FR3103904A1 - Ambient backscatter localization method and device - Google Patents

Abstract

The invention relates to a method for locating a transmitter device (D_TX) belonging to an ambient backscatter system (10) comprising a source (SO) and a receiver device (D_RX), and comprising also a surface (11) capable of reflecting signals towards the receiver device. Said method comprises steps of:- determining (E30) a difference, called "measured difference" (E_MES), between measurements of a power received by the receiver device when the transmitter device is respectively in a backscatter state and in a non-backscattering state, - determination (E40) of at least one position, called "candidate position" (T_i), of the transmitter device for which a function, evaluating the deviation (E_P) of power received by the receiver device depending on the backscattering or non-backscattering state, is equal to said measured difference, - determination (E50) of a position, called "final position" (T_FIN), as a function of said at least one candidate position. Figure for abstract: Fig. 4

Description

Ambient backscatter localization method and device

The present invention belongs to the general field of telecommunications. It relates more particularly to a method for locating a transmitter device belonging to a communication system by ambient backscatter, as well as an associated locating device. The invention finds a particularly advantageous application, although in no way limiting, for applications of the "Internet of Things" type ("Internet of Things" or IoT in the Anglo-Saxon literature).

Ambient backscatter communication technology is well known today. The technical principles on which this technology is based are described, in particular, in the document: “Ambient Backscatter Communications: A Contemporary Survey”, N. Van Huynh, D. Thai Hoang, X. Lu, D. Niyato, P. Wang , D. In Kim, IEEE Communications Surveys & Tutorials, vol. 20, no. 4, p. 2889-2922, Fourthquarter 2018.

Conventionally, the backscatter of an ambient signal takes place between a transmitting device and a receiving device occupying respective fixed positions.

The ambient signal concerned corresponds to a radio signal emitted, permanently or else recurrently, by at least one source in a given frequency band. For example, it can be a television signal, a mobile phone signal (3G, 4G, 5G), a Wi-Fi signal, a WiMax signal, etc.

To communicate with a receiver device separate from the source, a transmitter device uses the ambient signal to send data to said receiver device. More particularly, the transmitter device reflects the ambient signal towards the receiver device, possibly by modulating it. The signal thus reflected is called a “backscattered signal”, and is intended to be decoded by the receiver device (i.e. the receiver device extracts from the backscattered signal information transmitted by the transmitter device, for example in the form of bits).

The fact that no additional radio wave (in the sense of a wave other than that coming from the ambient signal) is emitted by the transmitting device makes the ambient backscatter technology particularly attractive. Indeed, the energy cost of a communication is thus optimized, which is particularly important in the current context of the IoT where each object of everyday life is intended to become a communicating object, and where many IoT applications , deployed both in an industrial and domestic context, offer location-based services based on the location of objects.

Thus, we know a localization solution based on the exploitation of communication technology by ambient backscatter, via the emission of an ambient signal of the W-Fi type, and described in the document: “Localizing Low-power Backscatter Tags Using Commodity Wifi”, M. Kotaru, P. Zhang, S. Katti, CoNext’17. This solution is based on the exploitation of an ambient Wi-Fi signal backscattered by a transmitter device, the backscattered signal being received and decoded by a plurality of receiver devices. More particularly, said receiver devices are configured to determine the angles at which the backscattered signal reaches them respectively, as well as the corresponding arrival times. Based on the angles and times of arrival thus determined, it is possible to locate the transmitting device by implementing a triangulation algorithm.

Such a localization solution thus takes advantage of the advantages of ambient backscatter in that the transmitter device does not need to be equipped with a dedicated Wi-Fi transmitter that consumes a lot of energy.

However, and despite a simplified implementation at the level of the transmitter device, the complexity and cost of implementing this location solution by ambient backscattering still remains high. In fact, to collect data useful for location estimates by triangulation but also to achieve an acceptable location accuracy (ie an accuracy of the order of a meter), this solution requires the use of at least three receiver devices.

The present invention aims to remedy all or part of the drawbacks of the prior art, in particular those set out above, by proposing a solution which makes it possible to locate a transmitter device configured to backscatter an ambient signal in a simpler, less costly and with better precision than the solutions of the prior art.

To this end, and according to a first aspect, the invention relates to a method for locating a transmitting device belonging to a communication system by ambient backscatter, said system also comprising a transmitting source according to at least one given transmission frequency and a receiver device, said system also comprising a surface capable of reflecting signals originating from the source and/or from the transmitter device towards the receiver device. Said method comprises steps of:
- determination of a difference, called "measured difference", between measurements of a power received by the receiver device when the transmitter device is respectively in a backscattering state and in a non-backscattering state,
- determination of at least one position, called "candidate position", of the transmitter device for which a calculation function is equal to said measured difference, said calculation function evaluating the difference in power received by the receiver device according to whether the transmitter device is in a backscatter or non-backscatter state depending on said transmit frequency, the respective positions of the source and the transmitter and receiver devices, and the influence of the surface on the signals intended to be received by the receiving device,
- determination of a position, called "final position of the transmitting device", as a function of said at least one candidate position.

Thus, according to said location method, it is proposed to locate the transmitter device by initially determining at least one candidate position of said transmitter device.

To determine said at least one candidate position, the location method advantageously relies on said calculation function which makes it possible to evaluate, via an analytical expression that the inventors have been able to establish, said power difference when a surface such that the surface mentioned above is present in the environment of the communication system by ambient backscatter.

Said calculation function is, from a mathematical point of view, a function admitting as arguments a plurality of parameters, including several fixed parameters (respective positions of the source and of the transmitting device, emission frequency of the source) and a variable parameter (position of the transmitting device). Access to such a calculation function is particularly remarkable since it makes it possible to very accurately evaluate variations in the power difference as a function of variations in the position of the transmitting device.

Also, in the context of the present invention, the determination of said at least one candidate position corresponds to the resolution of an equation with a single unknown, namely therefore the position of the transmitter device, the equation in question consisting in equaling the calculation function with the deviation measured by the transmitting device. The location of the transmitter device according to the invention is therefore very simple to implement since it involves acquiring power measurements and then solving an equation with one unknown on the basis of the torque thus acquired. In other words, the location process here makes it possible to avoid the use of a plurality of transmitting devices as well as the implementation of complex calculations (triangulation algorithm) as is proposed by the solutions of the prior art. This results in substantial material savings, and therefore a reduced implementation cost.

Furthermore, and secondly, the location method according to the invention proposes to determine a final position on the basis of said at least one candidate position.

Thus, if a single candidate position is determined, it is of course understood that said final position corresponds to this single candidate position. The inventors have observed that this configuration occurs when the measured difference is substantially equal to an absolute extremum of the calculation function. If, on the other hand, several candidate positions are determined (case where the measured difference is then sufficiently distinct from the absolute extrema of the calculation function), it is proposed to discriminate between them in order to obtain said final position.

In any case, the inventors have observed that the invention is particularly advantageous in that it makes it possible to achieve, for each candidate position, a much better location accuracy than in the state of the art. In addition, and even more advantageously, the maximum distance separating two candidate positions is less than the best location precision achievable by the solutions of the state of the art cited above, and this, whatever the value considered for the measured difference between the absolute extrema of the calculation function. In other words, when several candidate positions are determined, the latter are located in a very restricted area. Ultimately, the final position inherits the excellent results obtained for each candidate position in terms of location accuracy.

Furthermore, the invention is also advantageous in that the communication system comprises said surface. Indeed, the presence of such a surface in the environment of the source and of the transmitter and receiver devices implies that more signals are likely to be conveyed, by reflection on said surface, towards the receiver device. Such provisions contribute to increasing the maximum power difference achievable on the receiving device side in comparison with a configuration where the surface would not be present. In the end, communication between the transmitting device and the receiving device is facilitated since the operation of the latter is conditioned by the reception of a sufficient quantity of power.

