makes it possible to generate an OMTDR signal in the form of a "chirp" signal, which makes it possible to solve the problems identified above.

[0023] The invention relates to a method for generating a multi-carrier reflectometry signal intended to be injected into a transmission line to identify the presence of at least one possible fault on the line, the method comprising the steps of: Generate, in the frequency domain, a phase-modulated multi-carrier digital signal by: i. Generating a digital signal, ii. Modulating the digital signal in phase using a constellation of symbols, the phase θ k of each modulated symbol being determined from the following equality: θ k = ±( α * k 2< ) , where k is the index of each carrier of the signal and varies between 1 and the number of carriers N of the signal, iii. α is a coefficient taken from the interval π N − π 2 N ; π N + π 2 N iv. Rounding the phase θ k obtained at the phase of the symbol of the nearest constellation, Convert the generated signal into the time domain.

[0024] According to one particular aspect of the invention, the phase θ k of each modulated symbol is determined from the following equality; θ k = ± α ∗ k 2 + γ π N k + δ , Or γ is a relative integer and δ is a constant.

[0025] According to a particular aspect of the invention, the symbol constellation is a constellation of a PSK phase modulation or a QAM phase and amplitude modulation.

[0026] According to a particular aspect of the invention, the generated signal is a frequency-modulated pseudo-periodic signal of the "chirp" type.

[0027] In one embodiment, the method according to the invention further comprises the steps of: Convert the generated digital signal into an analog signal, inject the analog signal into a transmission line.

[0028] The invention also relates to a method for identifying the presence of at least one possible defect on a transmission line, the method comprising the steps of: Generate a multi-carrier reflectometry signal and inject it into a transmission line using the method according to the invention, Acquire and analyze the echo of said reflected reflectometry signal to deduce information relating to the detection and / or localization of characteristic impedance discontinuity of at least one fault.

[0029] According to one particular aspect of the invention, the step of analyzing the reflectometry signal echo comprises the substeps of: calculate the cross-correlation between the reflected reflectometry signal and the reflectometry signal injected into the line, to obtain a reflectogram, analyze the reflectogram to identify at least one peak of amplitude characteristic of the presence of a fault on the line.

[0030] According to one particular aspect of the invention, the step of analyzing the reflectometry signal echo comprises the substeps of: Multiply the acquired reflectometry signal by a pseudo-random sequence, apply a low-pass filter to the resulting signal, convert the filtered signal digitally, and reconstruct the signal.

[0031] The invention also relates to a computer program comprising instructions for executing the method of generating a reflectometry signal according to the invention, when the program is executed by a processor.

[0032] The invention also relates to a processor-readable recording medium on which is recorded a program containing instructions for executing the method of generating a reflectometry signal according to the invention, when the program is executed by a processor.

[0033] The invention further relates to a device for generating a reflectometry signal intended to be injected into a transmission line to identify the presence of at least one possible fault on the line, said device comprising means adapted to implement the method of generating a reflectometry signal according to the invention.

[0034] The device according to the invention may include a reflectometry signal generator configured to implement the method of generating a reflectometry signal according to the invention, a digital-to-analog converter (DAC) and a coupling device for injecting the analog reflectometry signal into a transmission line.

[0035] Other features and advantages of the present invention will become more apparent from the following description in relation to the following attached drawings. There Fig. 1 represents a diagram illustrating the known principle of time-domain reflectometry and its application to the detection of a non-obvious defect. Fig. 2 This represents an example of two reflectograms illustrating the appearance of the signature of a non-obvious defect, one obtained by injecting a signal with high-frequency components, the other by injecting a signal with low-frequency components. Fig. 3 represents a diagram of an OMTDR reflectometry system according to the prior art, The Fig. 4 illustrates, in several diagrams, the characteristics of an OMTDR signal according to the prior art, The Fig. 5 represents a flowchart detailing the implementation steps of the method for generating an optimized OMTDR signal according to an embodiment of the invention, The Fig. 6 represents two phase-modulated symbol constellation diagrams corresponding to two different stages of the invention, The Fig. 7 illustrates, in several diagrams, the characteristics of an example of an OMTDR signal obtained using the method according to the invention, The Fig. 8 represents a comparative reflectogram illustrating the results obtained with a conventional OMTDR signal and an OMTDR signal generated with the invention, The Fig. 9 It represents, on several diagrams, certain characteristics of a classic OMTDR signal allowing its compressibility to be evaluated. Fig. 10 represents the same diagrams as in the figure 9 but for an OMTDR signal obtained from an embodiment of the invention, The Fig. 11 represents a diagram of a compressed acquisition reflectometry system according to a second embodiment of the invention.

