Improved cluster management within an intelligent transport system

Abstract

Methods of communication in an intelligent transport system, ITS, comprising a plurality of ITS stations, ITS-Ss, at least one ITS-S being associated with a vulnerable road user, VRU, belonging to a c

Description

FIELD OF THE DISCLOSURE The present disclosure relates generally to Intelligent Transport Systems (ITS) and more specifically to Cooperative Intelligent Transport Systems (C-ITS). BACKGROUND OF DISCLOSURE Cooperative Intelligent Transport Systems (C-ITS) is an emerging technology for future transportation management that aims at improving road safety, traffic efficiency and driver experience. Intelligent Transport Systems (ITS), as defined by the European Telecommunications Standards Institute (ETSI), include various types of communication such as: - communications between vehicles (e.g., car-to-car), and - communications between vehicles and fixed locations (e.g., car-to-infrastructure). C-ITS are not restricted to road transport as such. More generally, C-ITS may be defined as the use of information and communication technologies (ICT) for rail, water and air transport, including navigation systems. Such various types of C-ITS generally rely on radio services for communication and use dedicated technologies. Such C-ITS are subject to standards, specified for each country and / or territory where C-ITS are implemented. Today in Europe, the European Telecommunications Standards Institute is in charge of the elaboration of the specifications forming the standards to which C-ITS are subjected. Cooperation within C-ITS is achieved by exchange of messages, referred to as ITS messages, among ITS stations (denoted ITS-Ss). The ITS-Ss may be vehicles, Road Side Units (RSUs), Vulnerable Road Users (VRUs) carrying an ITS equipment (for instance included in a smartphone, a GPS, a smart watch, or in a bicyclist equipment), or any other entities or infrastructure equipped with an ITS equipment, as well as central subsystems (back-end systems and traffic management centers). C-ITS may support various types of communications, for instance between vehicles (vehicle-to-vehicle or “V2V”), referring to all kinds of road users, e.g. car-to-car, or between vehicles and fixed locations such as vehicle-to-infrastructure or “V2I”, or between vehicles and pedestrians such as vehicle-to-pedestrian or “V2P” and infrastructure-to-vehicle or“l2V”, e.g., car-to-infrastructure. Such message exchanges may be performed via a wireless network, referred to as “V2X” (for “vehicle” to any kind of devices) networks, examples of which may include 3GPP LTE- Advanced Pro, 3GPP 5G, or IEEE 802.11p technology. Exemplary ITS messages include Collective Perception Messages (CPMs), Cooperative Awareness Messages (CAMs), and Vulnerable Road User (VRU) Awareness Messages (VAMs). The ITS-S sending an ITS message is named “originating” ITS-S and the ITS-S receiving an ITS message is named “receiving” ITS-S. ETSI TS 103 324 Specification (V0.0.35 of June 2022) defines the Collective Perception Service through which an ITS-S having on-board sensor systems detects objects in its vicinity and transmits, using broadcast CPMs, description information (e.g. dynamics such as position and / or kinematic information) thereof. The CPMs are sent periodically with a period from 100 ms to 1 s depending for example on the speed of the objects sensed by the originating ITS-S. EN 302 637-2 Specification (V1.4.1 of April 2019) defines the Cooperative Awareness Basic Service through which an ITS-S transmits, using broadcast CAMs, its ego-vehicle dynamics (e.g. position and speed). ETSI TS 103 300-3 Specification (V2.1.2 of April 2021) defines the Vulnerable Road User (VRU) Awareness Basic Service through which an ITS-S transmits, using broadcast VAMs, its ego-position and speed. In dense environment such as cities, many VRUs equipped with an ITS-S can send VAMs. To optimize the use of the radio communication channel while keeping the protection of VRUs offered by the VRU Awareness Basic Service (VBS), some VRU Clustering Operations are possible. Each ITS station has an environment model called Local Dynamic Map (LDM) that is regularly updated with highly dynamic data to locate vehicles, pedestrians, bicycles, etc. in the vicinity of the ITS station. The LDM is updated using information from on-board sensors and completed with information from received ITS messages such as: awareness messages containing the ego-position and the speed of connected vehicles (CAM) or of connected Vulnerable Road Users (VAM) and - collective perception messages (CPM) containing the objects perceived (e.g. vehicles, motorbikes, bicycles or pedestrians) by sensor-equipped ITS stations, referred to as the perceived objects. CPM improve the local perception ability (larger field of view, non-connected objects, etc.). In a case where an ITS station perceives several VRUs, a CPM can report a set of VRUs either as individual VRUs or as a set of VRUs (e.g., a set of VRUs moving in a coherent manner such as a set of pedestrians or bicyclists having coherent directions and speeds), the set being referred to as a VRU Group. This is advantageous to reduce the size of the CPM messages when there are many VRUs to report or when individual information about perceived VRUs is difficult to obtain and / or to analyze (e.g., in the case of a crowd). Similarly, a VRU ITS station may also report the current state of a set of VRUs containing its ITS-S. In such a case, the reporting is not individual but global given the current state of the set of VRUs such as its reference position, its dimension in the form of a bounding box shape, and its size (number of VRUs). This set is referred to as a VRU Cluster. Like a VRU Group, a VRU Cluster is a set of VRUs moving in a coherent manner, e.g., with coherent directions and speeds. One of the VRU ITS-Ss of the VRU Cluster acts as a VRU cluster leader, which is in charge of managing the set of VRUs. Only the VRU ITS-S cluster leader is transmitting VAMs on the radio communication channel, while the other VRU ITS-Ss of the VRU Cluster (referred to as cluster members) are set in a VRU passive state (i.e., they do not transmit their individual VAMs). This makes it possible to reduce the number of messages on the radio communication channel and to simplify the analysis that is carried out by the ITS-S receiving a single message containing items of information about the cluster rather than a set of messages for each individual VRU. Reporting items of information about a set of VRUs considered as a single VRU Group through CPMs or about a set of VRUs considered as a single VRU Cluster through VAMs instead of considering several individual VRUs makes it possible to reduce significantly the amount of radio communication resource used to transmit the C-ITS messages. In addition, at the receiving ITS-S end, it also reduces the amount of data to analyze, which can be crucial in very dense area location like an urban pedestrian crossing. While these mechanisms have proven to be efficient, there is a constant need to improve the mechanisms used for exchanging data among ITS stations, in particular to improve communication efficiency in terms of safety and in terms of resource consumption. SUMMARY OF THE DISCLOSURE The present disclosure has been devised to address one or more of the foregoing concerns. According to a first aspect of the disclosure, there is provided a method of communication in an intelligent transport system, ITS, comprising a plurality of ITS stations, ITS-Ss, at least one ITS-S being associated with a vulnerable road user, VRU, belonging to a cluster of VRUs managed by a first ITS-S and belonging to a group of VRUs managed by a second ITS-S, the method, to be carried out at the second ITS-S, comprising: determining that the VRU cluster and the VRU group may be managed jointly by a single ITS-S; and determining whether the second ITS-S is to manage the VRU cluster. Accordingly, the method of the disclosure makes it possible to manage a set of VRUs through CPMs and / or VAMs in order to improve the management of such a set of VRUs in particular circumstances. According to some embodiments, the method further comprises generating and transmitting an ITS message, the ITS message comprising an identifier of the VRU cluster and a managing indicator for management of the VRU cluster, the managing indicator being determined as a function of the determining whether the second ITS-S is to manage the VRU cluster. According to some embodiments, the managing indicator is an indication to inform ITS-Ss about an ITS-S in charge of managing the VRU cluster. According to some embodiments, the managing indicator is a request to an ITS-S different from the second ITS-S to manage the VRU cluster. According to some embodiments, the ITS-S requested to manage the VRU cluster is the first ITS-S, the method further comprising in response to transmitting the generated ITS message, receiving an ITS message from the first ITS-S, the received ITS message comprising an indicator to indicate that the first ITS-S accepts to manage the VRU cluster and associating a group state to the VRU group to indicate that the VRU cluster is managed by an ITS-S different from the second ITS-S. According to some embodiments, the method further comprises receiving an ITS message from the first ITS-S, the received ITS message comprising an indicator to request that the first ITS-S manages the VRU cluster. According to some embodiments, the method further comprises receiving an ITS message from the first ITS-S, the received ITS message comprising an indicator to indicate whether the first ITS-S manages the VRU cluster. According to some embodiments, the managing indicator indicates that the second ITS-S manages the VRU cluster, the method further comprising associating a cluster leader state to the VRU group. According to some embodiments, the method further comprises receiving an ITS message from an ITS-S, the received ITS message comprising a request to join the VRU cluster or a request to leave the VRU cluster and updating characteristics of the VRU cluster further to processing the received request. According to some embodiments, the managing indicator is indicative of a break up of the VRU cluster, the method further comprising associating a group state to the VRU group to indicate that the VRU cluster and the VRU group are not managed jointly by a single ITS-S. According to some embodiments, the generated ITS message further comprises an indicator of a reason of the break up. According to some embodiments, the generated ITS message is a Collective Perception