WO2022243318A1

WO2022243318A1 - Optimisation of geolocation of a terminal from one or more identifiers of neighbouring transmitting devices

Info

Publication number: WO2022243318A1
Application number: PCT/EP2022/063322
Authority: WO
Inventors: Renaud MARTY; Olivier Isson; Juan Carlos Zuniga; Julien BOITE
Original assignee: Sigfox
Priority date: 2021-05-17
Filing date: 2022-05-17
Publication date: 2022-11-24
Also published as: FR3122960A1; EP4342207A1

Abstract

The invention relates to a method for geolocating a terminal (20) of a wireless communication system (10). The terminal detects, for at least one transmitting device (40), an identifier (ID3) of the transmitting device. The terminal performs a lossy compression of the identifier of the transmitting device in order to obtain a reduced identifier (CID1), and then sends this reduced identifier to the access network in a message. The access network is capable of determining the identifier of the transmitting device from the reduced identifier, from discrimination information (Info3) determined from the received message, and from an ambiguity resolution database (33) making it possible to construct several lists of transmitting device identifiers each associated with a different value of the discrimination information. The geographical position of the terminal can then be estimated from the identifier of the transmitting device and using a geolocation database (51) storing a list of transmitting device identifiers and the respective geographical positions of said transmitting devices.

Description

Optimization of the geolocation of a terminal from one or more identifiers of neighboring transmitting devices

Field of the invention The present invention belongs to the field of the geolocation of a terminal of a wireless communication system. The geolocation of the terminal is implemented from one or more identifiers of neighboring emitting devices detected by the terminal and using a geolocation server comprising a database making the association between emitting devices and their geographical position.

State of the art

There are currently several geolocation systems based on a database making the association between an identifier of a transmitter device (for example a WiFi or Bluetooth access point, or an RFID tag) with the geographical position of the transmitter device.

In such a geolocation system, a terminal detects, for at least one transmitter device, an identifier of the transmitter device from a message transmitted by the transmitter device, for example on a beacon signal. The terminal then sends a query message to a geolocation server. The query message includes the identifier of the transmitting device. The geolocation server comprises a geolocation database making the association between identifiers of transmitting devices and their respective geographical positions. The geolocation server can then determine the geographical position associated with said transmitter device, then send this information to the terminal in a response message. The geographic position of the transmitter device corresponds to an estimated geographic position of the terminal.

The geographical position of the terminal can possibly be refined according to a power level with which the beacon signal is received by the terminal. It is also possible to estimate the geographical position of the terminal according to the geographical position of several different transmitting devices whose terminal has received the beacon signal at a given instant.

It is important in certain cases to limit the energy consumption of the terminal and/or the radio resources necessary for the transmission of an interrogation message by the terminal. This is particularly the case when the communication between the terminal and the geolocation server is ensured by a low-speed and low-consumption wide-area access network (“Low Power Wide Area Network” or LPWAN in the Anglo-Saxon literature). Such a wireless communication network is particularly suitable for applications of the loT type (Anglo-Saxon acronym for “Internet Of Things”, “internet of objects” in French) or M2M (Anglo-Saxon acronym for “Machine To Machine”, “machine-to-machine communication” in French).

It can then be envisaged to limit the size of an interrogation message by limiting the number of identifiers of transmitter devices included in the interrogation message or by compressing the identifier(s) of transmitter devices included in the interrogation message .

Limiting the number of sender device identifiers included in the interrogation message, however, leads to a degradation of the precision of the geolocation of the terminal. Compressing the sender device identifier(s) included in the query message may lead to ambiguity regarding the geographical position of the terminal because a compressed identifier may correspond to several identifiers in the geolocation server database (if one places oneself in the case of lossy compression). If an approximate position of the terminal is known, it is possible to resolve this ambiguity by limiting the geographical region in which one seeks to match the compressed identifier with the identifier of the transmitting device. However, an approximate position of the terminal is not always available.

It is therefore necessary to find a satisfactory solution to limit the energy consumption and/or the radio resources necessary for the transmission of an interrogation message by the terminal while maintaining satisfactory accuracy of the geolocation of the terminal. The geolocation server and the access network are generally managed by separate operators. Also, the chosen solution should preferably not require a change of interface with the geolocation server.

Disclosure of Invention

The object of the present invention is to remedy all or part of the drawbacks of the prior art, in particular those set out above, by proposing a solution for optimizing, in terms of energy consumption, and/or in terms of of use of radio resources, and/or in terms of precision, the geolocation of a terminal from one or more identifiers of neighboring transmitting devices.

To this end, and according to a first aspect, the present invention provides a method for geolocating a terminal of a wireless communication system. The terminal is adapted to exchange messages with an access network of the wireless communication system according to a first wireless communication protocol. The method comprises in particular the following steps:

- detection, by the terminal, for at least one transmitter device, of an identifier of the transmitter device from a message transmitted by the transmitter device according to a second wireless communication protocol,

- obtaining, by the terminal, a reduced identifier of the transmitter device by compression with loss of the identifier of the transmitter device,

- a transmission, by the terminal and intended for the access network, of a message comprising the reduced identifier of the transmitter device, - a determination of the identifier of the transmitter device from the reduced identifier, of a discrimination information determined from the message transmitted by the terminal, and from an ambiguity resolution database making it possible to construct several lists of identifiers of transmitter devices, each associated respectively with a different value of the discrimination information, - an estimate of the geographical position of the terminal from the identifier of the transmitting device and using a geolocation database storing a list of identifiers of transmitting devices and the respective geographical positions of said transmitting devices.

The geolocation database is stored by a geolocation server and the ambiguity resolution database is stored by one or more servers separate from the geolocation server. The ambiguity resolution database is for example stored by one or more servers of the access network, or by one or more third-party servers connected to the access network.

In the rest of the description, the term “identifier” corresponds to an identifier in its complete form (that is to say in a non-reduced form of the identifier). The term “reduced identifier” corresponds to a reduced form of the identifier obtained via compression with loss of information.

The fact of lossy compressing the identifier of the transmitter device makes it possible to obtain information of reduced size on the identifier. This therefore makes it possible to reduce the size of the message containing the information on the identifier transmitted by the terminal at the access network (or possibly reduce the number of messages needed to send the identifier information if this information cannot be contained in a single message). This therefore makes it possible to limit the energy consumption and the radio resources necessary for the transmission of the message(s) by the terminal. This compression can also have the advantage of making it possible to send more identifiers in a message for a given message size. This has the effect of improving the accuracy of the geolocation.