[0036] There figure 1 This diagram illustrates, as a reminder, the operating principle of a reflectometry diagnostic method applied to a transmission line L exhibiting a non-obvious fault (DNF). The example described below corresponds to a time-domain reflectometry method.

[0037] A reference signal S is injected into the transmission line at a point P. The reflected signal R is measured at the same point P (or at another point on the line). This signal propagates along the line and, during its propagation, encounters a first impedance discontinuity at the input of the non-bright fault DNF. The signal is reflected at this discontinuity with a reflection coefficient Γ₁. If the characteristic impedance Z c 2 in the non-obvious fault zone DNF is less than the characteristic impedance Z c1 before the appearance of the defect, then the reflection coefficient Γ 1 is negative and results in a negative amplitude peak in the reflected signal R. In the opposite case, the reflection coefficient Γ 1 is positive and results in a positive amplitude peak in the reflected signal R.

[0038] The transmitted portion T of the incident signal S continues to propagate in the line and then encounters a second impedance discontinuity, creating a second reflection of the incident signal with a reflection coefficient Γ2 of opposite sign to the first reflection coefficient Γ1. If Γ1 < 0 then Γ2 > 0. If Γ1 > 0 then Γ2 < 0.

[0039] Thus, by observing the reflected signal R, the signature of the non-obvious defect (DNF) is characterized by two successive peaks of opposite signs, as shown in the figure 2 .

[0040] There figure 2 represents a time reflectogram which corresponds either directly to the measurement of the reflected signal R, or to the cross-correlation between the reflected signal R and the signal injected into the cable S.

[0041] In the case where the injected reference signal is a time pulse, which corresponds to the case of a time-domain reflectometry method, the reflectogram can directly correspond to the measurement of the reflected signal R. In the case where the injected reference signal is a more complex signal, for example for methods of type MCTDR (Multi Carrier Time Domain Reflectometry) or OMTDR (Orthogonal Multi-tone Time Domain Reflectometry), then the reflectogram is obtained by inter-correlating the reflected signal R and the injected signal S.

[0042] On the figure 2 Two reflectograms, 201 and 202, are shown, corresponding to signals with two different maximum frequencies. Curve 201 corresponds to a pulse duration 2ΔT much greater than the signal's transit time through the non-obvious fault (DNF). Since the fault length is denoted Ld, this duration is equal to Ld / V, where V is the signal propagation speed in the cable. Curve 202 corresponds to a pulse duration 2ΔT much shorter than the signal's transit time through the non-obvious fault (DNF).

[0043] In both cases, the 203 signature of the non-obvious defect, in the reflectogram, is always composed of the succession of a first peak and a second peak whose signs are reversed.

[0044] The distance between the two peaks represents the length of the non-obvious defect, and their amplitude represents the severity of the non-obvious defect. Indeed, the greater the variation in characteristic impedance, the greater the amplitude of the non-obvious defect signature in the reflectogram.

[0045] As is known in the field of reflectometric diagnostic methods, the DNF position of the non-obvious fault on the cable, in other words its distance from the signal injection point P, can be directly obtained from the measurement on the time-domain reflectogram of the figure 2 , of the duration t DNF between the first amplitude peak recorded on the reflectogram (at abscissa 0.5 in the example of the figure 2 ) and the peak amplitude 203 corresponding to the signature of the non-frank default.