The advantages provided by the presence of the surface in the environment of the source and the transmitter and receiver devices are not limited to the increase in the maximum power difference achievable on the receiver device side. Indeed, the influence of the surface on the power received by the receiver device is taken into account by the calculation function, which results in particular in a contribution of a calculation order in said calculation function. This computational contribution is expressed as a function of the parameters of the communication system by ambient backscatter (respective positions of the source and of the transmitter and receiver devices, transmission frequency of the source), which offers the possibility, by varying at least one of these parameters, such as for example the emission frequency of the source, to more finely discriminate between the candidate positions, in comparison with a configuration where the surface would not be present.

Moreover, it should also be noted that the invention not only covers the case where the surface forms an element introduced manually and voluntarily into the environment of the source and the transmitter and receiver devices, but also the case where the surface is already fortuitously present in this environment and deliberately exploited to locate the transmitting device. Thus, the invention can be advantageously adapted to any type of spatial configuration.

In particular modes of implementation, the location method may also include one or more of the following characteristics, taken in isolation or in all technically possible combinations.

In particular modes of implementation, the step of determining the candidate positions is implemented by a location device, the method further comprising a step of obtaining, by the location device, said calculation function.

In particular modes of implementation, when a plurality of candidate positions are determined, the step of determining the final position comprises a set of sub-steps of:
- modification of the emission frequency of the source,
- determination of a measured deviation between other power measurements received by the receiver device when the transmitter device is respectively in a backscattering state and in a non-backscattering state,
- for each of the candidate positions, evaluation, for said modified transmission frequency, of the calculation function so as to obtain a power difference, called "auxiliary difference",
said set of sub-steps being iterated as long as the auxiliary deviations obtained during an iteration differ from the measured deviation determined during said iteration,
the final position being determined as a function of the position or positions respectively associated with the auxiliary deviation or deviations equal to the measured deviation determined during the last iteration of said set of sub-steps.

Such an implementation of the step of determining the final position makes it possible to remove the ambiguity as to the fact that the transmitting device could occupy a plurality of positions corresponding to said candidate positions, advantageously using the fact that the terms involved in the he analytical expression of the calculation function, and representative of the contribution of the surface, are sensitive to a modification of the emission frequency of the source.

These terms also make it possible to obtain, following a modification of the emission frequency of the source, a variation in the power difference greater than if the surface were absent. The fact that this variation is increased in the case where the surface is present contributes to facilitating the discrimination of the auxiliary deviation values between them, which ultimately makes it possible to facilitate the discrimination of the candidate positions between them to determine the final position. In this way, few iterations of said set of sub-steps are necessary, so that obtaining the final position of the transmitter device is very fast.

In particular modes of implementation, the final position is determined equal to the barycenter of said candidate positions.

In particular modes of implementation, two candidate positions are considered to coincide with each other if the distance separating them is less than a given measurement accuracy, and, if there is a plurality of candidate positions not coincident with each other, said final position is therefore determined according to said candidate positions which are not coincident with one another.

Proceeding in this way offers the possibility of reducing the number of candidate positions which should be considered as being able to be occupied by the transmitting device. Correlatively, the computational load linked to the determination of the final position is reduced.

In particular embodiments, the candidate positions are determined by a dichotomy method.

According to a second aspect, the invention relates to a computer program comprising instructions for implementing the location method according to the invention when said program is executed by a computer.

This program may use any programming language, and be in the form of source code, object code, or intermediate code between source code and object code, such as in partially compiled form, or in any other desirable form.

According to a third aspect, the invention relates to a computer-readable information or recording medium on which a computer program according to the invention is recorded.

The information or recording medium can be any entity or device capable of storing the program. For example, the medium may include a storage medium, such as a ROM, for example a CD ROM or a microelectronic circuit ROM, or even a magnetic recording medium, for example a floppy disk or a disk. hard.

On the other hand, the information or recording medium can be a transmissible medium such as an electrical or optical signal, which can be conveyed via an electrical or optical cable, by radio or by other means. The program according to the invention can in particular be downloaded from an Internet-type network.

Alternatively, the information or recording medium may be an integrated circuit in which the program is incorporated, the circuit being adapted to execute or to be used in the execution of the method in question.

According to a fourth aspect, the invention relates to a device for locating a transmitting device belonging to a communication system by ambient backscatter, said system also comprising a transmitting source according to at least one given transmission frequency and a receiving device, said system also comprising a surface able to reflect signals coming from the source and/or from the transmitter device towards the receiver device,
said location device comprising:
- a first determination module, configured to determine a difference, called “measured difference”, between measurements of a power received by the receiver device when the transmitter device is respectively in a backscattering state and in a non-backscattering state ,
- a second determination module, configured to determine at least one position, called "candidate position", of the transmitting device for which a calculation function is equal to said measured difference, said calculation function evaluating the difference in power received by the device receiver according to whether the transmitting device is in a backscattering or non-backscattering state depending on said transmission frequency, the respective positions of the source and the transmitting and receiving devices, as well as the influence of the surface on the signals intended to be received by the receiving device,
- a third determination module, configured to determine a position, called “final position”, as a function of said at least one candidate position.

In particular embodiments, the location device may also include one or more of the following characteristics, taken separately or in all technically possible combinations.

In particular embodiments, the location device further comprises an obtaining module, configured to obtain said calculation function.

In particular embodiments, said location device is included in the source or in the transmitter device or in the receiver device.

According to a fifth aspect, the invention relates to a communication system by ambient backscatter comprising a transmitting source according to at least one given transmission frequency, a transmitting device and a receiving device, said system also comprising a surface capable of reflecting signals from the source and/or the transmitting device to the receiving device,
said source or said transmitter device or said receiver device comprising a location device according to the invention.

According to a sixth aspect, the invention relates to a location system comprising a location device according to the invention, as well as a receiver device for an ambient backscatter communication system intended to comprise a source transmitting according to at least one frequency of given transmission as well as said receiver device and a transmitter device, said system also being intended to comprise a surface able to reflect signals originating from the source and/or from the transmitter device towards the receiver device.

Other characteristics and advantages of the present invention will emerge from the description given below, with reference to the appended drawings which illustrate an exemplary embodiment thereof which is devoid of any limiting character. In the figures:
FIG. 1 schematically represents, in its environment, a particular embodiment of a communication system by ambient backscatter according to the invention;
FIG. 2 corresponds to another representation of the system of FIG. 1, in which the respective positions of a transmitting device, of a receiving device, of a source and of a surface belonging to said system are indicated;
FIG. 3 schematically represents an example of hardware architecture of a location device for implementing a location method according to the invention;
FIG. 4 represents, in the form of a flowchart, a particular mode of implementation of the location method, said method comprising a step of determining a final position of the transmitter device D_TX as a function of candidate positions;
FIG. 5 schematically represents a particular mode of implementation of the step of determining a final position of FIG. 4.

FIG. 1 schematically represents, in its environment, a particular embodiment of a communication system 10 by ambient backscatter according to the invention.

As illustrated by FIG. 1, the communication system 10 comprises a transmitting source SO configured to transmit, according to at least one given transmission frequency F_E included in a frequency band called the "transmission band", a radioelectric signal called “ambient signal”. The emission of the ambient signal is carried out for example permanently or recurrently.

For the remainder of the description, and as illustrated by FIG. 1, the case in which the ambient signal is only emitted by a single source is considered in no way limiting. The choice consisting in considering a single source is made here for the purpose of simplifying the description only. Also, no limitation is attached to the number of sources that can be considered in the context of the present invention, the developments that follow being in fact generalizable without difficulty by those skilled in the art in the case of a plurality of sources that are not consistent between they.

By “radioelectric signal”, reference is made here to an electromagnetic wave propagating by non-wired means, the frequencies of which are included in the traditional spectrum of radioelectric waves (a few hertz to several hundred gigahertz).