Message, CPM. According to some embodiments, the received ITS message is a VRU Awareness Message, VAM. According to some embodiments, the method further comprises detecting, using sensors of the second ITS-S, a plurality of VRUs, the plurality of VRUs comprising the VRU associated with the at least one ITS-S. According to a second aspect of the disclosure, there is provided a method of communication in an intelligent transport system, ITS, comprising a plurality of ITS stations, ITS-Ss, at least one ITS-S being associated with a vulnerable road user, VRU, belonging to a cluster of VRUs managed by a first ITS-S and to a group of VRUs managed by a second ITS-S, the method, to be carried out at the first ITS-S, comprising: receiving an ITS message from the second ITS-S, the ITS message comprising characteristics of the VRU group, the characteristics of the VRU group comprising a reference to the VRU cluster, determining, from the received ITS message, that the VRU cluster and the VRU group may be managed jointly by a single ITS-S, generating and transmitting an ITS message, the ITS message comprising a managing indicator for management of the VRU cluster. Accordingly, the method of the disclosure makes it possible to manage a set of VRUs through CPMs and / or VAMs in order to improve the management of such a set of VRUs in particular circumstances. According to some embodiments, the received ITS message further comprises a second managing indicator for management of the VRU cluster, the managing indicator of the generated ITS message being referred to as the first managing indicator, the second managing indicator requesting the first ITS-S to manage the VRU cluster, the method further comprising setting an operating mode of the first ITS-S to a cluster leader mode comprising managing the VRU cluster and setting the first managing indicator to a value indicating that the first ITS-S manages the VRU cluster. According to some embodiments, the received ITS message further comprises a second managing indicator for management of the VRU cluster, the managing indicator of the generated ITS message being referred to as the first managing indicator, the second managing indicator indicating that the second ITS-S manages the VRU cluster. According to some embodiments, the method further comprises setting an operating mode of the first ITS-S to a VRU passive mode comprising refraining the first ITS-S from broadcasting VRU awareness messages, VAMs. According to some embodiments, the method further comprises generating and transmitting an ITS message to indicate a break up of the VRU cluster. According to some embodiments, the received ITS message is a Collective Perception Message, CPM. According to some embodiments, the generated ITS message is a VRU Awareness Message, VAM. According to other aspects of the disclosure, there is provided a device configured for carrying out each of the steps of the method described above and a non-transitory computer-readable medium storing a program which, when executed by a microprocessor or computer system in an Intelligent Transport System station, ITS-S, causes the ITS-S to perform each step of the method described above. These aspects of the disclosure have advantages similar to those mentioned above. At least parts of the methods according to the disclosure may be computer implemented. Accordingly, the present disclosure may take the form of an entirely hardware embodiment, an entirely software embodiment (including firmware, resident software, micro-code, etc.) or an embodiment combining software and hardware aspects that may all generally be referred to herein as a "circuit", "module" or "system". Furthermore, the present disclosure may take the form of a computer program product embodied in any tangible medium of expression having computer usable program code embodied in the medium. Since the solutions of the present disclosure can be implemented in software, the solutions of the present disclosure can be embodied as computer readable code for provision to a programmable apparatus on any suitable carrier medium. A tangible carrier medium may comprise a storage medium such as a floppy disk, a CD-ROM, a hard disk drive, a magnetic tape device or a solid state memory device and the like. A transient carrier medium may include a signal such as an electrical signal, an electronic signal, an optical signal, an acoustic signal, a magnetic signal or an electromagnetic signal, e.g., a microwave or RF signal. BRIEF DESCRIPTION OF THE DRAWINGS Further advantages of the present disclosure will become apparent to those skilled in the art upon examination of the drawings and detailed description. Embodiments of the disclosure will now be described, by way of example only, and with reference to the following drawings, in which: Figure 1 illustrates an example of an ITS system in which some embodiments of the disclosure may be implemented; Figure 2 illustrates a schematic representation of the architecture of an ITS station transmitting CPMs in accordance with embodiments of the disclosure; Figure 3 illustrates a schematic representation of the architecture of a VRU ITS station transmitting VAMs in accordance with embodiments of the disclosure; Figure 4 illustrates an example of a structure of a VRU awareness message, VAM, according to some embodiments of the disclosure; Figure 5 illustrates an example of a structure of a collective perception message, CPM, according to some embodiments of the disclosure; Figure 6 illustrates an example of a VRU ITS-S state diagram according to some embodiments of the disclosure; Figure 7 illustrates an example of a state diagram of a VRU Group managed by a CPM ITS-S according to some embodiments of the disclosure; Figure 8 illustrates an example of steps carried out in a CPM ITS-S according to some embodiments of the disclosure; Figure 9 illustrates an example of steps carried out in a VRU ITS-S according to some embodiments of the disclosure; Figure 10 illustrates an example of steps carried out in a CPM ITS-S to manage the transfer of the cluster leader role between two ITS stations, according to some embodiments of the disclosure; Figure 11 illustrates an example of steps carried out in a VRU ITS-S cluster leader to manage the transfer of the cluster leader role between two ITS stations, according to some embodiments of the disclosure; Figure 12 illustrates an example of a message sequence chart involving VRU ITS-S and CPM originating ITS-S, according to some embodiments of the disclosure; and Figure 13 is a schematic representation of an example of a communication ITS-S device configured to implement some embodiments of the disclosure. DETAILED DESCRIPTION OF THE DISCLOSURE It is noted that the names of the lists and elements (such as data elements) provided in the following description are only illustrative. Embodiments are not limited thereto and other names could be used. The embodiments of the disclosure are intended to be implemented in Intelligent Transportation Systems (ITS). It is to be recalled that ITS stations configured for transmitting CPMs, also referred to as CPM ITS-Ss, may be mounted on an infrastructure-based system having a wide field of view with an advanced sensor system and additional processing power whereas VRU ITS-Ss are often embedded devices (such as a smartphones). Accordingly, transferring the reporting of sets of VRUs from a VRU cluster leader to a CPM ITS-S may be advantageous in terms of processing resources. In addition, a CPM ITS-S may perceive VRUs not equipped with a VRU ITS-S and report such VRUs in a VRU Group or in a VRU Cluster. It is also to be recalled that according to ETSI TS 103 324 Specifications (V0.0.35), a VRU Group can be reported in a CPM by an ITS-S equipped with sensors and that, if this VRU Group corresponds to a VRU Cluster already reported by the ITS- S of a VRU cluster leader, the cluster identifier may be associated with the corresponding VRU Group and referenced within the CPM. In addition, as specified in ETSI 103 300-3 Specification (V2.1.1), a VRU cluster leader can break up its cluster if it receives a CPM containing its cluster identifier. Still according to ETSI 103 300-3 Specification (V2.1.1), it is possible for a VRU ITS-S to join or leave a cluster reported by a message of another type than VAM, by setting the cluster identifier to 0. To cope with this limitation, UK patent application GB2593225 discloses a solution that makes it possible to specify a cluster identifier (other than “0”) in a CPM. Therefore, it is possible to generate CPMs reporting several clusters. The inventors have observed that it may be difficult for an ITS station of a VRU belonging to a set of VRUs represented by a VRU Cluster, that is next represented by a VRU Group, to determine when it is no longer represented by this VRU Group. Indeed, if a VRU Cluster is associated with a VRU Group, the cluster identifier may be reported in CPMs and thus, a VRU ITS station should monitor the CPMs to determine that a cluster identifier previously reported in a CPM is no longer reported in a CPM. This may be made even more difficult since a CPM may include many objects in the same message and since not all the objects are reported in each generated CPM (CPM inclusion rules defined in ETSI TS 103 324 Specification create some variation in the periodicity of sending objects). Therefore, implementing a detection of VRU Cluster or VRU Group break up by monitoring VAMs and by monitoring CPM to determine whether cluster identifiers are still reported or not is difficult. According to a first general aspect of the disclosure, it is provided a solution to make it possible to signal explicitly VRU cluster breakup operations so that VRU ITS stations may implement the same mechanism to manage their cluster operation whatever is the type of the VRU cluster leader (i.e., whatever the type of messages, VAM or CPM, used for managing the cluster operations). According to another aspect of the disclosure, the transfer of the VRU cluster leader role between an ITS-S transmitting VAMs and an ITS-S transmitting CPM, or more generally between any type of ITS-Ss, depends on the circumstances. In particular, depending on the circumstances, a VRU ITS-S (e.g., an ITS-S of a bicycle VRU) may keep the role of a VRU cluster leader. It is also made possible to keep the VRU cluster leader role in a VRU ITS station while keeping the CPM ITS station as a complementary reporting of the VRU cluster and thus not disbanding the VRU cluster reported in a VAM. ITS system and ITS stations comprising VRU Cluster Management modules Figure 1 illustrates an example of an ITS system in which some