[0046] Several known methods can be used to determine the position ΔDNF. One method involves applying the relationship between distance and time: ΔDNF = VtDNF / 2, where V is the signal propagation speed in the cable. Another possible method is to apply a proportionality relationship of the type ΔDNF / tDNF = L / t₀, where L is the cable length and t₀ is the time interval, measured on the reflectogram, between the amplitude peak corresponding to the impedance discontinuity at the injection point and the amplitude peak corresponding to the signal reflection at the cable end.

[0047] There figure 3 represents a diagram of a 300 fault analysis system in a transmission line L, such as a cable or wire network, implementing an OMTDR reflectometry method according to the prior art.

[0048] Such a 300 system includes a GEN generator of a digital reference signal. The signal is modulated via a MOD phase modulator using PSK (phase shift keying). PSK modulation is associated with a CS symbol constellation. For example, the figure 3 A constellation of 16PSK modulation is shown. The bits of the digital signal are randomly associated with the symbols of the PSK constellation. In other words, the modulated symbols are, for example, directly generated randomly to construct the modulated digital signal. The modulated signal is then synthesized in the time domain using an Inverse Discrete Fourier Transform (IDFT) module. The synthesized signal is then converted to analog via a Digital-to-Analog Converter (DAC) and injected at a point on the transmission line L using a coupler or any other device that allows a signal to be injected into a line. The signal propagates along the line and is reflected off any singularities it contains. In the absence of faults on the line, the signal is reflected off the end of the line if the line termination is unmatched.In the presence of a fault on the line, the signal is reflected off the impedance discontinuity caused by the fault. The reflected signal is backpropagated to a measurement point, which may be the same as the injection point or different. The backpropagated signal is converted digitally by an analog-to-digital converter (ADC). A correlation coefficient (COR) is then performed between the measured digital signal and a copy of the digital signal generated before injection to produce a time-domain reflectogram R(t) corresponding to the cross-correlation between the two signals.

[0049] An OMTDR signal is based on OFDM technology and consists of using orthogonal frequency subcarriers to form the test signal.

[0050] Each amplitude, phase, or frequency is used to encode a certain number of bits called a symbol. Binary data can be random or carry an informational message.

[0051] The amplitude and phase of an OFDM subcarrier are fixed by the binary data to be transmitted according to the type of modulation chosen (M-PSK or M-QAM).

[0052] An example of a possible modulation for an OMTDR signal is M-PSK modulation because it exhibits good autocorrelation properties due to the fact that the signal spectrum is flat.

[0053] In phase-shift keying (PSK), M is the modulation order (4 for Q-PSK, 8 for 8-PSK, 16 for 16-PSK, etc.) and each subcarrier Sk is defined by its amplitude and phase as follows: s k = 1 ∀ f n et ϕ k = ϕ n = i 2 π M where i is between 0 and M - 1.

[0054] As with any digital modulation technique, the phase in question can only take on a finite number of values. Each phase value represents a unique binary number (also called a symbol), whose size (and therefore the amount of information transmitted) depends on the number of possible phase values. Generally, for a given PSK modulation, the binary numbers represented are all of the same size.

[0055] For example, for an 8-PSK modulation, the sequence of digital data to be sent: 000 001 011 010 101 corresponds to the sequence of symbols: 5 4 3 2 7 and to the sequence of phases 5π / 8,4π / 8, 3π / 8, 2π / 8, 7π / 8, according to an implementation example.

[0056] An OMTDR signal with randomly chosen symbols (and therefore phases) generally exhibits a poor ratio between peak power and average PAPR power. Furthermore, such a signal is incompressible in the time, frequency, and time-frequency planes, as illustrated in the... figure 4 .

[0057] The diagrams of the figure 4 represent different characteristics of a classic OMTDR signal modulated with 16-PSK modulation.