By way of non-limiting example, the ambient signal is a 4G mobile telephone signal transmitted by the source SO in the transmission band [811 MHz, 821 MHz], for example according to the transmission frequency F_E equal to 816 MHz , said source SO taking the form of a relay antenna.

It should however be specified that the invention remains applicable to other types of radioelectric signals, such as for example a mobile telephone signal other than 4G (for example 2G, 3G, 5G), a Wi-Fi signal, a WiMax signal , a DVB-T signal, etc. In general, no limitation is attached to the ambient radio signal that can be considered within the scope of the present invention. Consequently, it should be noted that the number of antennas equipping the source SO does not constitute a limiting factor of the invention.

The communication system 10 also comprises a transmitter device D_TX and a receiver device D_RX respectively configured in order to communicate with each other by ambient backscatter from the ambient signal emitted by the source SO. Furthermore, the source SO as well as the transmitter D_TX and receiver D_RX devices here occupy respective fixed positions.

Furthermore, and in accordance with the invention, the respective positions of the source SO and of the receiver device D_RX are given. In other words, it is considered that the respective positions of the source SO and of the receiver device D_RX are known.

In the remainder of the description, and as illustrated by FIG. 1, it is considered in a non-limiting manner that the communication system 10 comprises a single transmitter device D_TX and a single receiver device D_RX. It should however be specified that the invention is also applicable to a communication system comprising a plurality of transmitting devices and/or a plurality of transmitting devices, the developments necessary for such a generalization being able to be implemented without difficulty by man. of career.

In a manner known per se, communication by ambient backscatter consists of the exploitation of the ambient signal, by the transmitter device D_TX, to send data to said receiver device D_RX. More particularly, the transmitter device D_TX (respectively the receiver device D_RX) is configured to perform, from the ambient signal (respectively from the backscattered signal), processing aimed at backscattering said ambient signal (respectively aimed at decoding said backscattered signal) , by implementing a backscatter method (respectively a decoding method).

To this end, the transmitter device D_TX (respectively the receiver device D_RX) comprises for example one or more processors and storage means (magnetic hard disk, electronic memory, optical disk, etc.) in which data and a program are stored. computer, in the form of a set of program code instructions to be executed to implement the backscatter method (respectively the decoding method).

Alternatively or in addition, the transmitter device D_TX (respectively the receiver device D_RX) also comprises one or more programmable logic circuits, of the FPGA, PLD, etc. type, and/or specialized integrated circuits (ASIC), and/or a set of discrete electronic components, etc. adapted to implement the backscatter method (respectively the decoding method).

In other words, the transmitter device D_TX (respectively the receiver device D_RX) comprises a set of means configured in software (specific computer program) and/or hardware (FPGA, PLD, ASIC, etc.) to implement implements the backscatter method (respectively the decoding method).

The specific aspects concerning the transmission of data by backscatter intended for the receiver device D_RX, as well as those concerning the decoding techniques implemented by the latter, are known to those skilled in the art and fall outside the scope of the present invention. Therefore, they are not detailed here further.

In the present embodiment, the transmitter device D_TX is equipped with an antenna (not shown in the figures) configured, in a manner known per se, to receive the ambient signal but also to backscatter it towards the receiver device D_RX. It should be noted that no limitation is attached to the number of antennas that can equip the D_TX transmitter device.

In practice, the D_TX transmitter device is associated with a frequency band, called "influence band", which corresponds to the frequency band in which the antenna is able to receive / backscatter signals. When said influence band is included in the emission band associated with the SO source, it is called the “working band”. By “working band”, reference is made here to the fact that the transmitter device D_TX is compatible with the source SO, namely therefore that the backscattering can be performed for any frequency included in said working band.

However, nothing excludes considering an influence band that is not included in the emission band associated with the SO source. It is nevertheless implicit that for the transmitter device D_TX to be able to backscatter the ambient signal, said influence band and said emission band must have a non-empty intersection, the working band therefore corresponding to this intersection.

The transmitter device D_TX is also associated with operating states, namely at least one state called "backscatter" (the transmitter device D_TX backscatters the ambient signal) as well as a contrary state called "non-backscatter" (the device D_TX transmitter does not backscatter the ambient signal, or, in other words, is "transparent" to the ambient signal). These states correspond to configurations in which said antenna is connected to different impedances. This is typically a positive or even zero impedance for a backscatter state, and conversely a theoretically infinite impedance for the non-backscatter state.

For the remainder of the description, it is considered in no way limiting that the transmitter device D_TX is associated with a single backscattering state as well as a single non-backscattering state. The invention nevertheless remains applicable in the case where the transmitter device D_TX is associated with a plurality of backscatter states, these states being distinct from each other in that they are implemented thanks to respective impedances distinct from each other (l non-backscatter state remains unique). The following developments can be generalized without difficulty by those skilled in the art in the case where a plurality of backscatter states is considered.

In the present embodiment, the receiver device D_RX is equipped with a reception antenna (not shown in the figures) configured to receive signals in said working band. For example, said receiver device D_RX is a cell phone of the smartphone type.

It should however be noted that no limitation is attached to the number of antennas that can equip the D_RX receiver device.

In general, no limitation is attached to the structural forms that can be taken respectively by the source SO and the receiver device D_RX. By way of non-limiting examples, the following configurations are possible depending on the working frequency band considered:
- the source SO is a cell phone, for example of the smartphone type, and the receiver device D_RX is a base station,
- the source SO and the receiver device D_RX are both cellular telephones, for example of the smartphone type,
the source SO is a home gateway (also called an “Internet box”) emitting a Wi-Fi signal, and the receiver device D_RX is a cellular telephone, for example of the smartphone type, etc.

In accordance with the invention, the system 10 for communication by ambient backscatter also comprises a surface 11 able to reflect signals originating from the source SO and/or from the transmitter device D_TX towards the receiver device D_RX. Such a surface 11 has at least one face (i.e. the face towards which the incident signal waves are directed) whose roughness is for example adapted so as to allow such reflection.

By way of non-limiting example, said surface 11 has a face whose roughness is defined by asperities distributed periodically along a direction in which the surface 11 extends. According to a more specific example, said asperities are identical to each other. and take the form of circular half-cylinders whose respective axes are parallel to each other. For more information on the design of such a surface, those skilled in the art can refer to chapter 2.3 of the document: “Recommendation ITU-R P.2040-1 (07/2015): Effects of building materials and structures on radiowave propagation above about 100 MHz-P Series-Radiowave”. A surface 11 thus produced makes it possible to reflect an incident signal directed towards said asperities without privileged direction. In other words, such an incident signal is reflected, in the part of the environment positioned on the side of the asperities, in an omnidirectional manner.

However, nothing excludes having a surface with a face whose roughness is defined by asperities other than half cylinders parallel to each other. There is also nothing to preclude said asperities from being distributed non-periodically. In general, a person skilled in the art knows how to design a surface whose topology is capable of reflecting a radio signal towards the receiver device D_RX.

In the present embodiment, said surface 11 is positioned fixed, normal to the plane in which FIG. 1 is represented, to reflect signals originating from the source SO and from the transmitter device D_TX towards the receiver device D_RX. Such a configuration of the surface 11 therefore implies, in particular, that the source SO and the transmitter D_TX and receiver D_RX devices are positioned on the same side of the surface 11, more particularly facing a face whose roughness is adapted, as is mentioned above.

It will of course be understood that for the signals originating both from the source SO and from the transmitter device D_TX to be reflected towards the receiver device D_RX, the size of the surface 11 must be adapted accordingly.

For purely illustrative purposes, the surface 11 is flat and extends between two ends, denoted A and B, and separated by a distance of 1 meter. Furthermore, the distances between the source SO and the surface 11 on the one hand, and the source SO and the transmitter device D_TX on the other hand, are equal to 1 meter. The distance between the source SO and the receiver device D_RX is for its part equal to 2 meters.