This compression also has an advantage in terms of data confidentiality because the reduced identifiers transmitted in a message cannot a priori be decompressed by a third party who would intercept the message.

However, this compression leads to an ambiguity on the identifier of the transmitter device because a reduced identifier can potentially correspond to several identifiers of transmitter devices stored in the database of the geolocation server. The invention is based on the fact that the access network is capable of resolving this ambiguity using an ambiguity resolution database which makes it possible to determine the identifier of the transmitter device from the identifier reduced. The ambiguity resolution database makes it possible to construct lists of transmitter device identifiers each associated respectively with a different value of discrimination information. The access network determines the identifier of the transmitter device from the reduced identifier, from discrimination information determined from the message received from the terminal, and from the ambiguity resolution database. The discrimination information can be determined directly from parameters contained in the message transmitted by the terminal and/or from information available within the access network associated with the terminal which transmitted the message and derived from said message. The first step is to determine, in the ambiguity resolution database, a list associated with the value of the discrimination information determined from the message received from the terminal. Secondly, it is a question of determining, in this list, the identifier of the transmitter device which corresponds to the reduced identifier. The risk of there being an ambiguity in the list associated with the value of the discrimination information determined from the message received from the terminal is significantly reduced (there is an ambiguity if there remain in the list several identifiers which correspond to the same reduced identifier). Indeed, the size of a list built from the ambiguity resolution database is generally very small compared to the size of the list of all the identifiers of transmitter devices stored in the database of the server of geolocation. The size of the lists constructed from the ambiguity resolution database depends on the chosen discrimination information. In other words, by choosing the discrimination information suitably, it becomes possible for the access network, in the vast majority of cases, to remove the ambiguity on the identifier of the transmitter device. It should be noted that the “geographical position” of a transmitter device stored in the geolocation database can correspond, broadly, to position information representative of the exact geographical position of the transmitter device. Thus, it can be directly the coordinates (longitude, latitude and possibly altitude) of a geographical position of the transmitter device, but it can also be contextual information making it possible to estimate the exact geographical position of the transmitter device (such as a postal address, a store name, a district, region or country name, etc.).

In particular embodiments, the invention may also comprise one or more of the following characteristics, taken in isolation or in any technically possible combination.

In particular embodiments, the discrimination information includes information representative of a group of terminals to which the terminal belongs, and/or information representative of a geographical area in which the terminal is located, and/or information representative of at least one other identifier or reduced identifier of the transmitting device detected by the terminal.

In particular embodiments, obtaining the reduced identifier of the transmitter device includes a truncation of at least part of the identifier of the transmitter device.

In particular embodiments, obtaining the reduced identifier of the transmitter device comprises an estimation, for different parts of the identifier of the transmitter device, of a value representative of a discriminating power of each part for said identifier, and a selection of at least one part to be truncated according to the estimated values.

In particular embodiments, obtaining the reduced identifier of the transmitter device includes calculating a hash key using a hash function.

In particular embodiments, the hash function is implemented by an automatic learning algorithm previously trained to determine an optimal hash key for a sender device identifier. In particular embodiments, the first wireless communication protocol is a communication protocol of a wireless wide area network or a low-power wireless wide area network.

In particular embodiments, the second wireless communication protocol is a communication protocol of a wireless local area network, a communication protocol of a wireless personal area network, or a communication protocol of a system short distance communication.

According to a second aspect, the present invention relates to a method for updating an ambiguity resolution database as defined in any of the modes of implementation previously described. The updating method comprises in particular the following steps: - detection, by a terminal of the wireless communication system, of an identifier of a transmitter device from a message transmitted by the transmitter device,

- an evaluation, by the terminal, of a criterion to determine which of the identifier of the transmitter device or a reduced identifier of the transmitter device must be transmitted by the terminal in a message intended for the access network,

- an update of the ambiguity resolution database when the message sent by the terminal includes the identifier of the sender device in complete form.

In particular modes of implementation, the criterion is evaluated according to a number of identifiers of transmitting devices detected by the terminal during a predetermined period of time, and/or according to an indication given by a sensor of the terminal, and/or depending on a determination by the terminal whether the identifier of the sender device has already been sent, and/or depending on a configuration defined by the access network and sent to the terminal by the network access in a configuration message.

According to a third aspect, the present invention relates to a server connected to an access network of a wireless communication system (it may be a server belonging directly to the access network or a third-party server connected to the access network). The system comprises at least one terminal adapted to exchange messages with the access network according to a first wireless communication protocol and to detect, for at least one transmitter device, an identifier of said transmitter device from a message transmitted by said transmitter device according to a second wireless communication protocol. The server is configured to receive from the terminal a message comprising a reduced identifier of the transmitter device. The reduced identifier is the result of lossy compression of the identifier of the sending device. the server is also configured to determine the identifier of the transmitter device from the reduced identifier, from discrimination information determined from the message transmitted by the terminal, and from an ambiguity resolution database. The ambiguity resolution database makes it possible to construct several lists of transmitter device identifiers, each associated respectively with a different value of the discrimination information. The ambiguity resolution database is stored by the server or by one or more other servers. The server is further configured to estimate the geographic position of the terminal from the identifier of the transmitter device and using a geolocation database. The geolocation database stores a list of transmitter device identifiers and the respective geographical positions of said transmitter devices. The geolocation database is stored by a geolocation server separate from the server or servers storing the ambiguity resolution database. The server can be configured to implement any of the modes of implementation of the geolocation method previously described.

In particular embodiments, the server is further configured to receive from a terminal a message comprising an identifier of a transmitter device, and to update the ambiguity resolution database from the identifier received.

According to a fourth aspect, the present invention relates to an access network comprising a server according to any one of the preceding embodiments.

In particular embodiments, the access network is a wireless wide area network or a low power wireless wide area network. According to a fifth aspect, the present invention relates to a terminal of a wireless communication system. The terminal is suitable for exchanging messages with the access network according to a first wireless communication protocol. The terminal is configured to detect an identifier of a transmitter device from a message transmitted by the transmitter device according to a second wireless communication protocol. The terminal is further configured to evaluate a criterion to determine which of the identifier of the transmitter device or a reduced identifier of the transmitter device must be transmitted by the terminal in a message intended for the access network. When the reduced identifier must be transmitted, the terminal is configured to compress with loss the identifier of the transmitter device to obtain a reduced identifier of the transmitter device. Finally, the terminal is configured to transmit, to the access network, a message comprising either the identifier of the transmitter device, or the reduced identifier of the transmitting device, depending on the result of the evaluation of the criterion.