embodiments of the present disclosure may be implemented. According to this example, ITS station 110 is a road-side unit, RSU, that may generate and transmit CPMs such as CPM 130. It is observed that RSUs have generally more effective resources than other ITS stations such as VRU ITS-Ss to analyze a global situation. For example, a RSU may have a wider field of view than an ITS-S embedded within a vehicle, multiple fields of view, fast access to other information such as traffic conditions, traffic light status, knowledge of objects that populate the monitored area, etc. However, some embodiments of this disclosure can be implemented in any ITS-S equipped with sensors. More specifically, an ITS-S equipped with sensors can perceive objects that are not equipped with an ITS-S and thus, it may improve the situation awareness of other ITS-Ss in their vicinity through CPMs. Some embodiments of the disclosure may be implemented in any ITS-S. As illustrated, ITS 100 is implemented at an intersection and comprises fixed road side unit 110 and several entities that may carry or embed an ITS station (ITS-S) each, for transmitting and / or receiving ITS messages within the ITS. The several entities may be for example, bicycles 140, 150, 160, and 170 and pedestrian 180. Fixed road side unit 110 comprises a set of sensors, such as image sensors, here video cameras 120 and 121, and an analytical module to analyze data provided by the sensors, such as analysis module 111. Each of the video cameras 120 and 121 is configured to monitor or scan a portion of the area monitored by the RSU (here the road intersection), making it possible to reproduce images of the monitored area. For the sake of illustration, video camera 120 monitors area 125. Other sensors such as LIDARs (laser imaging detection and ranging devices) may also be used. The sensors are connected to the analysis module (e.g., video cameras 120 and 121 are connected to analysis module 111) so that the analysis module may process the data stream captured by the sensors (e.g., the video cameras), or some of the sensors, to analyze the traffic. The analysis module and the sensors may be separate from or embedded within the same physical road side unit. For example, the analysis module may be wire-connected to sensors that may be remote (i.e., not embedded in the road side unit). The processing of the data received from the sensors by the analysis module, e.g., analysis module 111, aims at detecting objects present in the monitored area, referred to as “perceived objects” or “detected objects” hereinafter. Mechanisms to detect such objects are well known by one skilled in the art. The analysis module is also configured to output a list of the perceived objects respectively associated with corresponding description information referred to as “state vector”. The state vector for a perceived object may include for instance parameters such as a position, a kinematic, temporal information, behavioral or object type classification information, etc. Therefore, the analysis module may identify, among the perceived objects, Vulnerable Road Users (VRUs) such as pedestrians, bicyclists as well as motorcyclists and also persons with disabilities or reduced mobility and may determine their position and movement. It may also identify objects such as trees, road construction / work equipment (e.g., road barriers), and so on. The VRUs may be considered as ITS-S when carrying an ITS equipment, for example an ITS equipment included in a smartphone, a satnav system, a smartwatch, or in a bicyclist equipment. According to the example illustrated in Figure 1, analysis module 111 may perceive the following objects when scanning monitored area 125: objects 142, 152, and 162, respectively corresponding to bicyclists 140, 150, and 160, and object 182 corresponding to pedestrian 180. It is to be observed that bicyclist 170 is not in detection area 125 of the sensors connected to analysis module 111. Accordingly, no corresponding object is detected. In addition, the perceived objects may be classified. According to ETSI TS 102 894-2 Specification that defines the Common Data Dictionary for ITS messages, perceived objects reported in a CPM can be of the following types: - vehicle (with various sub-types such as passenger, truck, bus, etc), - VRU, with various profiles such as pedestrian, bicyclist, motorcyclist, or animal, - VRU Group or VRU Cluster, and other class. Analysis module 111 comprises a VRU Group and Cluster Management module. As analyzed above, grouping VRUs makes it possible to reduce the size of a CPM when there are many VRUs to report in the same area. Another reason to report VRUs as a VRU Group instead of individual VRUs results from a difficulty of discriminating each VRU among several VRUs, for example in a crowded area. Criteria for grouping VRUs may be based on the distance from each other and their velocity. For instance, VRU Group 195 includes perceived VRUs 142, 152, and 162. As illustrated in Figure 1, road side unit 110 further comprises a Roadside ITS-S, R-ITS-S, 112, for example as specified in the reference architecture of an ITS station defined in version V1.1.1 of the ETSI EN 302 665 Specification. By the means of roadside ITS-S 112, RSU 110 can share information relative to the perceived objects. Typically, RSU 110 can share such information with receiving ITS stations by sending ITS messages, particularly the so-called Collective Perception Messages, CPMs, e.g. CPM 130, defined in ETSI TR 103 562 Specification and ETSI TS 103 324 Specification and usually sent periodically. An example of the format of a CPM is illustrated in Figure 5. More generally, any ITS-S in ITS 100 can share information on the objects it perceives, by sending CPMs, as well as information on itself, by sending so-called Cooperative Awareness Messages, CAMs, defined in ETSI EN 302 637-2 Specification. CAMs may include a position, a kinematic (or dynamics), a unique station identifier, temporal information, behavioral or object type classification information, etc. Similarly, VRU Awareness Messages, VAMs, defined in ETSI TS 103 300-3 Specification, can be sent by VRU ITS-S to share their own position and kinematic or to share information of a set of VRUs (i.e., a VRU Cluster). As illustrated in Figure 1, bicyclists 140, 150, and 170 and pedestrian 180 are equipped with VRU ITS-Ss 141, 151,171, and 181, respectively, and are able to transmit VAMs and / or to receive CPMs and other ITS messages (e.g., CAMs). Bicyclist 160 is not equipped with a VRU ITS-S and cannot transmit or receive any ITS message. The ITS messages are usually broadcast by their originating ITS-S, so that any other ITS-S can exploit them. All the messages exchanged over ITS 100 help each ITS-S to have a good level of knowledge of its environment in terms of which objects are present and where and how they behave. In the case of an urban area, many VAMs could be transmitted at the same time. This results in a high level of use of the radio channel and in a large number of ITS messages to be analyzed by each ITS station receiving the VAMs. To cope with such a situation, ETSI TS 103 300-3 Specification describes VRU Clustering Operation where a VRU ITS-S cluster leader (or VAM cluster leader, e.g., VRU ITS-S 141) transmits a VAM (e.g., VAM 131) regrouping information about a set of VRUs equipped with an ITS station (e.g., VRUs 140, 150, and 170 equipped with ITS stations 141, 151, and 171, respectively). After joining a VRU Cluster, an ITS-S of a VRU stops transmitting individual VAMs in order to limit the number VAMs exchanged between ITS-Ss. For example, ITS-Ss 151 and 171 stop sending individual VAMs after joining a cluster led by VRU ITS-S 141. It is noted that since VRU ITS-S 181 does not belong to any VRU Cluster, it continues transmitting individual VAMs (e.g., VAM 132) after the creation of VRU Cluster 145 grouping VRUs 140, 150, and 170 (VRU Cluster 145 does not comprise VRU 160). According to ETSI TS 103 300-3 Specification and upon determining that a VRU Cluster is reported in a CPM (e.g., a CPM comprising a cluster identifier), a VAM cluster leader can break up its cluster by sending a cluster VAM with a break up indication and preferably a reason for the break up, indicating that the cluster is broken up since it is reported in a CPM (e.g., using the reason “receptionOfCPMcontainingCluster” in the VRU Cluster Operation description). As a consequence, the originating CPM ITS-S replaces the VAM cluster leader to report the set of VRUs belonging to this cluster. Figure 2 illustrates a schematic representation of the architecture of an ITS station transmitting CPMs in accordance with embodiments of the disclosure. For the sake of illustration, it is considered here that the illustrated ITS station is the RSU referenced 110 in Figure 1. However, it may be another type of ITS-S-equipped entity. As mentioned above by reference to Figure 1, analysis module 111 is connected to one or more sensors monitoring an area such as a road intersection. These sensors may include cameras 120 and 121 but also other sensors such as LIDAR 210 or mere radar devices (not represented). The sensor raw data generated by the sensors are processed by the Perception and Tracking module 230. Perception and Tracking module 230 analyses perceived objects detected from each sensor raw data, and using sensor data fusion algorithms combines or merges the same objects detected by several sensors. Consideration of similarity between objects from different sensors can be based on their object types, positions, kinetics / dynamics (speed, acceleration), trajectories, etc. A level of confidence may also be computed when scrutinizing the similarities of these information items and the merging can be affected when the level of confidence is high enough. Newly perceived objects or updates about already-tracked objects are used to update Environment Model 220 of the ITS-S. CAMs, VAMs, and CPMs received from other ITS-Ss, conveying additional information, may also be used to update Environment Model 220 by the ITS Message Reception module 270. The Environment Model is also known as the Local Dynamic Map and contains a list of the perceived objects. Each ITS-S has its own Environment Model 220. An object in Environment Model 220 is defined together with multiple information items including, for example, all or part of the following: objectID, that is the identifier of the perceived (or detected) object, timeOfMeasurement, that represents the time at which the (last) measurement concerning the perceived object was made, - objectstate, that is a description of the object kinematic state and attitude. It