In general, a person skilled in the art knows how to determine the appropriate size for such signals to be reflected towards the receiver device D_RX. It should also be noted that it knows how to adapt this size in cases where only signals from the source SO or else only signals from the transmitter device D_TX are reflected, via the surface 11, towards the receiver device D_RX.

Moreover, nothing excludes considering a surface 11 which is not flat, such as for example a curved surface, since its position and its size are adapted to allow the reflection of signals from the source SO and/or of signals from the transmitter device D_TX to the receiver device D_RX. There is also nothing to exclude considering a surface 11 which is not normal to the plane in which FIG. 1 is represented, in other words an inclined surface since, here again, it is able to reflect signals coming from the source SO and/or signals from the D_TX transmitter device to the D_RX receiver device.

Moreover, it should also be noted that the invention not only covers the case where the surface 11 forms an element introduced manually and voluntarily into the environment of the source SO and of the transmitter devices D_TX and receiver D_RX, but also the case where the surface 11 is already present (fortuitously) in this environment and purposely exploited for the invention. In the latter case, and as already mentioned above, no limitation is attached to the nature of the surface 11 as long as it is configured to allow the reflection of signals from the source SO and/or signals from the device D_TX transmitter to D_RX receiver device. For example, it may be a plate fixed to a wall and indicating a street name or a street number.

The wave paths conveyed by the signals considered in the present invention are represented by dotted arrows in this figure 1. More particularly:
- the path P1 refers to a wave coming from the source SO and reaching directly (ie without being reflected by the diffusing surface 11) the receiver device D_RX;
- the path P2 (respectively the path P3) refers to a wave coming from the source SO and of which a reflection at the level of the end A (respectively at the level of the end B) of the surface 11 reaches the receiver device D_RX ;
- the path P4 refers to a wave coming from the source SO, then backscattered by the transmitter device D_TX and arriving directly at the receiver device D_RX;
- the path P5 refers to a wave coming from the source SO, then backscattered by the transmitter device D_TX and of which a reflection at the level of the surface 11, between the ends A and B, reaches the receiver device D_RX.

It should be noted that, in this figure 1, only the waves associated with the paths P4 and P5 carry data that the receiver device D_RX is intended to decode in the context of a communication by ambient backscatter. It is also important to note that said figure 1 is given for purely illustrative purposes. Thus, it does not include for example any element, other than the surface 11, capable of reflecting or diffracting the waves of the ambient signal emitted by the source SO. In this sense, FIG. 1 is intended to be a simplified representation of the environment in which the ambient backscatter communication system 10 is positioned. It should nevertheless be borne in mind that this environment is generally of a complex configuration and may include, in practice, various elements (walls, trees, ground, etc.).

In the context of the present invention, namely when a surface such as the surface 11 mentioned above is present in the environment of the source SO and of the transmitter devices D_TX and receiver D_RX, and remarkably, the inventors have managed to establish a precise analytical expression of the deviation E_P of power received by the receiver device D_RX depending on whether the transmitter device D_TX is in the backscatter or non-backscatter state. This analytical expression is expressed, in all generality, as a function of parameters of the communication system 10 by ambient backscatter. Specifically, the parameters in question are:
- the respective positions S, T, R of the source SO and of the transmitter devices D_TX and receiver D_RX,
- the emission frequency F_E of the SO source in the emission frequency band associated with the SO source.

In other words, the power difference E_P can be expressed in the form of a function which takes said parameters F_E, S, T and R as arguments. Said function also translates, as detailed below, the influence of the surface 11 on the signals intended to be received by the receiver device D_RX. This function is called “calculation function” for the remainder of the description. Of course, given the spatial configuration of the ambient backscatter communication system 10 considered in the present embodiment, it is understood that said calculation function here admits a single variable parameter, namely the position of the transmitter device D_TX.

We now describe calculation elements making it possible to arrive at the expression taken by said calculation function in the context of the embodiment illustrated in FIG. 1. To this end, and initially, we adopt, without loss of generality, the convention according to which said power difference E_P is evaluated according to the following general formulation:
E_P = P_R – P_NR.
In this formula, we have that:
- P_R corresponds to the power received by the receiver device D_RX when the transmitter device D_TX is in the backscatter state (with reference to figure 1, the waves which follow the paths P1 to P5 contribute to the value of P_R),
- P_NR corresponds to the power received by the receiver device D_RX when the transmitter device D_TX is in the non-backscatter state (with reference to figure 1, the waves which follow the paths P1, P2 and P3 contribute to the value of P_NR).

It is of course understood that, according to this writing convention, the power difference E_P is a positive number. If an opposite convention were to be adopted (i.e. E_P = P_NR – P_R), said deviation E_P would be a negative number, and the analytical expression presented below for the calculation function should be adapted accordingly (multiplication by -1 of the terms intervening in said analytical expression).

In addition, and in a second step, we introduce figure 2 which corresponds to another representation of the system 10 of figure 1, in which are indicated said positions S, T, R as well as the ends A and B of the surface 11 A straight line D1 and a straight line D2 are also represented in figure 2, the straight line D1 (respectively the straight line D2) corresponding to the straight line parallel to the segment [AB] and passing through the position S of the source SO (respectively passing through the position T of the transmitter device D_TX).

In addition, we denote by:
- P the transmission power of the SO source,
- A', B' and R' the points resulting from the orthogonal projections (symbolized in dotted lines in figure 2), on the line D1, of the points A, B and R respectively,
- A'', B'' and R'' the points resulting from the orthogonal projections (symbolized in dotted lines in figure 2), on the line D2, of the points A, B and R respectively,
- G ^S , G ^T and G ^R the respective gains of the source SO, of the transmitter device D_TX and of the receiver device D_RX, i.e. the gains of the antennas equipping the source SO, the transmitter device D_TX and the D_RX receiver device,
- G ^d a diffusion coefficient of the surface 11, (this is a dimensionless coefficient, analogous, for example, to the coefficient "Rν" indicated in formula (51) in the document "Recommendation ITU-R P. 2040-1 (07/2015) […]” already cited above),
- c the speed of light,
- λ = c / F_E the wavelength associated with the emission frequency F_E of the SO source,
- k = 2π / λ the wave vector associated with the wavelength λ,
- K ^SR = (G ^S x G ^R x λ ² ) / 4π,
- K ^SAR = (G ^S x G ^d x G ^R x λ ⁴ ) / 16π ² ,
- K ^TR = (G ^T x G ^R x λ ² ) / 16π ² ,
- K ^STR = (G ^S x (G ^T ) ² x G ^R x λ ⁴ ) / 64π ² ,
- α = 0 or 1 if the transmitting device D_TX is in the non-backscatter or backscatter state respectively,
- β = AB x [SA' / SA + (SA'-SR') / AR],
- γ = AB x [TA'' / TA + (TA''-TR'') / AR].

In order to obtain the analytical expression of the calculation function, it is considered that a signal S_RX received by the receiver device D_RX is written:
S_RX = S(SO, DIR )+ S(SO, DIFF) + S(D_TX, DIR),
expression that must be understood as being a vector relation over the field of complex numbers, and in which:
- S(SO, DIR) corresponds to the signal coming directly from the source SO (path P1 with reference to figure 1) and has the expression:
;
- S(SO, DIFF) corresponds to the signal coming from the source SO after a reflection on the surface 11 (path P2 and P3 with reference to FIG. 1) and has the expression:
;
- S(D_TX, DIR) corresponds to the backscattered signal coming directly from the transmitter device D_TX (path P4 with reference to figure 1) and has the expression:
.