Presentation of figures

The invention will be better understood on reading the following description, given by way of non-limiting example, and made with reference to Figures 1 to 4 which represent:

[Fig. 1] a schematic representation of an embodiment of a wireless communication system used to geolocate a terminal using a geolocation server,

[Fig. 2] a schematic representation of an embodiment of a terminal,

[Fig. 3] a schematic representation of the main steps of a mode of implementation of a geolocation process for a terminal,

[Fig. 4] an illustration of an example of implementation of a device geolocation method,

[Fig. 5] an example of a possible structure for an ambiguity resolution database,

[Fig. 6] a schematic representation of the main steps of an embodiment of a method for updating an ambiguity resolution database.

In these figures, identical references from one figure to another designate identical or similar elements. For reasons of clarity, the elements represented are not necessarily to the same scale, unless otherwise stated.

Detailed description of an embodiment of the invention

FIG. 1 schematically represents a wireless communication system 10, comprising at least one terminal 20 and an access network 30 comprising several base stations 31 .

The terminal 20 is adapted to send messages on an uplink to the access network 30. Each base station 31 is adapted to receive the messages from the terminal 20 when said terminal is within its range. So Conventionally, a message transmitted by the terminal 20 comprises an identifier of the terminal 20. Each message received by a base station is for example transmitted to a server 32 of the access network 30, possibly accompanied by other information such as an identifier of the base station 31 which received it, the reception power level of said received message, the time of arrival of said message, the frequency at which the message was received, etc. The server 32 processes for example all the messages received from the different base stations 31.

The wireless communication system 10 can be unidirectional, that is to say it only allows exchanges of messages on the uplink from the terminal 20 to the access network 30. However, nothing excludes, according to other examples, to allow two-way exchanges. If necessary, the access network 30 is also adapted to transmit, via the base stations 31, messages on a downlink intended for the terminal 20, which is adapted to receive them.

The exchanges of messages on the uplink intended for the access network 30 use a first wireless communication protocol.

In particular embodiments, the first wireless communication protocol is a wireless wide area network communication protocol (“Wireless Wide Area Network” or WWAN in the English literature). For example, the first wireless communication protocol is a standardized communication protocol of the UMTS (Universal Mobile Telecommunications System) type, LTE (Long Term Evolution), LTE-Advanced Pro, 5G, etc.

Alternatively, the first wireless communication protocol is a low power wireless wide area network communication protocol (“Low Power Wide Area Network” or LPWAN in the Anglo-Saxon literature). Such a wireless communication system is a long-range access network (greater than one kilometer, or even greater than a few tens of kilometers), with low energy consumption (for example energy consumption during transmission or reception of a message less than 100 mW, or even less than 50 mW, or even less than 25 mW), and whose data rates are generally less than 1 Mbits/s. Examples of LPWAN networks include Sigfox, LoRaWAN, Ingenu, Amazon Sidewalk, Helium, etc. Such wireless communication systems are particularly suitable for loT or M2M type applications.

In a communication system of the loT or M2M type, the data exchanges are essentially unidirectional, in this case on an uplink from the terminals 20 to the access network 30 of the system 10 of wireless communication. In order to minimize the risks of losing a message transmitted by a terminal 20, the Access network planning is often carried out in such a way that a given geographical area is covered simultaneously by several base stations 31, in such a way that a message transmitted by a transmitter device 20 can be received by several base stations 31 . In the remainder of the description, it is considered by way of non-limiting example that the first wireless communication protocol is a low-power, ultra-narrow band wireless wide area network communication protocol. By "ultra narrow band"("Ultra Narrow Band" or UNB in the Anglo-Saxon literature), it is meant that the instantaneous frequency spectrum of the radio signals transmitted by the terminals has a frequency width of less than two kilohertz, or even less than one kilohertz. .

As illustrated by FIG. 1, the terminal 20 is also suitable for receiving messages sent by at least one sender device 40, which is in the vicinity of said terminal 20. The messages sent by the sender device 40 use a second communication protocol. wireless communication, different from the first wireless communication protocol. It should be noted that the transmitter device 40 can be entirely independent of the wireless communication system 10, and it does not need to support the first wireless communication protocol.

In particular embodiments, the second wireless communication protocol is of lower range than the range of the first wireless communication protocol. In such a case, the geographical position of the transmitter device 40, within the range of which the terminal 20 is located, provides more precise information on the geographical position of the terminal 20 than, for example, the geographical position of a base station 31 which receives a message transmitted by the terminal 20. It should however be noted that it is also possible, according to other examples, to have a second wireless communication protocol whose range is not less than that of the first wireless communication protocol.

The second wireless communication protocol is for example a wireless local area network communication protocol (“Wireless Local Area Network” or WLAN in the Anglo-Saxon literature), for example of WiFi type (IEEE 802.11 standards), etc., or even a wireless personal network communication protocol (“Wireless Personal Area Network” or WPAN in the Anglo-Saxon literature), for example of the Bluetooth or BLE type (acronym for “Bluetooth Low Energy”, “Bluetooth with low consumption in French), etc. According to yet another example, the second wireless communication protocol can be a short distance communication protocol based for example on NFC (acronym for “Near Field”) technology. Communication”, “near field communication” in French) or on RFID technology (acronym for “Radio Frequency Identification”, “identification par radios” in French).

A geolocation server 50 includes a database, called a "geolocation database", comprising a table storing identifiers of transmitter devices 40. If it is present in the geolocation database, a transmitter device identifier 40 is associated in the table with at least one piece of position information representative of the geographical position of the transmitter device 40. An identifier of a transmitter device 40 corresponds for example to a MAC address of the transmitter device 40 (MAC is the acronym for "Media Access Control", "support access control" in French). Other parameters could however play the role of identifier for a transmitter device 40, such as for example an SSID (English acronym for “Service Set IDentifier”, “identifier d’ensemble de services” in French) or a BSSID (English acronym for "Base Service Set IDentifier", "basic service set identifier" in French) of a WiFi access point, an identifier of a Bluetooth or BLE access point, an identifier of an RFID tag , etc.

The position information can be direct coordinates (longitude, latitude and possibly altitude) of a geographical position of the transmitter device 40. The position information can however also be contextual information making it possible to estimate the geographical position of the transmitter device 40, such as a postal address, a store name, a district, region or country name, etc. The geolocation server 50 is for example connected to the server 32 of the access network 30 by an internet connection.