[0058] THE figures 4a) et 4b ) correspond respectively to the magnitude of the frequency response of the signal and the 16-PSK constellation diagram of the signal with randomly generated phases.

[0059] There figure 4c ) represents the time response of the signal and the figure 4d ) represents the signal distribution in the time-frequency plane, obtained by applying a Wigner-Ville transform to the signal.

[0060] We observe that the signal is dense in the time-frequency plane, in other words, it is not sparse. Such a signal is therefore not compressible.

[0061] The invention aims to propose a new method for generating an OMTDR signal that allows the construction of a signal having a "chirp" shape while respecting the constraints of the modulation adopted.

[0062] There figure 5 diagram, on a flowchart, the steps of implementation of the method according to the invention for an example in which the modulation used is an MPSK modulation.

[0063] At step 501, a digital signal in the form of a sequence of bits is generated.

[0064] The digital signal is modulated by MPSK modulation by associating each digital symbol with an MPSK constellation point.

[0065] To do this, in step 502, a phase is determined for each symbol to be modulated, using the following expression: θ k = ± α ∗ k 2 + γ π N k + δ k is the index of a signal carrier, k ranging from 1 to N, where N is the number of subcarriers. α is a coefficient taken from the interval π N − β ; π N + β , with β = π 2 N γ is a real number, positive, negative or zero, δ is a constant.

[0066] Relationship (1) has the effect, in particular, of generating a signal having the structure of a "chirp" signal.

[0067] However, the phases thus generated do not necessarily correspond to symbols of the chosen MPSK constellation.

[0068] At step 503, the phases generated at step 502 are rounded to the phases of the nearest MPSK constellation symbols.

[0069] Finally, in step 504, an inverse discrete Fourier transform step is applied to generate the OMTDR signal.

[0070] The choice of the coefficient α allows the signal to be structured so that it has the form of a chirp signal. In particular, a coefficient α chosen in the vicinity of π N , for example in the interval π N − β ; π N + β , with β = π 2 N allows obtaining a signal with a chirp signal shape while minimizing the peak factor (PAPR).

[0071] The coefficients γ and δ influence the reduction of the peak factor (PAPR) of the signal.

[0072] According to a first embodiment of the invention, the coefficient α is taken to be equal to − π N , the coefficient γ is taken to be equal to 1 and the coefficient δ is taken to be equal to 0.

[0073] In other words, in this example, the phases are generated using the following relationship: θ k = − π N k 2 + π N k

[0074] There figure 6 illustrates, on phase diagram 601, the phases generated using relation (2) (step 502) and on diagram 602, the symbols of the 16 PSK constellation obtained by rounding the phases of diagram 601.

[0075] There figure 7 represents, on the same diagrams as in the figure 4 , the characteristics of the OMTDR signal obtained with the first embodiment of the invention above.

[0076] We observe that the signal's representation in the time-frequency plane (diagram d) follows a linear evolution, which is characteristic of a chirp signal. This property allows us to obtain a sparse, and therefore compressible, signal with a reduced peak PAPR factor.

[0077] As an illustration, the following table 1 represents examples of peak power to average power ratio (PAPR) results for different PSK modulations and different numbers N of carriers, for a classic OMTDR signal.

[0078] The following table 2 represents the same results for an OMTDR signal obtained with the first embodiment of the invention.

[0079] We can see that the PAPR crest factor is greatly reduced thanks to the invention.

[0080] Other examples of phase determination laws achieve the same technical effects, for example the following laws: θ k = π N k 2 − 2 π N k + π N θ k = − π N k 2 θ k = π N k 2 + π N θ k = π N − 1 k 2 − 3 π N − 1 k + 2 π N − 1

[0081] The invention is applicable to M-PSK phase modulations but also to M-QAM phase and amplitude modulations.

[0082] According to a first embodiment, the invention can be implemented using the device of the figure 1 , the OMTDR digital signal generator being replaced by a generator configured to execute the method of constructing a modified OMTDR signal according to the invention.