preferably contains at least the object position and velocity at the time of measurement such as: o objectposition, that is the position of the perceived object reference point. The object position may include the following items of information: ■ worldPosition, that is the absolute position of the object e.g. using GPS position or UTM coordinates at the time of measurement, ■ xCoordinate, yCoordinate, zCoordinate, that represent the distance to the perceived object from the originating ITS-S’s reference position to object reference point in the x-direction, y-direction, and z-direction, respectively, at the time of measurement, in a predefined coordinate system, with a corresponding level of confidence. For instance, RSU 112 may use the World Geodetic System 84 (WGS84) as the predefined coordinate system, o objectvelocity (optional), that represents the velocity vector of the perceived object. It can be in a polar coordinate system or in a Cartesian coordinate system, o additional optional information may be added in the Environment Model for completing the description of the object kinematic state and attitude such as the object acceleration angles provided at the time of measurement from the ITS-S’s reference position in the predefined coordinate system, objectDimensionX, Y, Z (optional), that represent the dimensions of the perceived object in the x-direction, y-direction, and z-direction, respectively, at the time of measurement, - objectAge, that is the age of the perceived object, - objectPerceptionQuality, that represents a confidence level associated with the perceived object. The computation of the object confidence may be based on a sensor’s or fusion system’s specific detection confidence, on the binary detection success (i.e. the detection success of the object during the last measurement), and on the age of the object, - sensorlDList (optional), that is a list of the sensor identifiers which provided the measurement data of the perceived object, classification (optional), that provides the classification of the perceived object, with a corresponding level of confidence, and mapPosition, (optional), that indicates the position of the perceived object mapped onto the intersection topology description transmitted in MAP messages. Environment Model 220 is updated by the Perception and Tracking module 230 and the ITS Message Reception module 270, for example on a regular basis. The ITS Message Generation module 260 generates CPMs containing the perceived object information and estimated object information, for example on a regular basis. In addition, Environment Model 220 contains a list of individual objects and according to some embodiments of the disclosure, a list of VRU Groups and of VRU Clusters. The list of VRU Groups and VRU Clusters may be computed by the VRU Group and Cluster Management module 240. According to some embodiments, VRU Group and Cluster Management module 240 analyses the list of individual VRUs to regroup them in VRU Groups based on their characteristics, for example on their position and possibly their velocity. Criteria to create a VRU Group can be similar to the one used to create a VRU Cluster by a VRU ITS-S as specified in ETSI TS 103 300-3 Specification (V.2.1.2). For the sake of illustration, VRU Group and Cluster Management module 240 may create VRU Group 195 in Figure 1, that comprises VRU objects 142, 152, and 162. The VRU group is composed of perceived objects that may be equipped or not with an ITS-S. In the illustration in Figure 1, bicyclist 160 is not equipped with an ITS-S. ITS Message Reception module 270 receives ITS Messages from other ITS-Ss. When a VAM is received and the received VAM comprises items of information directed to a VRU Cluster, the items of information are provided to VRU Group and Cluster Management module 240. Upon reception of such items of information, VRU Group and Cluster Management module 240 may determine whether or not this VRU Cluster corresponds to an existing VRU Group. According to some embodiments, the analysis may be based on the VRU Cluster and VRU Group positions, shape, dimension, and velocity. For the sake of illustration, it may be considered that a VRU Cluster corresponds to a VRU Group if all or almost all the VRUs belonging to the VRU Cluster also belong to the VRU Group. The criterion may be based on ratios and thresholds as described with reference to Figure 10. When a VRU Cluster corresponds to a VRU Group, the identifier of the VRU Cluster (that may be provided within VRU Cluster information of a VAM transmitted by a VRU cluster leader) is added to characteristics of the corresponding VRU Group. Therefore, ITS Message Generation module 260, in charge of transmitting CPMs, may generate a CPM containing information about the VRU Group with a reference to the cluster identifier obtained from the received VAM. Upon receiving such a CPM, the VRU cluster leader and the VRU cluster members may become aware that there exists a VRU Group corresponding to their VRU Cluster. Figure 3 illustrates a schematic representation of the architecture of a VRU ITS station transmitting VAMs in accordance with embodiments of the disclosure. As illustrated, the VRU ITS-S (e.g., VRU ITS-S 141) comprises an ITS Message Generation module 360 to transmit VAMs, for example VAMs having a structure like the one described with reference to Figure 4. According to this example, the VRU ITS-S also comprises an ITS Message Reception module 370 that makes it possible to receive VAMs from other VRU ITS-Ss. In addition, the VRU ITS-S comprises a VRU Cluster Management module 340 that may be used to manage VRU cluster operations (such as creating a VRU Cluster, joining a VRU Cluster, leaving a VRU Cluster, or breaking up a VRU Cluster). Conditions to create, join, or leave a VRU Cluster may be the ones defined in ETSI TS 103 300-3 Specification, considering for example the distance between VRUs and their velocities. In addition, according to some embodiments of the disclosure, ITS Message Reception module 370 is configured to receive CPMs from a CPM originating ITS-S and to process such messages. According to some embodiments, ITS Message Reception module 370 may process such CPMs in order to identify indications of VRU Groups and / or VRU Clusters contained in a CPM. VAM structure Figure 4 illustrates an example of a structure of a VRU awareness message, VAM, according to some embodiments of the disclosure. As illustrated, VAM 400 contains an ITS PDU header 410 and a “VAM Parameters” field 420. ITS PDU header 410 includes a VRU identifier of the originating ITS station (i.e., an identifier of the VRU sending the VAM). VAM Parameters field 420 contains a Basic Container 430, optionally a Cluster Information Container 440, and also optionally a Cluster Operation Container 450. Basic Container 430 may comprise the following fields: - stationType, that indicates the type of the originating ITS station of the VAM. Examples of possible values are the following ones: 0 for 'unknown', 1 for 'pedestrian', and 2 for 'bicyclist', - referenceposition, that represents the reference position of the originating ITS station of the VAM within a geographic coordinate system. For the sake of illustration, the reference position can contain the latitude, longitude, and altitude information together with a position confidence ellipse. According to some embodiments, when the VAM originating ITS station is taking the role of a cluster leader (such as VRU ITS-S 142 in Figure 1), the VAMs it generates contain a VRU Cluster Information Container (e.g., VRU Cluster Information Container 440) that may comprise all or some of the following fields: - clusterld, that is a locally unique identifier of the VRU cluster (i.e., the identifier is unique in the radio coverage area of the VRU), - clusterBoundingBoxShape, that defines the shape of the cluster bounding box. Various types of shape are possible such as rectangular, circular, polygonal, etc. Each shape may be defined using a specific value, - clusterCardinalitySize, that is an estimation of the number of VRUs in the VRU cluster, - clusterProfiles, that identifies all the VRU Profiles that are known to be within the cluster. It may consist in a bitmap encoding VRU profiles, to allow multiple profiles to be indicated in a single cluster. An example of VRU Cluster Profiles may be defined as follows: bit 0 is used to encode the presence of one or more pedestrians, bit 1 is used to encode the presence of one or more bicyclists, bit 2 is used to encode the presence of one or more motorcyclists, and bit 3 is used to encode the presence of one or more animals. Accordingly, if the cluster contains at least one pedestrian, bit 0 is set to 1. Likewise, if the cluster contains at least one bicycle, bit 1 is set to 1 and so on. As illustrated and according to some embodiments of the disclosure, an optional field may be used (it is an additional field compared to ETSI 103 300-3 Specification (V2.1.2)): - clusterLeaderRole, that indicates whether the VRU ITS-S is leading or not the cluster. Its value may be ON (true) or FALSE (false). Optionally the VAM originating ITS station can include a Cluster Operation Container (e.g., Cluster Operation Container 450) to make it possible to perform VRU cluster operations, as described with reference to Figure 6. According to some embodiments, four types of cluster operations are possible: - clusterjoininfo, that may be used by a VRU ITS station intending to join an existing cluster. The clusterjoininfo may contain an identifier (clusterld) of the cluster the VRU ITS station intends to join and the time after which the VRU ITS station will stop transmitting individual VAMs, - clusterLeavelnfo, that may be used by a VRU ITS station belonging to a cluster to inform it has recently left the cluster. The clusterLeavelnfo may contain an identifier (clusterld) of the cluster the VRU ITS station has left and the reason of leaving the cluster, - clusterBreakupInfo, that may be used by a VRU ITS station leading a cluster to inform of its intention to break up its cluster. The clusterBreakupInfo may contain a reason of breaking up the cluster and a time after which the VRU ITS station will stop transmitting cluster VAMs, and - clusterldChangeTimelnfo, that may be used by a VRU ITS station leading a cluster to inform the cluster members that it will change its cluster identifier (clusterld) at a specified time. Such a change of cluster identifier may be used for safety and / or privacy reasons. According to some embodiments of the disclosure, a particular VRU cluster operation is used (additional cluster operation compared to ETSI 103 300-3 