Figure 2 schematically represents an embodiment of a terminal 20.

As illustrated by FIG. 2, the terminal 20 comprises a first communication module 21 suitable for exchanging messages with the base stations 31 according to the first wireless communication protocol. The first communication module 21 is for example in the form of a radio circuit comprising equipment (antenna, amplifier, local oscillator, mixer, analog filter, etc.). The terminal 20 also comprises a second communication module 22 adapted to receive messages transmitted by the transmitter device of interest 40, according to the second wireless communication protocol. The second communication module 22 is for example in the form of an electric radio circuit comprising equipment (antenna, amplifier, local oscillator, mixer, analog filter, etc.). In addition, the terminal 20 also comprises a processing circuit 23, connected to the first communication module 21 and to the second communication module 22. The processing circuit 23 comprises for example one or more processors and storage means (magnetic hard disk , electronic memory, optical disk, etc.) in which a computer program product is stored, in the form of a set of program code instructions to be executed to implement at least certain steps of a method of geolocation of the terminal and of a method for updating an ambiguity resolution database (see below).

The server 32 of the access network 30 also comprises one or more processors and storage means in which a computer program product is stored, in the form of a set of program code instructions to be executed in order to minus certain steps of a process for geolocation of the terminal and of a process for updating an ambiguity resolution database (see below). FIG. 3 schematically represents the main steps of an example of implementation of a method 100 according to the invention for geolocating a terminal 20.

The geolocation method 100 comprises a step 101 of detecting, by the terminal 20, for at least one transmitter device 40, an identifier of the transmitter device 40 from a message transmitted by the transmitter device 40 according to the second communication protocol. wireless communication.

The geolocation method 100 then comprises a step of obtaining

102, by the terminal 20, of a reduced identifier of the transmitter device 40. The reduced identifier is the result of a compression with loss of the identifier of the transmitter device 40. The reduced identifier therefore corresponds to information representative of the identifier of the transmitter device 40 which has a smaller size than the identifier of the transmitter device 40. For example, the identifier of the transmitter device 40 is coded on sixty-four bits, whereas the reduced identifier is coded on only twelve bits. This information can however be ambiguous insofar as several different identifiers of transmitting devices could share the same reduced identifier. The geolocation method 100 then comprises a step of transmitting

103, by the terminal 20, intended for the access network 30, of a message comprising the reduced identifier of the transmitter device 40. This message is transmitted according to the first wireless communication protocol.

The geolocation method 100 then includes a step 104 of determining, by the access network 30, the identifier of the transmitter device 40 from the reduced identifier and using a database, called " ambiguity resolution database”. The ambiguity resolution database makes it possible to construct different lists of identifiers of emitting devices associated respectively with different values of discrimination information. A particular value of discrimination information can be determined from the message transmitted by the terminal 20. A list associated with the value of the discrimination information determined from the message received from the terminal 20 can then be constructed using using the ambiguity resolution database. It then remains to determine, in this list, the identifier of the transmitting device which corresponds to the reduced identifier. The risk of there being an ambiguity in the list associated with the value of the discrimination information determined from the message received from the terminal is significantly reduced. Indeed, the size of a list built from the ambiguity resolution database is smaller than the size of the list of all the identifiers of transmitting devices stored in the database of the geolocation server. The geolocation method 100 then comprises an estimation step

105 of the geographical position of the terminal 20 from the identifier of the transmitting device and using the geolocation database of the server 50 of geolocation.

The estimated geographical position of the terminal 20 corresponds for example to the position of the transmitter device 40 stored in the geolocation database. The geographical position of the terminal 20 can optionally be refined according to a power level with which the beacon signal is received by the terminal 20, or using metadata known to the access network 30. It is also possible to estimating the geographical position of the terminal 20 as a function of the geographical position of several different transmitting devices 40 detected by the terminal 20 at a given instant.

In the example considered, the step 104 of determining the identifier of the transmitter device 40 from the reduced identifier and the ambiguity resolution database is implemented by the server 32 of the network of access 30. The step 105 of estimating the geographic position of the terminal 20 from the identifier of the transmitter device 40 and the geolocation database can be implemented by the server 32 of the access network 30 and/or by the geolocation server 50. According to a first example, the server 32 of the access network 30 sends to the geolocation server 50 the identifier of the transmitter device 40 or, when several transmitter devices have been detected by the terminal, all of the available identifiers, and the server 50 of geolocation estimates the position of the terminal 20 from the geographical positions associated with the various identifiers in the geolocation database. According to a second example, the geolocation server 50 merely returns the geographical position associated with each identifier in the geolocation database, and it is the server 32 which estimates the position of the terminal 20 from the geographical positions associated with the different identifiers.

The ambiguity resolution database is for example stored by the server 32, and/or by several other servers of the access network 30, or else by servers not belonging to the access network 30. place however in the case where the server or servers which store the ambiguity resolution database are distinct from the geolocation server 50 . These servers include information which is not known by the geolocation server 50 and which makes it possible to resolve any ambiguity in the determination of the identifier of a transmitter device 40 from the reduced identifier. This allows the operator of the ambiguity resolution database to maintain its database without depending on the operator of the geolocation database. In addition, the discrimination information may be associated with private information known only to the access network and which must not be communicated to the geolocation server 50. FIG. 4 illustrates an example of implementation of the geolocation method 100 according to the invention. In this example, we consider nine transmitter devices 40 respectively associated with the identifiers ID1, ID2, ..., ID9. The correspondence table 21 indicates the reduced identifier associated with each of the identifiers ID1, ID2, ..., ID9. In the example considered, there are six reduced identifier values: CID1, CID2, ..., CID6. It appears that the reduced identifier CID1 is associated both with the identifier ID1 and with the identifier ID3; the reduced identifier CID2 is associated both with the identifier ID2 and with the identifier ID6; the reduced identifier CID3 is associated with the identifier ID4; the reduced identifier CID4 is associated both with the identifier ID5 and with the identifier ID7; the reduced identifier CID5 is associated with the identifier ID7; the reduced identifier CID6 is associated with the identifier ID9. There are therefore ambiguities since the same reduced identifier is associated with several different identifiers, and it is therefore not possible to determine certain the identifier of a transmitter device from a reduced identifier.

In the example illustrated in Figure 4, the terminal 20 detects the transmitter device 40 whose identifier is ID3.