It should be noted that this vector relationship is based on the assumption that the power of a backscattered signal S(D_TX, DIFF) reaching the receiver device D_RX after reflection on the surface 11 (path P5 with reference to FIG. 1 ) is negligible in comparison with the powers respectively associated with the signals S(SO, DIR), S(SO, DIFF) and S(D_TX, DIR). It is in fact understood that such a signal S(D_TX, DIFF) has undergone, throughout its course, two reflections: a first reflection at the level of the transmitter device D_TX due to backscattering, as well as a second reflection at the level of the surface 11. This plurality of reflection implies that the distance traveled by the signal S(D_TX, DIFF) is greater than those traveled respectively by S(SO, DIR), S(SO, DIFF) and S(D_TX, DIR), so that its amplitude is lower in comparison with the respective amplitudes of the latter. In other words, each reflection induces a weakening of the corresponding signal. It should be noted that this modeling hypothesis on which the formulation of the S_RX signal is based has been validated by digital simulations by the inventors.

Finally, we also introduce the following series of notations:
- P(D_TX):

where A(D_TX) = α x [K ^STR / (ST ² x TR ² ) x P] ^1/2 ,
- P(D_TX, DIR):

where A(DIR) = [K ^SR / SR ² x P] ^1/2 ,
- P(D_TX, DIFF):
where A(DIFF) = [K ^SAR / (AS ² x AR ² ) x P] ^1/2 ,
- P(DIR):

- P(DIFF):

- P(DIR, DIFF):
.

From this series of notations, we have that:
- P_R = P(DIR) + P(DIFF) + P(DIR, DIFF) + P(D_TX, DIFF | α = 1) + P(D_TX | α = 1) + P(D_TX, DIR | α = 1) , expression in which the values of the terms P(D_TX, DIFF), P(D_TX), P(D_TX, DIR), as described above, are considered for α equal to 1 (ie the values of these terms are obtained for A(D_TX) = [K ^STR / (ST ² x TR ² ) x P] ^1/2 );
- P_NR = P(DIR) + P(DIFF) + P(DIR, DIFF) + P(D_TX, DIFF | α = 0) + P(D_TX | α = 0) + P(D_TX, DIR | α = 0) , expression in which the values of the terms P(D_TX, DIFF), P(D_TX), P(D_TX, DIR), as described above, are considered for α equal to 0 (ie the values of these terms are obtained for A(D_TX) = 0).

Ultimately, the calculation function making it possible to evaluate the power difference E_P, according to said parameters, is written:

According to this expression, the calculation function involves three terms which add together, including:
- a first term representative of the contribution, in terms of power, of signals backscattered by the transmitter device D_TX and reaching the receiver device D_RX without reflection on the surface 11 (term without trigonometric function),
- a second term representative of the contribution, in terms of power, of a coupling between signals backscattered by the transmitter device D_TX and reaching the receiver device D_RX without reflection on the surface 11 and signals emitted by the source SO and reaching the receiver device D_RX without reflection on the surface 11 (term involving only the cos trigonometric function),
- a third term representative of the contribution, in terms of power, of a coupling between signals backscattered by the transmitter device D_TX and reaching the receiver device D_RX without reflection on the surface 11 and signals emitted by the source SO and reaching the receiver device D_RX after reflection on surface 11 (term involving a product of trigonometric functions sin and cos).

It is then important to note that said third term is that which is representative of the influence (i.e. of the contribution) of the surface 11 in the evaluation of the power difference E_P. It is therefore understood, in view of the analytical expression of the calculation function, that the voluntary use of the surface 11 in the environment of the source SO and of the transmitter devices D_TX and receiver D_RX advantageously makes it possible to increase the maximum achievable power difference (due to the presence of said third term) in comparison with a configuration where the surface 11 would not be present.

As described below in detail, it is possible, from the expression of said calculation function and via a location method according to the invention, to determine at least one possible position of the transmitter device D_TX for which said calculation function is equal to a given value of E_P. It is indeed understood that if E_P, S, R and F_E (i.e. the wave vector k) are fixed, the expression of said calculation function is read as an equation in the variable T, the solutions of this equation corresponding from at possible positions (i.e. in the sense that they satisfy said T-equation) of the transmitter device D_TX.

In the present embodiment, said location method is implemented by a location device D_LOC which is included in the receiver device D_RX.

Furthermore, within the meaning of the present invention, and to implement said localization method, said calculation function typically corresponds to a set of code instructions. For example, said code instructions are written in the MATLAB language, so that said calculation function corresponds to a MATLAB script. The software environment of the localization device D_LOC is therefore adapted, in a manner known per se, to the execution of such a MATLAB script.

In general, no limitation is attached to the programming language in which the code instructions of said calculation function are written, which may be in the form of source code, object code, or intermediate code between source code and code object, such as in a partially compiled form, or in any other desirable form.

FIG. 3 schematically represents an example of hardware architecture of the localization device D_LOC for the implementation of the localization method according to the invention.

For this purpose, the D_LOC localization device has the hardware architecture of a computer. As illustrated by FIG. 3, the location device D_LOC comprises, in particular, a processor 1, a random access memory 2, a read only memory 3, a non-volatile memory 4, communication means 5 integrating the antenna of the receiver device D_RX as well as acquisition means 6.

Said acquisition means 6 are configured to acquire measurements of an electromagnetic power received by said location D_LOC, and therefore ultimately said receiver device D_RX. A measurement thus acquired therefore corresponds to an electromagnetic power level received by the receiver device D_RX at the time of the corresponding acquisition.

It should be noted that said acquisition means are configured to acquire a power measurement regardless of the state (non-backscatter or backscatter) in which the transmitter device D_TX is located.

Conventionally, said acquisition means 6 comprise an acquisition chain connected to a sensitive element configured to supply an analog electrical signal representative of the measured electromagnetic power. In the present embodiment, said sensitive element corresponds to the reception antenna fitted to the receiver device D_RX.

Said acquisition chain comprises for example an acquisition card configured to condition said electrical signal. The conditioning implemented by the acquisition card comprises for example, in a manner known per se, an amplification and/or a filtering and/or a current-power conversion. In general, the configuration of such acquisition means is well known to those skilled in the art, and is therefore not detailed here further.

The read only memory 3 of the location device D_LOC constitutes a recording medium in accordance with the invention, readable by the processor 1 and on which is recorded a computer program PROG in accordance with the invention, comprising instructions for the execution steps of the location method according to the invention. The PROG program defines functional modules of the localization device D_LOC, which are based on or control the hardware elements 2 to 6 of the localization device D_LOC mentioned above, and which include in particular:
- a first determination module MOD_1, configured to determine a difference, called "measured difference", between measurements acquired by the receiver device D_RX, including a first measurement and a second power measurement received when the transmitter device D_TX is respectively in a backscatter state and in a non-backscatter state,
- a second determination module MOD_2, configured to determine positions, called "candidate positions", of the transmitter device D_TX for which said calculation function is equal to said measured difference,
- a third determination module MOD_3, configured to determine a position, called “final position”, according to said candidate positions.

For the rest of the description, it is considered in no way limiting that the code instructions defining the calculation function are initially stored in a memory equipping an entity other than the location device D_LOC, for example a memory equipping the transmitter device D_TX, or else the source SO, or even an entity external to the system 10 and able to memorize the instructions of said calculation function (eg: database server). Consequently, in the example of hardware architecture considered here, the localization device D_LOC also comprises a module MOD_OBT for obtaining configured to obtain said calculation function. In other words, the obtaining of the calculation function code instructions, by the localization device D_LOC, is carried out via a data exchange (transmission/reception) controlled by the obtaining module MOD_OBT and implemented by the communication means 5 of said localization device D_LOC and communication means of said entity.

In general, the means of communication considered for such an exchange of data are based on a communication interface. No limitation is attached to the nature of this communication interface, which can be wired or wireless, and can implement any protocol known to those skilled in the art (Ethernet, Wifi, Bluetooth, 3G, 4G, 5G, etc.).