Specification (V2.1.2)) to transfer the role of a cluster leader: - clusterTransferlnfo, that is used by a VRU ITS station to request or to accept the role of cluster leader. It is be noted that the organization of the fields illustrated in Figure 4 are provided for the sake of illustration. It is not limitative and the fields may be organized differently. CPM structure Figure 5 illustrates an example of a structure of a collective perception message, CPM, according to some embodiments of the disclosure. The illustrated CPM structure, referenced 500, is based on ETSI TS 103 324 Specification (V0.0.35 of June 2022). It comprises an ITS PDU header 510, a CPM reference time field 515, and a “CPM Parameters” field 520. ITS PDU header 510 is a common header that includes the information of the protocol version, the message type, and the ITS-S identifier (ID) of the originating ITS-S. CPM reference time (cpmReferenceTime) field 515 is the absolute reference time of the message. As illustrated, “CPM Parameters” field 520 comprises a management container referenced 530 and a perception data container referenced 540 that in turn contains an optional set of sensor information containers referenced 550 and an optional set of perceived object containers referenced 560. It is observed that “CPM Parameters” field 520 comprises other containers that are not shown since they are not particularly relevant for this disclosure (e.g., the free space addendum container or the perceived region container). Each container includes some data elements (DE) and / or data frames (DF). ETSI TS 102 894-2 V2.1.1 Specification defines conventional data elements and data frames used in ITS messages. Regardless of the type of the ITS-S generating the considered CPM, the management container provides information regarding the station type and the reference position of the originating ITS station. The message can be transmitted either by an ITS station, such as a vehicle, or by a stationary RSU. Sensor information container 550 (that is optional), contains information regarding the set of sensors of the ITS station. It provides information about the sensory capabilities of an ITS station. Depending on the station type of the originating ITS station, different sensor information specifications are available to encode the properties of a sensor. The sensor information container is attached to CPMs at a lower frequency than the other containers, as defined in ETSI TS 103 324 Specification. A sensor information container may include: sensorld, that is an identifier of the sensor, sensorType, that represents the type of the sensor. Various types of sensors may be defined such as radar, lidar, monovideo, stereovision, nightvision, ultrasonic, pmd, fusion, inductionloop, sphericalCamera, itssaggregation and uwb, - detectionArea, that represent the area of the detection (e.g., a field of view of the camera sensor), and freeSpaceConfidence, that represents a confidence level of the information indicating that an area is not occupied. Perceived object container 560 (that is optional) contains a set of objects perceived by the sensors of the CPM originating ITS station. It comprises a sequence of optional or mandatory data elements (DEs) and / or data frames (DFs) which give a detailed description of the dynamic state and properties of a perceived (or detected) object. More precisely, each object may be described using the dedicated perceivedObject structure referenced 560 as defined by ETSI TS 102 894-2 Specification (V2.1.1) and may comprise various fields including the following ones: objectld, that is an identifier assigned to a perceived object by the originating ITS-S. It remains constant as long as the object is perceived by the originating ITS-S, - measurementDeltaTime, that corresponds to the time difference between the time of measurement of the items of information provided in the CPM, that are directed to the perceived object, and the reference time provided in CPM reference time 515, - position, that represents the position defined by xCoordinate, yCoordinate, and zCoordinate, corresponding to the distance between the perceived object and the originating CPM ITS-S's reference point the in x-direction, y-direction, and z-direction of the ITS-S coordinate system, respectively, at the time of measurement, velocity, that represents the velocity defined by xVelocity, yVelocity, and zVelocity, corresponding to the speed of the perceived object in the originating CPM ITS-S’s reference system in the x-direction, y-direction, and z-direction, respectively, at the time of measurement, - objectDimensionX.Y.Z (optional), that represents the dimension of the perceived object in the x-direction, y-direction, and z-direction, respectively, objectAge, that is the age of the perceived object, objectPerceptionQuality, that is the confidence level associated with the perceived object. The computation of the object confidence level may be based on a sensor’s or merging system confidence, on the binary detection success (i.e. the detection success of the object during the last measurements), and on the age of the perceived object, sensorldList (optional), that is a list of sensor identifiers which provided the measurement data used to perceive the object. It refers to sensorld in sensor information container 550. If the sensor information container is never provided by the originating ITS-S, the list may be populated with random numbers, where each number is assigned to a sensor of the originating ITS-S, and classification, that provides the classification of the perceived object. It comprises a class of objects (e.g., Object Class 565) and possibly a subclass (e.g., a vehicle class may have passengerCar, bus, etc. as subclasses) with a class confidence value. According to some embodiments of the disclosure, an ITS-S transmitting CPMs may regroup items of information of VRUs into a VRU Group, using the VRU Group and Cluster Management module 240 (as described with reference to Figure 2). To report a VRU Group in a CPM, the classification structure associated with the object (i.e., with the VRU Group) may use the data structure groupSubClass of type VruClusterlnformation. The VruClusterlnformation data structure may be similar to the one of the VAM Cluster Information Container 440 described with reference to Figure 4. According to some embodiments of the disclosure, a new field is used to indicate whether or not the originating ITS-S of the CPM (the CPM ITS-S) is acting as a VRU cluster leader. As illustrated, such a new field may be denoted clusterLeaderRole and may be added to the VruClusterlnformation data structure referenced 566. This new field may be used to inform VRU ITS-Ss that the CPM ITS-S is handling or not a VRU Group as a VRU Cluster. For the sake of clarity, if the VRU Cluster is managed by the CPM ITS-S, it may be referred to as the CPM VRU cluster. Still according to some embodiments of the disclosure, a data structure 567, for example denoted VruClusterOperation, that may be similar to VruClusterOperation 450 described with reference to Figure 4 with regard to a VAM structure, may be added to the VruClusterlnformation to make it possible to perform an operation on the CPM VRU cluster. For the sake of illustration, the VRU cluster operation in a CPM may be signalled using the following items of information to support VRU cluster operations for cluster break up, for cluster leader role transfer, or for cluster id change: - clusterBreakupInfo (referenced 570), that contains a cluster break up reason (e.g., clusterBreakupReason) and the cluster breakup time (e.g., breakupTime), clusterTransferlnfo (referenced 571), that contains the cluster leader role transfer type (e.g., clusterTransferType) and the cluster transfer time (e.g., transferTime), and - clusterldChangeTimelnfo, that is used by a CPM ITS station leading a VRU Cluster to inform the cluster member that it will change its cluster identifier at a specified time, for example for safety or for a privacy reason of ITS messages. It is observed that in the context of a CPM ITS-S, the operations known as clusterjoininfo and clusterLeavelnfo are useless. Regarding the cluster break up reasons, the same reason values as the ones used by a VRU ITS-S cluster leader can be used by a CPM ITS-S cluster leader, such as: 0 (notProvided) to indicate that no reason is provided, 1 (clusteringPurposeCompleted) to indicate that the purpose of clustering some VRUs has been reached, - 2 (leaderMovedOutOfClusterBoundingBox), to indicate that the cluster leader has moved outside the bounding box of the VRU Cluster, 3 (JoiningAnotherCluster), to indicate that the VRUs belonging to the VRU Cluster join another VRU Cluster, - 4 (enteringLowriskareaBasedonMAPs), to indicate that the area wherein the VRUs of the VRU Cluster are located is a low risk area, and 5 (receptionOfCPMcontainingCluster), to indicate that the identifier of the VRU cluster has been transmitted in a CPM. In addition to the reason values used by a VRU ITS-S cluster leader, the following reasons may be used by CPM ITS-Ss: - 6 (switchingTolndividualObjectReporf), to indicate that the CPM ITS-S will stop reporting the set of VRUs using groupSubclass classification after the time specified by breakupTime, instead, the set of VRUs will be reported as individual objects in CPMs using the corresponding vruSubClass for each VRU. VRU ITS-Ss of the VRU Cluster should move into a mode according to which individual VAMs are to be generated and transmitted and - 7 (safetycondition), to indicate that the VRU Cluster should break up for a safety reason. Thanks to such a cluster break up indication, a VRU ITS-S can handle a cluster break up operation in the same manner, whether the cluster break up instruction is sent via a VAM or via a CPM. Regarding the cluster transfer indications, the data structure is used to manage the transfer of the cluster leader role between a CPM ITS-S reporting a VRU Cluster and another ITS-S reporting the same VRU Cluster (this other ITS-S being a VRU ITS-S cluster leader or another CPM ITS-S). According to some embodiments of the disclosure, the cluster leader role could be transferred from a CPM ITS-S to another ITS-S able to manage the cluster. To that end, the cluster leader role transfer type (clusterTransferType) may use the following values: 0 (notProvided), to indicate that the cluster leader role transfer type is not provided, 1 (clusterLeaderTransferRequest), to indicate that a transfer of the cluster leader role, from the CPM ITS-S to another ITS-S is requested, - 2 (clusterLeaderTransferAccepted), to indicate that a transfer of the cluster leader