The terminal 20 then performs a compression of the identifier ID3 to obtain the reduced identifier CID1 associated with the identifier ID1 (to this end, it is assumed for example that the terminal 20 knows the compression function which makes it possible to obtain a identifier reduced from an identifier of a transmitter device, or that the terminal 20 knows the correspondence table 21).

The terminal 20 then transmits a message comprising the reduced identifier CID1 intended for the access network 30. The access network 30, and more particularly the server 32, then seeks to determine the identifier of the transmitter device 40 from the reduced identifier CID1 and using an ambiguity resolution database 33.

In the example considered, the ambiguity resolution database 33 makes it possible to construct different lists of identifiers of emitting devices associated respectively with different values of discrimination information: infol, info2, ..., info5. By way of non-limiting example, the discrimination information corresponds to the name of a client company of the access network 30 to which the terminal 20 belongs. In other words, in the example considered, the resolution database 33 of ambiguity is capable of constructing, for each of several client companies of the access network 30 (infol, info2, ..., info5), a list of identifiers of transmitter devices 40 which have already been detected by terminals belonging to said client company. The identifiers of transmitter devices 40 which have already been detected by terminals belonging to the client company whose name is infol are ID1, ID4 and ID5; the identifiers of transmitter devices 40 which have already been sent by terminals belonging to the client company whose name is info2 are ID2 and ID9; etc

In the example considered, the server 32 is capable of determining that for the message received from the terminal 20, the value of the discrimination information is info 3. In other words, the server 32 is capable of determining that the client company to which belongs to the terminal 20 which has just sent the message comprising the reduced identifier CID1 is the client company info 3. This information is for example explicitly contained in the message received from the terminal 20, or else this information can be derived from the message received from the terminal 20 (for example the message also includes an identifier of the terminal 20 and the server 32 is capable of determining which client company the terminal 20 belongs to from the identifier of the terminal 20). It then remains to determine, in the list associated with the discrimination information having the value info3, which transmitter device identifier 40, among the identifiers ID3, ID7 and ID8 of the list, corresponds to the reduced identifier CID1. Only identifier ID3 has reduced identifier CID1 in this list. The server 32 thus determines that the identifier of the transmitter device 40 detected by the terminal 20 is very probably ID3. It is of course assumed that the server 32 knows the decompression function which makes it possible to obtain a transmitter device identifier from a reduced identifier, or that the access network 32 knows the correspondence table 21).

The access network can then send a message comprising the identifier ID3 to the geolocation server 50. The geolocation server 50 comprises a geolocation database 51 which stores identifiers of transmitter devices 40 each associated respectively with at least one piece of position information representative of the geographical position of the transmitter device 40. In the example considered, the identifier ID1 is associated with the position information pos1, the identifier ID2 is associated with the position information pos2, ..., the identifier ID9 is associated with the position information pos9. The geographical position of the terminal 20 can then be estimated as being the geographical position of the transmitter device 40 whose identifier is ID3, namely the geographical position associated with pos3.

In the example illustrated in FIG. 4, it is considered that a single sender device identifier 40 is detected by the terminal 20. The invention could however also apply to the case where several identifiers corresponding to different sender devices 40 are detected. detected by the terminal 20 for a certain period of time then transmitted to the access network 30 in a single message or in several messages. The geographical position of the terminal 20 can then be determined according to the various position information associated in the geolocation database 51 with the various transmitting devices detected.

In the example illustrated in FIG. 4, without the ambiguity resolution database 33, it would not have been possible for the access network 30 and for the geolocation server 50 to determine with certainty what is the identifier of the transmitter device 40 detected by the terminal 20 among the identifiers ID1 and ID3 (that is to say among the identifiers having the reduced identifier CID1). It would then not have been possible to estimate the geographical position of terminal 20 (which could then correspond to pos1 as well as pos3).

The invention thus makes it possible to reduce the quantity of information to be transmitted by the terminal 20 (by compressing the identifier) while limiting the risk of ambiguity in the determination of the identifier of the transmitter device 40 (thanks to the base of data 33 of ambiguity resolution).

It should be noted that the resolution of ambiguity is not always guaranteed, in particular when several identifiers presenting the same reduced identifier respond to the same value of the discrimination information. The risk of an ambiguity persisting, however, can generally be greatly reduced by choosing the discriminating information appropriately.

The discrimination information may correspond to information representative of a group of terminals to which the terminal 20 belongs (such as for example the name of the client company to which the terminal 20 belongs, a type of device corresponding to the terminal 20, etc. .).

The discrimination information may also correspond to a geographical area in which the terminal 20 is located. In this case, the access network 30 must be able to determine position information representative of the geographical position of the terminal 20 without using the geolocation server 50. The position information of the terminal 20 can correspond to coordinates

(longitude, latitude and possibly altitude) of an estimated geographical position of terminal 20, possibly with an indication of the accuracy of this estimated position. The terminal 20 position information can however also be contextual information making it possible to estimate the approximate geographical position of the terminal 20, such as for example a postal address, a store name, a district, region or country name, etc This contextual information can in particular be obtained from a parameter associated with the message received from the terminal 20. By way of example, it is possible to determine in which region or in which country the terminal 20 is located from an identifier of the terminal if the access network is aware that this terminal belongs to a client company which operates only in a particular region or country.

The access network 30 is for example configured to estimate the geographical position of the terminal 20 according to messages received from said terminal 20. In particular modes of implementation, the geographical position is estimated from the message received comprising the identifier of the transmitter device 40. However, nothing excludes, according to other examples, estimating the geographical position of the terminal 20 from other messages previously transmitted by the terminal 20.

Different methods for estimating the geographical position of the terminal 20 can be implemented. For example, the access network 30 can estimate the geographical position of the terminal 20 as being the geographical position of a base station 31 which has received a message transmitted by the terminal 20. If several stations 31 can receive a message transmitted by the terminal 20, it is possible to estimate the geographical position of the terminal 20 according to the geographical positions of all the base stations 31 which have received the message transmitted by the terminal 20 (for example by defining a barycenter of these geographical positions). According to another example, the access network 30 can estimate the distances which separate the terminal 20 from one or more base stations 31 by calculating the propagation time of a message sent by the terminal 20 to the base stations 31 from TOA measurements or measurements of differences in times of arrival (“Time Difference of Arrival” or TDOA in the Anglo-Saxon literature) of this message at the different base stations 31. It is then possible to estimate the position of the terminal 20 by multilateration if the geographical positions of the base stations 31 are known.