[0083] There figure 8 represents two examples of reflectograms 801,802 respectively obtained with a prior art OMTDR system and an OMTDR system according to the first embodiment of the invention.

[0084] In both examples, an electrical fault is located 30 m from the signal injection point. This fault is characterized by a peak of 803 in the measured reflectogram. This can be seen on the figure 8 The OMTDR signal improved according to the invention amplifies peak 803 compared to the conventional OMTDR signal. Due to the improved PAPR crest factor of the signal, the detection gain of peak 803 is enhanced.

[0085] The invention thus makes it possible to reduce the ratio between peak power and average power of an OMTDR signal in order to improve the accuracy of fault detection by analysis of a reflectogram.

[0086] The invention also makes it possible to make an OMTDR signal parsimonious and compressible in the time-frequency domain.

[0087] There figure 9 illustrates, on three diagrams, the fact that a classic OMTDR signal with random generation of modulated symbols is neither parsimonious nor compressible.

[0088] A signal is compressible in a domain if the magnitudes of its coefficients sorted in that domain decrease rapidly. Subsequently, we consider the domain of the Discrete Cosine Chirp Transform (DCCT), which is used as a basis for representing sparse signals as indicated in reference [1].

[0089] There figure 9a ) illustrates the time-frequency representation of a classic OMTDR signal and shows that this signal is dense in the time-frequency plane.

[0090] There figure 9b ) is a diagram of the representation of the same OMTDR signal after application of a DCCT transform. It can also be noted that the distribution of the DCCT coefficients is dense and not sparsiminal.

[0091] Finally, the figure 9c ) represents a curve of the moduli of the DCCT coefficients obtained at the figure 9b (sorted in descending order) We can see that these coefficients have a relatively slow decrease towards zero. The distribution of the coefficients is spread over a wide range of values.

[0092] Thus, a classic OMTDR signal is not parsimonious and cannot be compressed in the DCCT domain.

[0093] There figure 10 illustrates the same diagrams as in the figure 9 but this time for an OMTDR signal generated using the invention.

[0094] There figure 10a ) shows that the time-frequency representation of the signal follows a linear evolution.

[0095] THE figures 10b) et 10c ) shows that the distribution of the magnitudes of the DCCT coefficients of the signal is sparse, that is, a large number of the coefficients are close to zero.

[0096] The OMTDR signal generated using the invention is therefore compressible in the DCCT domain.

[0097] Document [1] describes in detail a method for reducing the sampling frequency of an analog-to-digital converter in a reflectometry system when the signal used is compressible in the DCCT domain.

[0098] There figure 11 represents a diagram of a reflectometry system 110 according to a second embodiment of the invention.

[0099] The system 110 includes a module 100 for generating an OMTDR signal according to the invention comprising at least a module 101 for generating phases and a module 102 for rounding the generated phases to the symbols closest to the chosen constellation.

[0100] System 110 then includes a MOD modulator, an IDFT inverse Fourier transform module, and a DAC digital-to-analog converter. These three elements are identical to those already described in the figure 1 .

[0101] System 110 differs from the reflectometry system described in the figure 1 in that the analog-to-digital converter (ADC) is replaced by a compressed acquisition system (ACQ) which exploits the sparse nature of the generated OMTDR signal.

[0102] The ACQ system is described in detail in document [1] to which the person skilled in the art may refer for further information regarding its implementation.

[0103] The ACQ system includes a multiplier or mixer MUL which multiplies the signal measured at the output of the coupler with a pseudo-random sequence of + / -1 values. The mixer MUL operates at a frequency fp which may be higher than the Nyquist frequency fs.

[0104] The ACQ system then includes a low-pass FIL filter whose cutoff frequency depends on the desired compression factor, and then an analog-to-digital converter ADC that operates at a sampling frequency fm that is lower than the frequency fp of the signal.

[0105] An RS signal reconstruction module allows the signal to be reconstructed before performing the COR cross-correlation with a copy of the generated signal.