role, from another ITS-S to the CPM ITS-S is accepted, and 3 (keepVAMCIusterLeaderActive), to indicate that the current VRU cluster leader should keep its role of VRU cluster leader. Cluster break up management Figure 6 illustrates an example of a VRU ITS-S state diagram according to some embodiments of the disclosure. As illustrated, a VRU ITS-S may be in one of four different VRU basic awareness service (VBS) clustering states denoted VRU-IDLE (referenced 600), VRU-ACTIVE-STANDALONE (referenced 605), VRU-ACTIVE-CLUSTER-LEADER (referenced 610), and VRU-PASSIVE (referenced 615). According to the VRU-IDLE state, the user of the VRU ITS-S is in a state in which he / she is not considered as being a VRU (i.e., the role of the ITS-S is not set to VRU, e.g., it is set to VRU_ROLE_OFF). In the operating mode corresponding to this state, no VAM is generated and transmitted by the VRU ITS station. In the VRU-ACTIVE-STANDALONE state, VAMs are transmitted periodically with information relating to the VRU only (the generated VAM are individual VAMs specific to the VRU). In such a state, the VRU ITS-S may indicate an intention to join a cluster or may indicate that it has just left a cluster, for example using the Cluster Operation Container 450 described in reference to Figure 4. The VRU-ACTIVE-CLUSTER-LEADER state corresponds to the state of the VRU ITS-S when the latter is a VRU cluster leader that generates and transmits cluster VAMs, that is to say VAMs comprising information relating to a set of VRUs moving in a coherent manner. In the VRU-PASSIVE state, the VRU ITS-S stops sending VAMs (either cluster VAMs or individual VAMs). When the ITS station is in VRU-IDLE state 600, it may enter VRU-ACTIVE-STANDALONE state 605 (step 620) when its user is to be considered as a VRU (“entering VRU Role”, e.g., when the VRU Role is set to VRU_ROLE_ON). This may be the case, for example, of a pedestrian, having an ITS station, exiting from a bus. When the ITS station is in VRU-ACTIVE-STANDALONE state 605, its operating mode makes it possible to transmit individual VAMs periodically. When being in this operating mode, its user may stop being considered as a VRU (“leaving VRU Role”, e.g., the role of the user of the ITS-S changes to VRU_ROLE_OFF), for example if the user of the ITS-S enters a bus. In such a case, the ITS-S returns to the VRU-IDLE state (step 625). Alternatively, from VRU-ACTIVE-STANDALONE state 605, the ITS-S may determine, based on VAMs received from other VRU ITS-Ss, that it would be beneficial to create a cluster (step 630), for example using VRU Cluster Management module 340 in Figure 3. To that end, the ITS-S may obtain a cluster identifier (clusterld), for example it generates a random cluster identifier, that is locally unique (i.e., it is unique in the radio coverage area of the VRU). Next, the ITS-S may determine a cluster initial dimension and shape (Cluster Bounding Box Shape), a cluster initial size (Cluster Cardinality Size), and a cluster VRU profiles (Cluster Profiles), that are determined as a function of the VRU cluster leader only. After creating the cluster, the ITS-S moves to VRU-ACTIVE-CLUSTER-LEADER state 610. After entering in the corresponding operating mode, a cluster VAM is generated and transmitted with cluster information enabling other ITS-Ss to get knowledge about the VRU Cluster so that they can join it if the conditions are fulfilled. As described with reference to Figure 4, the created VRU Cluster may be signaled by VRU Cluster Management module 340 using the Cluster Information Container 440. While being in VRU-ACTIVE-CLUSTER-LEADER state 610, the cluster leader may update the description of the cluster (step 635) regarding the VRUs that joined the cluster and the VRUs that left the cluster, based on join and / or leave VAMs received from other VRU ITS-Ss, and regarding the reference position of the cluster, its velocity, etc. Information about VRUs joining and / or leaving the cluster may be obtained by VRU Cluster Management module 340, for example using the Cluster Operation Container 450 with the parameter clusterjoininfo and / or clusterLeavelnfo. According to some embodiments and as described with reference to Figure 4, cluster VAMs may contain a VRU Cluster Information Container (e.g., VRU Cluster Information Container 440 in Figure 4) with specific data elements relating to the VRU Cluster. It is also possible for the cluster leader to change the clusterld, for example for safety and / or privacy reasons, as described above. In such a case, the modifications brought to the cluster may be signaled by VRU Cluster Management module 340 to inform the VRU cluster members of the change, for example using the Cluster Operation Container 450 with the parameter clusterldChangeTimelnfo. When the ITS station is in VRU-ACTIVE-STANDALONE state 605, transmitting VAMs periodically, it may decide to join a cluster upon receiving a cluster VAM from a cluster leader. This determination may be carried out by its VRU Cluster Management module (e.g., VRU Cluster Management module 340 in Figure 3) based on characteristics of the VRU Cluster and of the VRU (e.g., based on its position and velocity). In such a case, the VRU ITS-S may send an individual VAM with a VRU Cluster Operation Container (e.g., VRU Cluster Operation Container 450 in Figure 4) with Clusterjoininfo comprising the identifier of the cluster to join (Clusterld) and the time to join. When the VRU ITS-S determines that it has successfully joined the cluster (e.g., by analyzing a cluster VAM received from the cluster leader and determining that the cluster VAM designates the VRU ITS-S as a VRU member), its state moves to VRU-PASSIVE state 615 (step 640). As described above, when entering in the corresponding operating mode, the VRU ITS-S stops transmitting individual VAMs when it joins the cluster (only the cluster leader sends cluster VAMs representing all the cluster members). When the VRU Cluster Management module (e.g., VRU Cluster Management module 340 in Figure 3) determines that the cluster is to be ended, a cluster VAM is generated and transmitted to forewarn the VRU Cluster members of the end of the VRU Cluster. This cluster VAM preferably include Cluster Operation information (e.g., Cluster Operation information 450 in Figure 4) containing information about the cluster break up (Cluster Break up Info). Such cluster break up information may indicate the reason of the cluster break up reason and the time of the beak up. After transmitting the cluster VAM informing of the beak up of the VRU Cluster, the VRU cluster leader returns to VRU-ACTIVE-STANDALONE 605 (step 640). As explained with reference to Figure 5, a CPM may report a group of VRUs using the groupSubClass classification. When a CPM originating ITS-S determines that one of its VRU Group may be associated with a VRU Cluster described in a received cluster VAM, for example in a case where all or almost all the members of the VRU Cluster belong to the VRU group (e.g., as described with reference to Figure 10), the cluster identifier obtained from the cluster VAM may be reported in the Clusterld field of the CPM VRU Group. According to ETSI TS 103 300-3 Specification, a VRU cluster leader may break up its cluster after determining that this cluster is referenced within a CPM and preferably after determining that the cluster leader is located within the VRU Group (e.g., using its bounding box). To that end, the VRU cluster leader may send a cluster VAM with a Cluster Operation Container with a Cluster Break up Info indicating the clusterBreakupReason to be “receptionOfCpmContainingCluster”. As a consequence, VRU cluster members may resume sending individual VAMs. As described above, in such a case, the VRU cluster leader moves to VRU-ACTIVE-STANDALONE state 605 after breaking up the cluster (step 645). According to some embodiments, the VRU cluster leader may move to VRU-PASSIVE state 615 after breaking up the cluster (not represented), if the reason of the break up is that the cluster is reported in a CPM, to avoid transmitting numerous individual VAMs. In such a case, the cluster leader and the cluster members preferably stay in VRU-PASSIVE state 615 until the cluster is no longer reported in a CPM. According to some embodiments of the disclosure, the cluster leader may receive a request to keep the cluster leader role in the VAM cluster (“Keeping VRU cluster role”, step 645). Such a request may be received in a CPM, typically from the CPM ITS-S managing the corresponding VRU Group, for example using the clusterTransferlnfo in a VRU cluster operation. It is recalled that according to ETSI TS 103 300-3 Specification, a VRU Cluster Management module may decide to leave a cluster at any time, for example because the cluster leader is lost, the cluster member is above a certain distance threshold from its cluster, or for safety reason. In such a case, the VRU ITS-S should resume sending individual VAMs. When the VRU ITS-S is leaving the cluster, it sends an individual VAM with the Cluster Operation Container containing the Cluster Leave Info indicating the cluster identifier and the reason of leaving (e.g. cluster leader lost, outside of the cluster bounding box, etc.). After leaving the VRU Cluster, the ITS-S moves to the VRU-ACTIVE-STANDALONE state (step 655). In particular, an ITS-S may leave a cluster after receiving a cluster break up, for example in a cluster VAM. According to some embodiments, the ITS-S determines whether the VRU Cluster to be broken up is identified in a CPM previously received from a CPM originating ITS-S. If the VRU Cluster to be broken up is identified in a CPM previously received, the ITS-S may stay in VRU-PASSIVE state 615 until a CPM received from the same CPM originating ITS-S does not identify the VRU Cluster any longer (not represented). In other words, a VRU cluster member receiving a cluster break up with the reason “receptionOfCpmContainingCluster’ reason may stay in the VRU-PASSIVE state instead of returning to the VRU-ACTIVE-STANDALONE state if its cluster is already reported in a CPM. The VRU ITS-S may also join a CPM VRU Cluster. Transferring the reporting of a VRU cluster from VAMs to CPMs may be advantageous, in particular when CPMs are generated by a fixed infrastructure that can have a wider field of view and complementary sensing means to perceived objects. However, according to the state-of-the art, when a VRU Cluster is reported in a CPM, the VRU Cluster is no longer managed by a VRU ITS-S, in particular when the CPM ITS-S breaks up the VRU Cluster. In addition, there are also some cases where it may be advantageous to keep both the VRU Cluster to be reported