According to another example, it is possible to estimate the position of the terminal 20 by multilateration by determining the distances which separate the terminal 20 from several base stations 31 from an RSSI measurement for each base station 31 for a message sent by the terminal 20 to the access network 30.

According to yet another example, the method for estimating a geographical position of the terminal 20 by the access network 30 can be based on automatic learning techniques (“Machine Learning” in the Anglo-Saxon literature) which associate a radio fingerprint at a geographical position of the geographical area considered.

In particular embodiments, the estimation of the geographical position of the terminal 20 is carried out by the access network 30 without any explicit information contributing to this estimation being sent by the terminal in a message intended for the access network (in other words, the terminal does not send messages to the access network whose binary data includes information making it possible to estimate the geographical position of the terminal). Such provisions make it possible to limit the quantity of data exchanged between the terminal and the access network to geolocate the terminal 20. The discrimination information can also correspond to information representative of at least one other identifier (or at least one other reduced identifier) of transmitter device 40 detected by terminal 20. Indeed, transmitter devices 40 which are geographically close to each other are generally detected simultaneously by a terminal 20. This information can make it possible to resolve an ambiguity on an identifier of transmitting device: for example, if two transmitting devices respond to the same reduced identifier, but only one of these two devices transmitters has already been detected simultaneously with another transmitting device indicated in the message transmitted by the terminal 20, then it is very likely that this transmitting device is the one detected by the terminal 20.

Numerous other examples of discrimination information could be envisaged and determined from information explicitly contained in the message received from the terminal 20, or from metadata of the message known to the access network 30. The choice of a particular discrimination information is only one variant of the invention.

The discrimination information can also correspond to the combination of several information of different natures.

Different structures can be envisaged for storing the data in the ambiguity resolution database 33 . The choice of a particular structure of the ambiguity resolution database 33 is only one variant of the invention. FIG. 4 illustrates a first example according to which the ambiguity resolution database 33 stores different lists of identifiers, each list being associated respectively with a different value of the discrimination information.

Figure 5 illustrates, by way of non-limiting example, another possible structure for the ambiguity resolution database 33. In the example considered and illustrated in FIG. 5, each line corresponds to a message received from a terminal 20 of the communication system 10 . For each message received, several pieces of information relating to said message are stored. For example, the information C1, C2, ..., CK correspond to characteristics relating to the terminal 20 determined by the access network 30 from the message. This may in particular be explicit data contained in the message or metadata associated with the message or the terminal that sent the message (for example, a terminal identifier, the name of the client company to which the terminal belongs, a particular type of terminal, a particular service associated with the terminal, information on the geographical position of the terminal determined by the access network, etc.). For each message received, the ambiguity resolution database 33 also stores the reduced identifier CID of the sender device indicated in the message and, when it can be determined, the identifier ID of the sender device. When several transmitter devices are detected by the terminal 20 and entered in the message sent to the access network 30, then it is possible to memorize, for this message, the reduced identifiers and, when they can be determined, the identifiers of these different transmitting devices. Discrimination information can then correspond to a particular characteristic determined for the message received, or else to a combination of particular characteristics determined for the message received.

For example, if the access network 30 is able to determine that the terminal belongs to client company A (C1=A), and that the terminal is located in country B (C2=B), then it is possible to make a request to the ambiguity resolution database 33 to construct a list of identifiers ID associated in the database with previously received messages for which the conditions (C1=A) and (C2=B) are also satisfied. It then remains to determine in this list, which identifier corresponds to the reduced identifier indicated in the message being processed.

According to another example, a characteristic (for example C3) can correspond to coordinates of an approximate geographical position of the terminal 20 determined by the access network 30 (C3=C). In this case, it is possible to make a request to the ambiguity resolution database 33 to construct a list of identifiers ID associated in the database with previously received messages for which the position indicated for the characteristic C3 belongs to a region geographical centered on C and presenting a certain diameter.

It should be noted that, if the reduced identifier indicated in the message being processed is D, it is also possible to directly include the condition (CID=D) in the request made to the database 33 for resolving d 'ambiguity. In this case the list obtained will contain only one element if there is no ambiguity.

If there remains an ambiguity in the list obtained after the request made to the ambiguity resolution database 33 (that is to say if there remain in this list several identifiers corresponding to the same reduced identifier), then it is possible to make another more selective request.

It is thus possible to develop different strategies to determine the identifier of the transmitter device 40 in step 104 of the method 100 of geolocation. A first strategy could consist in making a first query that is not very selective to the ambiguity resolution database 33, then making other more selective queries only if necessary, that is to say if it is remains an ambiguity in the list obtained in response to previous queries. A second strategy could consist of making a very selective request from the first request in order to ensure that there is no ambiguity from the response to the first request. The strategy can possibly be refined over the messages received to gradually reduce the complexity of the request as long as there is no ambiguity in the list obtained in response to the request. Optionally, a machine learning algorithm can be put in place to determine the best query in terms of a compromise between the complexity of the query and the risk of an ambiguity remaining in the list obtained in response to the query. This can for example be done using a decision tree forest algorithm (“random forest” in the Anglo-Saxon literature). This algorithm is known to be efficient in distributing data in a partition set, even when the number of criteria reaches a large value. In this case, each leaf of a decision tree constitutes a list of identifiers and the path to arrive at a leaf corresponds to the discrimination information. The discrimination information can then change according to the message received according to potentially very complex rules.

There is indeed a compromise to be found between the complexity (and therefore the selectivity) of the request, the risk that there remains an ambiguity in the list obtained in response to the request, and the risk of not finding an identifier responding to the reduced identifier in the list obtained in response to the request. Indeed, the more selective the query, the lower the number of elements in the list obtained in response to the query, and the lower the risk that there will remain an ambiguity for these identifiers. On the other hand, the more the request is selective, the greater the risk of not finding an identifier corresponding to the reduced identifier.

Different strategies are also possible with regard to the step of obtaining 102 the reduced identifier via lossy compression of the identifier of the transmitter device 40.

In particular embodiments, a reduced identifier can be constructed by truncating at least part of the identifier of the transmitter device 40. The part to be truncated can correspond for example to a certain part of the most significant bits or of the most significant bits low identifier.