FIG. 4 represents, in the form of a flowchart, a particular mode of implementation, by the location device D_LOC, of the location method.

As illustrated by FIG. 4, the location method comprises a step E10 of obtaining said calculation function. This step E10 is implemented by said obtaining module MOD_OBT equipping the location device D_LOC.

Once the obtaining step E10 has been executed, the location device D_LOC has the calculation function making it possible to evaluate the deviation E_P as a function of the parameters defining the system 10 for communication by ambient backscatter, namely more precisely here in function of the position T (considered as a variable) of the transmitting device D_TX as well as of the fixed parameters S, R and F_E. The localization device D_LOC is therefore able to calculate values of said deviation E_P by assigning values to parameter T.

The location method also includes a step E20 of acquiring a pair C_MES of measurements (ie a first measurement and a second measurement) of received power. This step E20 is implemented by the acquisition means 6 equipping the location device D_LOC.

More particularly, said first measurement (respectively said second measurement) is acquired when the transmitter device D_TX is in the backscattering state (respectively in the non-backscattering state). It is also obvious that the acquisitions are carried out in said working band, since it is in this frequency band that the transmitter D_TX and receiver D_RX devices are compatible with the SO source, as mentioned above.

In a particular mode of implementation, the transmitter device D_TX is associated with given and mutually distinct time periods, as well as configured to backscatter during said time periods. The localization device D_LOC, for its part, is aware of said time periods, and is configured to synchronize with at least one of these periods, so as to be able to carry out the first measurement (respectively the second measurement) when the transmitter device D_TX backscatters (respectively does not backscatter) the ambient signal from the SO source.

No limitation is attached to the way in which the localization device D_LOC acquires knowledge of said backscatter time periods. For example, said time periods are communicated by the transmitter device D_TX to the location device D_LOC prior to the acquisition step E20.

Alternatively, the location device D_LOC performs power measurement acquisitions in a recurring manner, for example following a constant time step. Moreover, according to this alternative, the transmitter device D_TX is configured to transmit to the location device D_LOC a message informing it of the passage from the non-backscatter state to the backscatter state. In this way, a measurement acquired after receiving such a message can be associated with said backscatter state.

Once the pair C_MES has been acquired, the location method comprises a step E30 of determining a difference, called “measured difference” E_MES, between the measurements of said pair C_MES. This step E30 is implemented by said first determination module MOD_1.

The objective of determining such a measured difference E_MES is to obtain an exploitable value to determine at least one position of the transmitter device D_TX. For this purpose, said measured deviation E_MES is determined to correspond to a value that can be taken by the calculation function.

Thus, in the present mode of implementation, and in accordance with the convention adopted for writing the power difference E_P (i.e. EP = P_R – P_NR), the measured difference E_MES corresponds to the subtraction of said second measurement to said first measurement, or even to the absolute value of the difference between said first and second measurements.

It is of course understood that if the opposite convention were adopted, the measured deviation E_MES would be determined equal to the subtraction of said first measurement from said second measurement.

The location method also includes a step E40 of determining at least one position, called “candidate position” T_i (i being an integer index greater than or equal to 1), of the transmitter device D_TX for which the calculation function is equal to said deviation measured E_MES. This step E40 is implemented by said second determination module MOD_2.

In other words, step E40 consists in solving an equation, called “general equation”, in the variable T defined by an equality between said measured difference E_MES and the value of the power difference E_P as given by the analytical expression of the calculation function. This general equation in the variable T is written:

the solution(s) to this general equation corresponding to said candidate positions T_i.

The inventors have observed, by digital simulations, that there are one or more candidate positions T_i solutions to the general equation above as a function of the value of the measured difference E_MES. More specifically, when the value of the measurement deviation E_MES is equal to or close to the absolute maximum that can be reached by the calculation function, it has been found that said general equation admits a single solution. This finding remains true when the measured deviation E_MES is equal to or close to the absolute minimum attainable by the calculation function. On the other hand, when the measured difference takes a value situated between said absolute maximum and said absolute minimum, it has been observed that said general equation generally admits a plurality of solutions.

As observed by the inventors, the closer the value of E_MES is to an absolute extremum, the closer the determined candidate solutions T_i are to each other. In addition, and in a particularly advantageous manner, it has also been observed that whatever the value of the measured deviation E_MES between the absolute extrema of the calculation function, the maximum distance separating two candidate positions T_i is systematically lower than the best accuracy of location attainable by state-of-the-art solutions.

For the remainder of the description, it is considered that a plurality of candidate positions T_i are determined during step E40 of determination. Of course, it is understood that if a single candidate position were determined at the end of step E40, this candidate position would constitute the final position that should be retained for the transmitter device D_TX within the meaning of the present invention.

By way of non-limiting example, the candidate positions T_i are determined by a dichotomy method.

Such a dichotomy method is conventionally implemented to iteratively test possible values of the variable T in a given range of values, thus making it possible to determine for which value of this range the measured difference E_MES is reached.

Nothing excludes considering other methods of solving the general equation, the choice of the dichotomy method only constituting a variant implementation of the invention. For example, the method implemented can be a Newton method, a fixed point method, a Lagrange method, a golden section method, etc.

Even more generally, it is possible to consider, during step E40, a cost function corresponding to the calculation function from which said measured difference E_MES is subtracted. Therefore, the minimization of such a cost function can be implemented according to any optimization method known to those skilled in the art.

In the present mode of implementation, the location method proposes to discriminate between the candidate positions T_i solutions of the general equation by taking into account a measurement precision. For this purpose, two candidate positions are considered to coincide with each other if the distance separating them is less than a measurement accuracy with which the measurements of the couple C_MES are acquired. Proceeding in this way offers the possibility of reducing the number of candidate positions T_i which should be considered as being able to be occupied by the transmitting device D_TX.

Conventionally, such measurement precision is inherent to the operation of the acquisition means 6 equipping the receiver device D_TX. Thus, the measurements of the couple C_MES are acquired with said given measurement accuracy, this accuracy then propagating to the candidate positions T_i solutions of the general equation.

Consequently, if the maximum distance separating any two candidate positions is less than said measurement precision, it is considered that all of said candidate positions T_i coincide with one another. This may be the case, for example, if said maximum distance is of the order of a millimeter while the precision is itself of the order of a centimeter. In this way, the location method according to the invention makes it possible to obtain a single position for the transmitter device D_TX, this single position corresponding to any one of the candidate positions considered to coincide with each other.

More generally, in the case where a subset of candidate positions (among all the candidate positions T_i obtained by solving the general equation) are confused with each other due to the taking into account of the measurement precision, no limitation is associated with the position chosen in said subset to act as a representative of the latter.

In practice, when the value of the measured difference E_MES is far from an absolute extremum, the inventors have found that taking into account said measurement accuracy does not systematically allow the set of candidate positions T_i to be reduced to a position unique. In other words, a plurality of candidate positions T_p_NID (p being an integer index greater than 1) not coincident with each other (in the sense that the distance separating any two candidate positions from among said plurality is greater than the measurement accuracy) generally subsist despite the taking taking into account the accuracy of measurement.

Also, in the present mode of implementation, and as illustrated by FIG. 4, the location method comprises a step E50 of determining a position, called “final position of the transmitter device D_TX” T_FIN, according to said positions candidates T_p_NID not confused with each other. This step E50 is implemented by said third determination module MOD_3.

The objective targeted by such a step E50 is therefore to remove the ambiguity as to the fact that the transmitter device D_TX could occupy a plurality of positions corresponding to said candidate positions T_p_NID not coincident with each other. Of course, if at the end of step E40, a single candidate position is obtained, this step is considered as implicit, the single candidate position therefore being determined as the final position of the transmitter device D_TX. Similar considerations apply to the third determination module MOD_3 which can be confused with the second determination module MOD_2.