[0106] Various signal reconstruction techniques can be implemented by the RS module. One possible implementation involves applying a greedy algorithm to iteratively construct a sparse approximation of the signal. An example of a greedy algorithm is the Orthogonal Matching Pursuit algorithm described in reference [2]. Références

[0107] [1] “Ajamian, T. (2019). Exploration of Compressive Sampling for Wire Diagnosis Systems Based on Reflectometry (Doctoral dissertation, École centrale de Nantes) » [2] Joel A Tropp and Anna C Gilbert, "Signal recovery from random measurements via orthogonal matching pursuit", in: IEEE Transactions on information theory 53.12 (2007), pp. 4655-4666.

Claims

1. A method for generating a multi-carrier reflectometry signal intended to be injected into a transmission line to identify the presence of at least one possible fault on the line, the method comprising the steps of: - generating, in the frequency domain, a phase-modulated multi-carrier digital signal by: i. generating (501) a digital signal, ii. phase-modulating (502) the digital signal based on a constellation of symbols, the phase θk of each modulated symbol being determined based on the following equation: θk = ± (α * k2), where k is the index of each carrier of the signal and varies between 1 and the number of carriers N of the signal, iii. α is a coefficient within the interval π N − π 2 N ; π N + π 2 N iv. rounding (503) the obtained phase θk to the phase of the symbol of the nearest constellation, - converting (504) the generated signal into the time domain.

2. The method for generating a multi-carrier reflectometry signal according to claim 1, wherein the phase θk of each modulated symbol is determined based on the following equation: θ k = ± α ∗ k 2 + γ π N k + δ, where γ is a relative integer and δ is a constant.

3. The method for generating a multi-carrier reflectometry signal according to any one of the preceding claims, wherein the constellation of symbols is a constellation of a phase modulation PSK or of a phase and amplitude modulation QAM.

4. The method for generating a multi-carrier reflectometry signal according to any one of the preceding claims, wherein the generated signal is a frequency-modulated pseudo-periodic "chirp" signal.

5. The method for generating a multi-carrier reflectometry signal according to any one of the preceding claims, further comprising the steps of: - converting the generated digital signal into an analogue signal, - injecting the analogue signal into a transmission line.

6. A method for identifying the presence of at least one possible fault on a transmission line, the method comprising the steps of: - generating a multi-carrier reflectometry signal and injecting it into a transmission line by way of the method according to claim 5, - acquiring and analysing the echo of said reflected reflectometry signal to deduce therefrom a piece of information relating to the detection and / or the location of an impedance discontinuity which is characteristic of at least one fault.

7. The method for identifying the presence of at least one fault according to claim 6, wherein the step of analysing the echo of the reflectometry signal comprises the substeps of: - computing the intercorrelation between the reflected reflectometry signal and the reflectometry signal injected into the line, to obtain a reflectogram, - analysing the reflectogram to identify at least one amplitude peak which is characteristic of the presence of a fault on the line.

8. The method for identifying the presence of at least one fault according to claim 6, wherein the step of analysing the echo of the reflectometry signal comprises the substeps of: - multiplying the acquired reflectometry signal by a pseudo-random sequence, - applying a low-pass filter to the obtained signal, - converting the digitally filtered signal, - reconstructing the signal.

9. A device (300) for generating a reflectometry signal intended to be injected into a transmission line to identify the presence of at least one possible fault on the line, said device comprising means designed to implement the method for generating a reflectometry signal according to any one of claims 1 to 5.

10. The device (110) for generating a reflectometry signal according to claim 9, comprising a reflectometry signal generator configured to implement the method for generating a reflectometry signal according to any one of claims 1 to 5, a digital-to-analogue converter (DAC) and a coupling device for injecting the analogue reflectometry signal into a transmission line.

11. A computer program comprising instructions which cause the device according to any one of claim 9 or 10 to execute the steps of the method of generating a reflectometry signal according to any one of claims 1 to 4.

12. A processor-readable recording medium on which the computer program according to claim 11 is recorded.