According to another example, the part to be truncated can be chosen by estimating a discriminating power of each part of the identifier, and by selecting the part to be truncated having the smallest discriminating power. For example, the identifier may include a part with several bit bytes allocated to the identification of the company that built the transmitter device 40. This part allows millions of possibilities while the number of companies actually discriminated by this part is in the thousands. Therefore, it is advantageous to truncate this part rather than another to construct a partial identifier. In general, it is possible to take advantage of the difference between the theoretical space and the effective space of identifiers and their representation bit by bit. In the case mentioned above, the bits identifying the companies have a lower discriminating power than the others. It is conceivable to carry out simulations and experimentally measure a collision rate according to the truncated parts, in order to determine the best encoding of the reduced identifier in terms of discriminating power (by collision, we mean that the same reduced identifier corresponds to one or more existing identifiers). In particular embodiments, the reduced identifier of the transmitter device 40 is determined from a calculation of a hash key using a hash function. It should be noted that the hash function used, and the size of the hash key, may be different for different groups of terminals. However, the terminal should know the hash function to be used to decompress a reduced identifier according to the terminal which carried out the compression.

In particular embodiments, the hash function is implemented by a machine learning algorithm previously trained to determine an optimal hash key for a sender device identifier. For example, one can use a deep neural network to construct an optimal hash key by integrating the original identifiers into the latent vector space. Such an approach makes it possible to minimize the collision rate. The hash function thus determined is then more efficient since it is adapted to the data.

It is now appropriate to consider how the ambiguity resolution database 33 can be built and kept up to date.

FIG. 6 schematically represents the main steps of an exemplary implementation of a method 200 for updating an ambiguity resolution database 33 (or for creating the ambiguity resolution database 33 ambiguity if it does not yet exist). The updating method 200 comprises a step 201 of detecting, by a terminal 20 of the wireless communication system 10, for at least one transmitter device 40, an identifier of the transmitter device 40 from a message transmitted by the transmitter device 40. It should be noted that this may be the same terminal 20 or another terminal 20 as that previously mentioned in the description of the method 100 of geolocation. Also, it may be the same transmitter device 40 or another transmitter device 40 as that mentioned in the description of the method 100 of geolocation.

The updating method 200 then includes a step 202 of evaluating, by the terminal 20, a criterion to determine which of the identifier of the transmitter device 40 or a reduced identifier of the transmitter device 40 must be transmitted by the terminal. 20 in a message to the access network 30. This step 202 evaluation of a criterion for an identifier can of course be repeated for several identifiers when several identifiers are detected by the terminal 20. The terminal can then transmit in the message intended for the access network 30 a combination comprising identifiers under their full form and/or identifiers in their reduced form.

Finally, the method 200 includes a step 203 of updating, by the access network 30 (and more particularly by the server 32), the ambiguity resolution database 33 when the message transmitted by the terminal 20 includes the identifier of the transmitter device 40. This step may in particular consist of storing in the ambiguity resolution database 33 an association between the identifier in its complete form, the identifier in its reduced form, and one or more discrimination information.

With such provisions, it becomes possible to store identifiers of transmitter devices in their complete (non-reduced) form in the ambiguity resolution database 33, and to associate these identifiers with discrimination information determined by the network access 30.

Several methods can be envisaged for evaluating the criterion making it possible to determine whether the full identifier or the reduced identifier must be sent by the terminal 20. In particular embodiments, the criterion is evaluated according to a number of identifiers transmitter devices detected by the terminal 20 for a certain predetermined period of time. By way of non-limiting example, if a single identifier is detected by the terminal, then the complete identifier is sent; if two identifiers are detected, one of the two identifiers is sent in its complete form while the other identifier is sent in its reduced form; if three or more identifiers are detected, all identifiers are sent in their reduced form.

In particular embodiments, the terminal 20 is configured to transmit a certain proportion of the identifiers in their complete form. By way of non-limiting example, the terminal 20 is configured to send one out of two identifiers in its full form and one out of two identifiers in its reduced form.

In particular embodiments, the terminal 20 is configured to gradually reduce the proportion of identifiers sent in their complete form. By way of non-limiting example, the terminal 20 is configured to send all the identifiers in their complete form during a first period, then one identifier out of two during a second period, then one identifier out of four during a third period, etc. In particular embodiments, the terminal 20 is configured to evaluate the criterion according to an indication given by a sensor of the terminal 20. This may be for example an indication relating to the start or the end of a mobility phase of the terminal 20 (it is advantageous to send identifiers in their complete form when the terminal 20 moves to promote the success of its geolocation), or an indication relating to a certain temperature or a certain pressure undergone by the terminal 20 (it is advantageous to send identifiers in their complete form when such an alert indication is detected, to promote the success of the geolocation of the terminal 20, despite the disadvantages of having to send identifiers in their form complete). The sensor can therefore be a motion sensor (accelerometer, gyroscope, magnetometer, etc.), a temperature sensor, a pressure sensor, a smoke sensor, a noise sensor, etc.

In particular embodiments, the terminal 20 is configured to evaluate the criterion according to whether or not the identifier of the transmitter device 40 has already been sent previously by the terminal 20 (when an identifier has been sent in its complete form , it is possible to simply send it afterwards in its reduced form). It should be noted that a notion of time can also be introduced: the criterion can in particular take into account the date on which the identifier of the transmitter device 40 has already been sent previously. If the date is far away, then it is advantageous to promote the issuance of the identifier in its complete form. If the date is recent, then it is advantageous to send the identifier in its reduced form.

In particular embodiments, the terminal 20 is configured to evaluate the criterion according to a configuration defined by the access network 30 using a configuration message sent by the access network 30 to destination terminal 20. In this case the communication system must be bidirectional. Different configurations can be envisaged as a function of time and/or as a function of the geographical position of the terminals 20.

Of course, the criterion can also be assessed on the basis of a combination of the examples cited above. Also, other methods can be considered for evaluating the criterion making it possible to determine whether the full identifier or the reduced identifier must be sent by the terminal 20. The choice of a particular method is only a variant of the 'invention.

Any type of interface can be envisaged between the server 32 of the access network 30 and the geolocation server 50 and between the server 32 and the server hosting the ambiguity resolution database (when the resolution database of ambiguity is hosted by a server different from the server 32). In particular, it is possible to consider the use of a file, a programming interface, a callback function, etc.

The above description clearly illustrates that, through its various characteristics and their advantages, the present invention achieves the set objectives. In particular, the invention makes it possible to limit the energy consumption of the terminal and/or the necessary radio resources while maintaining good geolocation performance.