FIG. 5 schematically represents a particular mode of implementation of step E50 in which a set of sub-steps are implemented to determine said final position T_FIN.

As illustrated by FIG. 5, said set of sub-steps firstly comprises a sub-step E50_1 for modifying the transmission frequency F_E of the source SO.

To do this, the location device D_LOC is for example configured to transmit to the source SO a message informing it of the modification of the transmission frequency F_E, so that said source SO then transmits according to a modified transmission frequency corresponding F_E_NEW. No limitation is attached to the form taken by this message, this form being for example compliant with a given telecommunications standard.

The modified transmission frequency F_E_NEW resulting from said modification is of course chosen by the location device D_LOC in the working band since, as mentioned before, the ambient backscatter communication system 10 is configured to operate in said working band.

By way of non-limiting example, the modified transmission frequency F_E_NEW is equal to half of the transmission frequency F_E used by the system 10 before said modification sub-step E50_1. It should however be noted that any other value of the working band can be chosen for said modified transmission frequency F_E_NEW, the choice of this value only constituting an implementation variant of the particular mode of FIG. 5.

Once the transmission frequency F_E has been modified, step E50 includes a sub-step E50_2 for acquiring another pair C_MES_NEW of power measurements received when the transmitter device D_TX is respectively in the backscatter state and in the backscatter state. non-backscatter state. The implementation of this sub-step 50_2 is similar to that of step E20 described previously.

Once said other pair C_MES_NEW has been acquired, step E50 includes a sub-step E50_3 for determining a measured difference E_MES_NEW between the measurements of said other pair C_MES_NEW. The implementation of this sub-step 50_3 is similar to that of step E30 described previously.

Following the determination of said measured deviation E_MES_NEW, step E50 comprises, for each of the candidate positions T_p_NID not coincident with each other, a sub-step E50_4 of evaluation, for said modified transmission frequency F_E_NEW, of the function of calculation so as to obtain a power difference, called “auxiliary difference” E_p_AUX.

In other words, for a fixed index p, the calculation function is evaluated by considering that the transmitter device D_TX occupies a fixed position given by T_p_NID (the positions S, R respectively of the source SO and of the receiver device D_RX being invariant regardless of the index p considered) and that the source transmits according to the modified transmission frequency F_E_NEW. In this way, for each of said candidate positions T_p_NID not merged with each other, an auxiliary difference E_p_AUX associated with it is obtained.

Moreover, and as illustrated by FIG. 5, said set of sub-steps E50_1, E50_2, E50_3, E50_4 is iterated as long as the auxiliary deviations E_p_AUX obtained during an iteration differ from the measured deviation E_MES_NEW determined during said iteration. It should be noted that the value of the frequency F_E_NEW (respectively of the couple C_MES_NEW) obtained during an iteration obviously differs from the corresponding value(s) obtained prior to this iteration.

Consequently, the final position T_FIN is determined according to the position or positions T_p_NID respectively associated with the auxiliary deviation(s) E_p_AUX equal to the measured deviation E_MES_NEW determined during the last iteration of said set of sub-steps E50_1, E50_2, E50_3 , E50_4.

For example, if during the last iteration of the set of sub-steps E50_1, E50_2, E50_3, E50_4, a plurality of auxiliary deviations E_p_AUX are equal to the measured deviation E_MES_NEW determined during said last iteration, the position final T_FIN is determined equal to the barycenter of the candidate positions T_p_NID respectively associated with said auxiliary deviations E_p_AUX.

However, nothing excludes that the final position T_FIN is determined other than by a barycenter calculation. For example, said final position T_FIN is determined equal to a weighted combination of the candidate positions T_p_NID respectively associated with said auxiliary deviations E_p_AUX, the weighting coefficients involved being able for example to be chosen randomly, or even deterministically on the basis one or more external information (information as to the probability that the transmitter device D_TX is positioned in a determined geographical sector, etc.).

According to yet another example, the final position T_FIN is determined equal to any one of the candidate positions T_p_NID respectively associated with said auxiliary deviations E_p_AUX, the choice of a position among said candidate positions T_p_NID being carried out by means of a random draw .

Of course, it is also understood that if a single auxiliary deviation E_p_AUX is equal to the measured deviation E_MES_NEW determined during said last iteration, the final position T_FIN is determined equal to the candidate position T_p_NID associated with said single auxiliary deviation E_p_AUX.

In practice, the inventors have observed that it is generally necessary to iterate the set of sub-steps E50_1, E50_2, E50_3, E50_4 at most twice to obtain the final position T_FIN of the transmitter device D_TX. In other words, a maximum of two modifications E50_1 of the transmission frequency of the source are to be considered in this particular mode of implementation of step E50.

Moreover, the fact of considering a surface such as surface 11 in the environment of the communication system 10 by ambient backscatter is particularly advantageous for the implementation of the mode of FIG. 5. Indeed, and initially, it can be noted that all the terms of the analytical expression of the calculation function are sensitive to a modification of the emission frequency of the SO source. This is true for the first term of said analytical expression due to the presence of the coefficient K ^STR which depends on the wavelength λ. This is also true for the second and third terms of said analytical expression due, in particular, to their dependence on the wave vector k.

However, and in a second step, the fact that the calculation function includes terms depending on the emission frequency of the source SO, including in particular said third term representative of the contribution of the surface 11, makes it possible to obtain, following to a modification of said transmission frequency, a variation in the power difference greater than if the surface 11 were absent. This contributes to differentiating the auxiliary deviation values E_p_AUX from each other, which ultimately facilitates the discrimination of the candidate positions T_p_NID not confused with each other to determine the final position T_FIN.

Consequently, the mode of implementation of Figure 5 requires few iterations, thus making it possible to very quickly obtain the final position T_FIN of the transmitter device D_TX.

Other modes of implementing step E50, as an alternative to the FIG. 5 mode, can also be envisaged. For example, nothing excludes that, following the taking into account of the precision of measurement and the obtaining of the candidate positions T_p_NID not confused between them, the final position T_FIN is determined equal to the barycenter of the said candidate positions T_p_NID. In other words, according to this example, it is not envisaged to discriminate between these candidate positions T_p_NID by taking into account one or more modifications of the emission frequency of the source SO, but rather to evaluate said barycenter directly after taking into account the measurement accuracy.

There is also nothing to exclude considering other modes of implementation of step E50, according to arrangements similar to those mentioned previously in the context of FIG. 5 (weighted combination of candidate positions T_p_NID, choice of a any position among said candidate positions T_p_NID), but for which, again, said provisions are applied directly after taking into account the measurement accuracy.

The location method has been described so far considering that the measurement accuracy associated with the acquisition means 6 was taken into account to discriminate between the candidate positions T_i. This being said, said method remains of course applicable without taking into account said measurement accuracy. More particularly, all the different modes of implementation described above and applied to the candidate positions T_p_NID not coincident with each other can be applied in a similar manner to the candidate positions T_i not discriminated against each other thanks to said measurement accuracy.

The location method has also been described by considering that a signal S_RX received by the receiver device D_RX is written:
S_RX = S(SO, DIR )+ S(SO, DIFF) + S(D_TX, DIR).
As mentioned, this vector relationship is based on the assumption that the power of a backscattered signal S(D_TX, DIFF) reaching the receiver device D_RX after reflection on the surface 11 is negligible in comparison with the powers respectively associated with the signals S(SO, DIR), S(SO, DIFF) and S(D_TX, DIR). The fact remains that the invention remains applicable in the case where such a signal S(D_TX, DIFF) is taken into account in the expression of the signal S_RX. To this end, the inventors have been able to establish that the signal S(D_TX, DIFF) has the expression:

where K ^STR = (G ^S x (G ^T ) ² x G ^R x λ ⁴ ) / 64π ² .
From the expression of S(D_TX, DIFF), a person skilled in the art knows how to update the expression of the power P_R (respectively of the power P_NR) received by the receiver device D_RX when the transmitter device D_TX is in the backscatter state (respectively in the non-backscatter state). Correlatively, he also knows how to update the analytical expression of the calculation function, and determine the candidate positions T_i solutions of this updated calculation function.

The invention has also been described so far considering that the code instructions defining the calculation function are initially stored in a memory equipping an entity other than the location device D_LOC. This being so, the invention remains of course applicable in the case where said code instructions are implemented in the location device D_LOC from its design, therefore making optional the fact that the location device D_LOC comprises a module for obtaining and that the location method comprises a step E10 of obtaining.

For example, the calculation function code instructions are implemented in the ROM 3 of the localization device D_LOC so as to be integrated into the program PROG. Nothing, however, excludes the code instructions of the calculation function from being implemented, during design, in another memory of the localization device D_LOC, the program PROG therefore comprising instructions allowing access to the function of calculation.

Furthermore, the invention also remains applicable when the steps E10 of obtaining the calculation function, E30 of determining a measured difference E_MES, E40 of determining candidate positions and E50 of determining a final position T_FIN (at except for acquisition substep E50_2 if step E50 is implemented in accordance with the mode of FIG. 5) are implemented by a location device not included in the receiver device D_RX. In this implementation variant of the invention, the receiver device D_RX is configured to transmit to said localization device the pair C_MES of acquired measurements (and possibly the pair or pairs C_MES_NEW if applicable) so that the latter perform the above-mentioned steps.

For example, the location device D_LOC is included in the source SO or else in the transmitter device D_TX.

According to another example, said localization device D_LOC corresponds to a device external to the system 10 for communication by ambient backscatter. No limitation is attached to the nature of this external device, provided it is configured in hardware and software to implement said steps E10, E30, E40 and E50 (with the exception of sub-step E50_2 acquisition if step E50 is implemented in accordance with the mode of FIG. 5). For example, it may be a PC type computer equipped with means for receiving the C_MES pair (and possibly the C_MES_NEW pair(s) if applicable).

It should also be noted that in the context where the location process is not executed in its entirety by the location device D_LOC, there is also nothing to exclude considering that the modification of the emission frequency of the source SO forming the subject of sub-step E50_1 (step E50 is implemented in accordance with the mode of FIG. 5) is carried out manually.

Finally, and as has already been mentioned before, the invention is also applicable in the case where the ambient backscatter communication system comprises a plurality of mutually non-coherent sources. The fact that the sources are not coherent between them implies in particular that the power differences induced by each source at the level of the receiver device D_RX are added together. In other words, it is possible to determine an overall power difference at the receiver device D_RX by adding said power differences respectively associated with the sources (i.e. by adding calculation functions determined appropriately for each of the subsystems composed of a source, the transmitting device and the receiving device).

Claims (13)

Method for locating a transmitting device (D_TX) belonging to a communication system (10) by ambient backscatter, said system also comprising a transmitting source (SO) according to at least one given transmission frequency (F_E) and a receiving device (D_RX), said system also comprising a surface (11) capable of reflecting signals originating from the source and/or from the transmitter device towards the receiver device,
said method comprisingsteps of:
- determination (E30) of a difference, called “measured difference” (E_MES), between measurements of a power received by the receiver device when the transmitter device is respectively in a backscattering state and in a non-backscattering state ,
- determination (E40) of at least one position, called "candidate position" (T_i), of the transmitter device for which a calculation function is equal to said measured difference, said calculation function evaluating the difference (E_P) of power received by the receiver device depending on whether the transmitter device is in a backscatter or non-backscatter state as a function of said transmission frequency, the respective positions of the source and of the transmitter and receiver devices, as well as the influence of the surface on the signals intended to be received by the receiving device,
- determination (E50) of a position, called “final position of the transmitter device” (T_FIN), as a function of said at least one candidate position.
Method according to claim 1, in which the step (E40) of determining the candidate positions (T_i) is implemented by a location device (D_LOC), the method further comprising a step (E10) of obtaining, by the location device, of said calculation function. Method according to any one of Claims 1 to 2, in which, when a plurality of candidate positions (T_i) are determined, the step (E50) of determining the final position (T_FIN) comprises a set of sub-steps of:
- modification (E50_1) of the transmission frequency (F_E) of the source (SO),
- determination (E50_3) of a measured deviation (E_MES_NEW) between other measurements (C_MES_NEW) of power received by the receiver device (D_RX) when the transmitter device (D_TX) is respectively in a backscatter state and in a non-backscatter,
- for each of the candidate positions (T_p_NID), evaluation (E50_4), for said modified transmission frequency (F_E_NEW), of the calculation function so as to obtain a power difference, called “auxiliary difference” (E_p_AUX),
said set of sub-steps being iterated as long as the auxiliary deviations obtained during an iteration differ from the measured deviation determined during said iteration,
the final position being determined as a function of the position or positions respectively associated with the auxiliary deviation or deviations equal to the measured deviation determined during the last iteration of said set of sub-steps. Method according to any one of Claims 1 to 2, in which the final position (T_FIN) is determined equal to the barycenter of the said candidate positions (T_p_NID). Method according to any one of Claims 1 to 4, in which two candidate positions (T_i) are considered to coincide with each other if the distance separating them is less than a given measurement accuracy, and, if there is a plurality of positions candidates (T_p_NID) not coincident with each other, said final position (T_FIN) is therefore determined as a function of said candidate positions not coincident with each other. A method according to any of claims 1 to 5, wherein the candidate positions are determined by a dichotomy method. Computer program comprising instructions for implementing a location method according to any one of Claims 1 to 6 when said program is executed by a computer. A computer-readable recording medium on which a computer program according to claim 7 is recorded. Locating device (D_LOC) of a transmitting device (D_TX) belonging to a communication system (10) by ambient backscatter, said system also comprising a transmitting source (SO) according to at least one given transmission frequency (F_E) and a receiver device (D_RX), said system also comprising a surface (11) capable of reflecting signals originating from the source and/or from the transmitter device towards the receiver device,
said location device comprising:
- a first determination module (MOD_1), configured to determine a difference, called “measured difference” (E_MES), between measurements of a power received by the receiver device when the transmitter device is respectively in a backscatter state and in a non-backscatter state,
- a second determination module (MOD_2), configured to determine at least one position, called “candidate position” (T_i), of the transmitter device for which a calculation function is equal to said measured difference (E_MES), said calculation function evaluating the deviation (E_P) of power received by the receiver device depending on whether the transmitter device is in a backscatter or non-backscatter state as a function of said transmission frequency, of the respective positions of the source and of the transmitter and receiver devices , as well as the influence of the surface on the signals intended to be received by the receiver device,
- a third determination module (MOD_3), configured to determine a position, called “final position” (T_FIN), as a function of said at least one candidate position. Locating device (D_LOC) according to claim 9, said locating device further comprising an obtaining module (MOD_OBT), configured to obtain said calculation function. Location device (D_LOC) according to any one of claims 9 to 10, said location device being included in the source (SO) or in the transmitter device (D_TX) or in the receiver device (D_RX). Ambient backscatter communication system (10) comprising a transmitting source (SO) according to at least one given transmission frequency (F_E), a transmitting device (D_TX) and a receiving device (D_RX), said system also comprising a surface ( 11) capable of reflecting signals coming from the source and/or from the transmitter device towards the receiver device,
said source or said transmitting device or said receiving device comprising a locating device (D_LOC) according to claim 11. Locating system comprising a locating device (D_LOC) according to any one of Claims 9 to 10, as well as a receiver device (D_RX) for an ambient backscatter communication system (10) intended to comprise a transmitting source (SO ) according to at least one given transmission frequency (F_E) as well as said receiver device and a transmitter device (D_TX), said communication system also being intended to comprise a surface (11) capable of reflecting signals coming from the source and /or from the transmitting device to the receiving device.