Claims

1. Method (100) for geolocating a terminal (20) of a wireless communication system (10), said terminal (20) being adapted to exchange messages with an access network (30) of said system ( 10) for wireless communication according to a first wireless communication protocol, said method comprising:

- detection (101), by the terminal (20), for at least one transmitter device (40), of an identifier of said transmitter device (40) from a message transmitted by said transmitter device (40) according to a second wireless communication protocol,

- obtaining (102), by the terminal (20), of a reduced identifier of the transmitter device (40) by compression with loss of the identifier of the transmitter device (40), said compression resulting in an ambiguity because the identifier reduced can correspond to several identifiers of different transmitting devices (40),

- a transmission (103), by the terminal (20) and intended for the access network (30), of a message comprising the reduced identifier of the transmitter device (40),

- a determination (104) of the identifier of the transmitter device (40) from the reduced identifier, from discrimination information determined from the message transmitted by the terminal (20), and from a database (33) ambiguity resolution making it possible to construct several lists of transmitter device identifiers each associated respectively with a different value of the discrimination information,

- an estimate (105) of the geographical position of the terminal (20) from the identifier of the transmitting device and using a geolocation database (51) storing a list of identifiers of transmitting devices and the respective geographic positions of said transmitter devices (40), wherein the geolocation database (51) is stored by a geolocation server (50) and the ambiguity resolution database (33) is stored by one or several servers (32) separate from the geolocation server (50).

2. Method (100) according to claim 1 in which the discrimination information comprises:

- information representing a group of terminals to which the terminal (20) belongs, and/or - information representative of a geographical area in which the terminal (20) is located, and/or

- information representative of at least one other identifier or reduced identifier of the transmitting device (40) detected by the terminal (20).

3. Method (100) according to any one of claims 1 to 2 wherein the obtaining (102) of the reduced identifier of the transmitter device (40) comprises a truncation of at least part of the identifier of the device transmitter (40).

4. Method (100) according to claim 3 wherein obtaining (102) the reduced identifier of the transmitter device (40) comprises:

- an estimate, for different parts of the identifier of the transmitter device (40), of a value representative of a discriminating power of each part for said identifier, and

- a selection of at least one part to be truncated according to the estimated values.

5. Method (100) according to any one of claims 1 to 4 wherein the obtaining (102) of the reduced identifier of the transmitter device (40) comprises a calculation of a hash key using a hash function.

6. The method (100) according to claim 5 wherein the hash function is implemented by a pre-trained machine learning algorithm to determine an optimal hash key for a sender device identifier.

A method (100) according to any of claims 1 to 6 wherein the first wireless communication protocol is a wireless wide area network or low power wireless wide area network communication protocol.

8. A method (100) according to any one of claims 1 to 7 wherein the second wireless communication protocol is a wireless local area network communication protocol, a wireless personal area network communication protocol, or a communication protocol of a short distance communication system.

9. A method (200) for updating an ambiguity resolution database (33) as defined in any one of claims 1 to 8, said method (200) comprising:

- detection (201), by a terminal (20) of the wireless communication system (10), of an identifier of a transmitter device (40) from a message transmitted by the transmitter device (40),

- an evaluation (202), by the terminal (20), of a criterion to determine which of the identifier of the transmitter device (40) or a reduced identifier of the transmitter device (40) must be transmitted by the terminal (20) in a message to the access network (30),

- an update (203) of the ambiguity resolution database (33) when the message sent by the terminal (20) includes the identifier of the sender device (40) in a complete form.

10. Method (200) according to claim 9 in which the criterion is evaluated:

- according to a number of transmitter device identifiers detected by the terminal (20) during a predetermined period of time, and/or

- according to an indication given by a sensor of the terminal (20), and/or

- depending on a determination by the terminal (20) if the identifier of the transmitter device (40) has already been sent, and/or

- according to a configuration defined by the access network (30) and transmitted to the terminal (20) by the access network (30) in a configuration message.

11. Server (32) connected to an access network (30) of a wireless communication system (10), said system (10) comprising at least one terminal (20) adapted to exchange messages with the network. access (30) according to a first wireless communication protocol and to detect, for at least one transmitter device (40), an identifier of said transmitter device (40) from a message transmitted by said transmitter device (40) according to a second wireless communication protocol, said server (32) being characterized in that it is configured to:

- receive from the terminal (20) a message comprising a reduced identifier of the transmitter device (40), said reduced identifier being the result of a compression with loss of the identifier of the transmitter device (40), said compression resulting in ambiguity because the the reduced identifier can correspond to several identifiers of different transmitter devices (40), - determining the identifier of the transmitter device (40) from the reduced identifier, discrimination information determined from the message transmitted by the terminal (20), and a resolution database (33) of ambiguity making it possible to construct several lists of identifiers of transmitter devices each associated respectively with a different value of the discrimination information, the ambiguity resolution database (33) being stored by the server (32) or by one or more other servers,

- estimate the geographical position of the terminal (20) from the identifier of the transmitting device and using a geolocation database (51) storing a list of identifiers of transmitting devices and the respective geographical positions of said transmitting devices (40), the geolocation database (51) being stored by a geolocation server (50) distinct from the server or servers (32) storing the ambiguity resolution database (33).

12. Server (32) according to claim 11, said server (32) being further configured to:

- receiving from a terminal (20) a message comprising an identifier of a transmitter device (40), - updating the ambiguity resolution database (33) from the received identifier.

13. Access network (30) comprising a server (32) according to any one of claims 11 to 12.

14. An access network (30) according to claim 13, said access network (30) being a wireless wide area network or a low power wireless wide area network.

15. Terminal (20) of a wireless communication system (10), said terminal (20) being adapted to exchange messages with the access network (30) according to a first wireless communication protocol, said terminal ( 20) being characterized in that it is configured for:

- detect, for at least one transmitter device (40), an identifier of said transmitter device (40) from a message transmitted by the transmitter device (40) according to a second wireless communication protocol, - evaluating a criterion to determine which of the identifier of the transmitter device (40) or a reduced identifier of the transmitter device (40) must be transmitted by the terminal (20) in a message intended for the access network (30),

- when the reduced identifier must be transmitted, lossy compressing the identifier of the transmitter device (40) to obtain a reduced identifier of the transmitter device (40), the reduced identifier being ambiguous because it can correspond to several identifiers of transmitter devices (40) different,

- send, to the access network, a message comprising either the identifier of the sender device (40), or the reduced identifier of the sender device (40), depending on the result of the evaluation of the criterion.

16. Terminal (20) according to claim 15, in which the criterion is evaluated:

- according to a number of identifiers of transmitting devices detected by the terminal (20) during a predetermined period of time, and/or - according to an indication given by a sensor of the terminal (20), and/or