in VAMs and to refer to the corresponding cluster identifier within a VRU Group reported in a CPM. For the sake of illustration and considering the example provided in Figure 1, a queue of bicyclists may be perceived only during a short period of time by the CPM ITS-S, and incompletely, while the cluster leader may continuously report relevant information about the VRU Cluster. In such a case, it is advantageous to keep both the VRU Cluster to be reported in VAMs and to refer to the corresponding cluster identifier within a VRU Group reported in a CPM. Figure 7 illustrates an example of a state diagram of a VRU Group managed by a CPM ITS-S according to some embodiments of the disclosure. According to some embodiments of the disclosure and as illustrated, a VRU Group managed by a CPM ITS-S (or a VRU Cluster managed by a CPM ITS-S, referred to as a CPM VRU Cluster) can take one of four different states denoted GROUP-IDLE (referenced 700), GROUP-ACTIVE (referenced 705), GROUP-CLUSTER-ACTIVE (referenced 710), and CLUSTER-LEADER (referenced 715). GROUP-IDLE 700 corresponds to an initial state from which a VRU Group may start to be reported in a CPM. In GROUP-ACTIVE state 705, a set of VRUs is reported within a VRU Group (also referred to as a CPM VRU Group) instead of being reported as individual objects in the CPM perceived object container. In GROUP-CLUSTER-ACTIVE state 710, the VRU Group is associated with a VRU Cluster reported in a received cluster VAM. In such a case, the cluster identifier (e.g., clusterld) received from the cluster VAM may be referenced within the VRU Group description (e.g., it may be assigned to the clusterld of the VRU Group in the data structure VruClusterlnformation). In CLUSTER-LEADER state 715, the CPM ITS-S is having the role of the cluster leader of the VRU Cluster that is identified within the VRU Group description (e.g., clusterld). From GROUP-IDLE state 700, a CPM ITS-S equipped with sensors may decide to group VRUs that move in a coherent manner (i.e., that have common perceived characteristics), for example that have close relative positions and about the same velocity. As a result, the CPM ITS-S creates a VRU Group (step 720). Accordingly, the CPM ITS-S reports information about a set of VRUs, in a CPM, as a VRU Group instead of reporting information for each of the VRUs individually. Then, the state of the VRU Group is set to GROUP-ACTIVE state 705. When a VRU Group is in this state, the VRUs of the VRU Group are reported as a whole within a CPM VRU group (instead of being reported as individual objects in the CPM perceived object container). In such a case, a VRU Group identifier (e.g., objectld) is assigned to the VRU Group by the CPM ITS-S. The items of information about the VRU Group may be reported using the data structure VruClusterlnformation 567 in Figure 5. In this state, the data structure does not comprise any VRU Cluster identifier (e.g., clusterld) identifying a VRU Cluster associated with the VRUGroup. From GROUP-ACTIVE state 705, the state of the VRU Group moves back to GROUP-IDLE state 700 when the VRU Group is disbanded (step 725). This may happen, for example, when the VRUs of the VRU Group break up. Still from GROUP-ACTIVE state 705, the VRU Group and Cluster Management module of the CPM ITS-S (e.g., VRU Group and Cluster Management module of 240 in Figure 2) may determine that a VRU Cluster reported in a VAM corresponds to the VRU Group (e.g., all or almost all the VRUs of the VRU Cluster belong to the VRU Group). In such a case, the VRU Cluster may be associated with the VRU Group (“Associating VRU Cluster to a VRU group”, step 730). Next, the state of the VRU Group is set to GROUP-CLUSTER-ACTIVE state 710. As described above, in such a case, the cluster identifier (e.g., clusterld) of the associated VRU Cluster may be referenced within the VRU Group description of a CPM (e.g., it may be assigned to the clusterld of the VRU Group in the data structure VruClusterlnformation, e.g., VruClusterlnformation 567 in Figure 5). Accordingly, at this stage, the VRU ITS-S cluster leader and VRU ITS-S cluster members are aware that items of information about their VRU Cluster are now reported by this CPM ITS-S. From GROUP-CLUSTER-ACTIVE state 710, the CPM ITS-S may determine that it is able to manage the VRU cluster and thus decide to do so (“Becoming cluster leader”, step 735). Accordingly, the state of the VRU Group moves to CLUSTER-LEADER state 715. As described above, the CPM ITS-S has, in this state, the role of a cluster leader for the VRU Custer identified by clusterld, to update the VRU Cluster. The CPM ITS-S preferably informs the other ITS stations through a CPM that it is to be considered as the cluster leader. For the sake of illustration, the clusterLeaderRole field of the VruClusterlnformation data structure (e.g., VruClusterlnformation data structure 566 in Figure 5) may be set to ON (or to the true value). Accordingly, the CPM ITS-S is in charge of updating the VRU Cluster (“Updating a CPM cluster”, step 740) based on join and / or leave operations sent by VRU ITS-Ss and / or based on data acquired by the sensors of the CPM ITS-S (or connected, directly or indirectly, to the CPM ITS-S), making it possible to update the VRU Cluster about VRUs that are not provided with an ITS-S. From CLUSTER-LEADER state 715, the CPM ITS-S may determine that it is not the best ITS-S to manage the VRU Cluster. In such a case, it may use the VRU operation container with the clusterTransferlnfo to transfer the cluster leader role to another ITS-S (“Transferring cluster leader role”, step 745, for example to a VRU ITS-S or to a RSU). This may be the case, for example, for a group of bicyclists passing quickly in an area monitored by a CPM ITS-S. In such a situation, a VRU ITS-S of the VRU Cluster is in a better position than a CPM ITS-S to play the role of the cluster leader. Still from CLUSTER-LEADER state 715, the VRU Group and Cluster Management module (e.g., VRU Group and Cluster Management module 240 in Figure 2) of the CPM ITS-S may determine that cluster break up conditions are met (e.g., the behavior of the VRU members lacks consistency in terms of position and / or velocity, VRLIs outside the field of view of sensors of the CPM ITS-S, safety conditions are arising, etc.). In such a case, the CPM ITS-S may decide to break up the VRU Cluster (“Breaking up a CPM cluster”, step 750). As a consequence, the CPM ITS-S may report a cluster break up, for example using the Cluster Operation Container 567 and using the clusterBreakupInfo 570 to inform VRU ITS-Ss belonging to this cluster. Next, the state of the VRU Group moves to GROUP-ACTIVE state 705. Accordingly, the VRU cluster identifier (e.g., clusterld) is removed from the VRU cluster information 566 of the VRU Group (i.e., it is not reported any longer in CPMs generated and transmitted by the CPM ITS-S and the VRU cluster leader role is set to OFF (or to a false value). Thanks to this mechanism, the VRU ITS stations that were belonging to this cluster are able to resume sending individual VAMs immediately, avoiding a discontinuity in the reporting of the VRUs in the ITS system. From GROUP-CLUSTER-ACTIVE state 710, the CPM ITS-S may determine that the VRU Cluster should no longer be associated with the VRU Group (e.g., all the VRUs belonging to the VRU Cluster do not belong any longer to the corresponding VRU Group). In such a case, the VRU Cluster is disassociated from the VRU Group (“End of association”, step 755). As a consequence, the state of the VRU Group moves to GROUP-ACTIVE and the VRU cluster identifier (e.g., clusterld) is removed from the VRU cluster information 566 of the VRU Group (i.e., it is not reported any longer in CPMs generated and transmitted by the CPM ITS-S. Figure 8 illustrates an example of steps carried out in a CPM ITS-S according to some embodiments of the disclosure. As illustrated, if a CPM ITS-S equipped with sensors perceives a new set of VRUs moving in a coherent manner (step 800), for example VRUs that are close to each other and have about the same velocity, it may decide to create a VRU Group (step 805). This may be done by a VRU Group and Cluster Management module (e.g., VRU Group and Cluster Management module 240 in Figure 2). The new VRU Group may be added to its list of VRU Groups. Its state is preferably set to GROUP-ACTIVE. In parallel, an ITS Message Reception module (e.g., ITS Message Reception module 270 in Figure 2) of the CPM ITS-S is configured to receive VAMs from VRU ITS-S. In particular, the cluster VAMs received from VRU ITS-S cluster leaders are analyzed by the VRU Group and Cluster Management module. If a cluster VAM is received by the Message Reception module (step 810) and if the VRU Group and Cluster Management module determines that the VRU Cluster reported in the received cluster VAM may be associated with one of the created VRU Groups (step 815), the identifier of the VRU Cluster received in the cluster VAM (e.g., clusterld) is added to the VRU Group characteristics (step 820). The state of the VRU Group to which is associated the identifier of the VRU Cluster is set to GROUP-CLUSTER-ACTIVE. Next, a CPM comprising the identifier of the VRU Cluster within the characteristics of the VRU Group is sent by the CPM ITS-S (e.g., by its Messgage Generation module) over the radio communication channel to report the VRU Group with the corresponding clusterld. As illustrated in Figure 9 and according to ETSI TS 103 300-3, a VRU ITS-S cluster leader receiving a CPM containing its cluster may break up the VRU Cluster by sending a cluster break up with the reason “receptionOfCpmContainingCluster”. When the CPM ITS-S receives a cluster break up operation in a cluster VAM (step 830), it checks whether the identifier of the VRU Cluster contained in the cluster VAM corresponds to one of the VRU Groups previously created (step 835). If the identifier of the VRU Cluster corresponds to one of the VRU Groups previously created, the state of the corresponding VRU Group is set to CLUSTER-LEADER (step 840). In this state, the CPM ITS-S is able to manage the VRU Cluster, in particular to update the VRU Cluster (step 845) with the joining VRUs and / or the leaving VRUs, based on joining and / or leaving requests received from VRU ITS-Ss (step 850), that are received as individual VAMs, for example in their Cluster Operation Container (e.g., in Cluster Operation Container 450 in Figure 4). According to TS 103 300-3 Specification (V2.1.2), a VRU ITS-S may determine that it is within a cluster bounding box indicated by a message other than a VAM (for example a CPM). In that case, it may follow a cluster join process (e.g., using Cluster Operation Container 450 in Figure 4), preferably providing the special value "0" as the identifier of the cluster it joins. Similarly, a VRU ITS-S that is a member of a VRU Cluster may decide to resume sending individual VAMs because it has determined it was within the cluster indicated by the other message, but is now going to leave or has left that cluster bounding box. In such a case, it may follow a cluster leave process (e.g., using the Cluster Operation Container 450 in Figure 4), indicating the special cluster identifier value "0". In the case according to which the CPM ITS-S receives a joining or leaving request with the special cluster identifier value “0”, as a result of the cluster join or leave process, the CPM ITS-S could first determine for which of its VRU Groups or Cluster the VRU ITS-S belongs to before updating the corresponding VRU Group or Cluster. According to some embodiments of the disclosure, a VRU ITS-S is able to refer to the cluster identifier (e.g., clusterld) indicated within a VRU Group in the VRU Cluster Information (e.g., VRU Cluster Information 566) of a received message (for example in a received CPM). Therefore, a VRU device may determine that it is within a cluster bounding box indicated by a message other than a VAM (for example a CPM). In such a case, it may follow the cluster join process, preferably providing the special value "0" as the identifier of the cluster it requests to join, if there is no cluster identifier in the received message (if it is not of the VAM type). Otherwise, it should indicate the cluster identifier indicated in the message (if it is not of the VAM type, for example if it is a CPM). Similarly, a VRU device that is in VRU-PASSIVE state and within a cluster indicated by a message other than a VAM (for example a CPM) may decide to resume sending the VAM because it has determined it was within the cluster indicated by the other message, but is now going to leave or has left that cluster bounding box. In that case, it may follow the cluster leave process, indicating the special cluster identifier value “0” if there is no cluster identifier in the received message (if it is not of the VAM type). Otherwise, it should indicate the cluster identifier indicated in the received message (if it is not of the VAM type, for example if it is a CPM). Additionally, the CPM ITS-S may update the cluster information (such as the cluster dimension and the number of VRUs in the cluster and their profiles) based on the VRUs perceived by its sensors. When the cluster conditions are not met any longer (step 855), the CPM ITS-S may generate a cluster break up message such as a CPM (step 860), for example using the clusterBreakup / nfo 570 in the VruClusterOperation data structure 567 in Figure 5. Accordingly, the state of the VRU Group is set to GROUP-ACTIVE and the message containing the cluster break up is sent (step 865). Figure 9 illustrates an example of steps carried out in a VRU ITS-S according to some embodiments of the disclosure. As illustrated, if the VRU ITS-S is a cluster leader (step 900) and if the VRU ITS-S receives a CPM with a cluster identifier corresponding to the identifier of the cluster of the VRU ITS-S (step 905), a cluster VAM comprising a cluster break up operation is generated and transmitted (step 910). On the contrary, if the VRU ITS-S is a cluster member (step 915), if the VRU ITS-S receives a CPM with a cluster identifier corresponding to the identifier of the cluster to which the VRU ITS-S belongs and if the received CPM comprises break up information (step 920), the state of the VRU ITS-S is set to VRU-ACTIVE-ALONE (step 925), meaning that the VRU ITS-S leaves the VRU Cluster and resumes transmitting individual VAMs. Figure 10 illustrates an example of steps carried out in a CPM ITS-S to manage the transfer of the cluster leader role between two ITS stations, according to some embodiments of the disclosure. As illustrated, upon reception of a cluster VAM (step 1000), a CPM ITS-S determines whether the cluster leader may be perceived by its sensors (step 1005). If the cluster leader may be perceived by the sensors of the CPM ITS-S, it is determined whether the VRU Cluster may be associated with one VRU Group created by the CPM ITS-S (each VRU Group may be considered one after the other). To that end, a test may be carried out based on the ratio of the number of VRUs belonging to the VRU Group and to the VRU Cluster with regard to the number of VRUs belonging to the VRU Group and with regard to the number of VRUs belonging to the VRU Cluster (a VRU belonging to the VRU Group or to the VRU Cluster if it is located within its bounding box), also referred to as the intersection over the union (loU). If it is determined that both ratios are above a first threshold (step 1010), for the sake of illustration, the value of the first threshold may be set to 90%, the VRU Cluster identifier (Clusterld) contained in the received cluster VAM is associated with the CPM VRU Group (e.g., as described with reference to step 820 in Figure 8). In addition, the CPM cluster leader role is set to ON (step 1015). This corresponds to CLUSTER-LEADER state 715 in Figure 7. Next, the CPM ITS-S generates and sends a CPM with the clusterLeaderRole set to ON (step 1020) so that the VRU ITS-S cluster leader may break up its cluster (moving to VRU-ACTIVE-STANDALONE state 605 in Figure 6) or moves directly to VRU-PASSIVE state 615 in order to stop sending VAMs (either cluster VAMs or individual VAMs). If the CPM ITS-S determines that at least one of the ratios is below the first threshold (step 1010), a second test may be carried out to determine whether the ratios are both above a second threshold that is lower than the first threshold (still for the sake of illustration, the value of the second threshold may be set to 75%). If the ratios are both above the second threshold (step 1025), the VRU Cluster identifier (ClusterlcT) contained in the received cluster VAM may be associated with the CPM VRU Group (e.g., as described with reference to step 820 in Figure 8). In addition, the CPM cluster leader role may be set to OFF (step 1030). Accordingly, although the VRU Cluster and the VRU Group match only partially, receiving ITS stations having less power than the CPM ITS-S may benefit from the analysis carried out by the CPM ITS-S, taking into consideration that the VRU group in CPM and the VRU cluster in VAM have some common objects. Next the CPM ITS-S generates and sends a CPM with the clusterLeaderRole set to OFF (step 1020) to inform the VRU ITS-S cluster leader that it is still the cluster leader (corresponding to the state GROUP-CLUSTER-ACTIVE 710 in reference to Figure 7) and that it should continue to send cluster VAMs. To that end, the CPM sent by the CPM ITS-S may comprise a specific item of information, e.g., clusterTransferlnfo 571 in Figure 5, set to the value “3” that may represent a request to keep the cluster VAM active. When receiving such a CPM, the VRU ITS-S cluster leader determines that it should continue to send cluster VAM and that it should stay in the VRU-ACTIVE-CLUSTER-LEADER state. If the CPM ITS-S determines that the VRU cluster leader is not perceived (step 1005) or if at least one of the ratios is lower than the second threshold, the VRU Cluster and the VRU Group should not be associated (step 1035), e.g., the VRU Group and Cluster Management module 240 in Figure 2 of the CPM ITS-S does not associate the VRU Group to the cluster VAM (corresponding to GROUP-ACTIVE state 705 in Figure 7). According to another example, steps 1010 and 1025 are based on the bounding box of the VRU Group and on the bounding box of the VRU Cluster, the ratio being the ratio of the overlapping part of the bounding boxes of the VRU Group and of the VRU Cluster with regard to the bounding box of the VRU Group and with regard to the bounding box of the VRU Cluster. Such a way of computing the ratios may be convenient in the case where the cardinality of the VRU Cluster and / or VRU Groups is unknown. Figure 11 illustrates an example of steps carried out in a VRU ITS-S cluster leader to manage the transfer of the cluster leader role between two ITS stations, according to some embodiments of the disclosure. As illustrated, if the VRU ITS-S receives a CPM with a cluster identifier corresponding to the identifier of the cluster to which the VRU ITS-S belongs and if the received CPM comprises an indication indicating that the cluster leader role of the originating CPM ITS-S is set to ON (step 1100), the state of the VRU ITS-S is set VRU-PASSIVE, meaning that the VRU ITS-S should stop generating and transmitting cluster VAMs (and should not generate and transmit individual VAMs), or to VRU-ACTIVE-STANDALONE, meaning that the VRU ITS-S should resume generating and transmitting individual VAMs (step 1105). If the VRU ITS-S receives a CPM with a cluster identifier corresponding to the identifier of the cluster to which the VRU ITS-S belongs and if the received CPM comprisesan indication indicating that the cluster leader role of the originating CPM ITS-S is set to OFF (step 1110), the VRU ITS-S stays in the VRU-ACTIVE-CLUSTER LEADER (step 1115), meaning that the VRU ITS-S keeps the role of cluster leader (and continues generating and transmitting cluster VAMs). For the sake of illustration, the process of transferring or not the role of the cluster leader, described in reference to Figures 10 and 11, is directed to a transfer between a CPM ITS-S and a VRU ITS-S (sending VAM). However, it is to be noted that any ITS station able to lead a cluster may use the data structure clusterTransferlnfo 571 in Figure 5 or a similar data structure to request the transfer of the cluster leader role. When a cluster leader ITS-S is willing to let another ITS-S to lead the cluster, it may send a VAM or a CPM with the clusterTransferType set to a value “1” (or any other predetermined value) requesting to transfer the role of cluster leader. Accordingly, an ITS-S accepting to transfer this role may answer with the clusterTransferType set to a value “2” (or any other predetermined value) representing the acceptance of the transfer of the cluster leader role. Moreover, the active cluster leader should set the clusterLeaderRole to ON (or true value) to inform other ITS-S when it is able to manage the cluster (accepting join / leave cluster operations). Figure 12 illustrates an example of a message sequence chart according to some embodiments of the disclosure, involving VRU ITS-S and CPM originating ITS-S. For the sake of illustration, this sequence of messages may apply to the ITS system 100 illustrated in Figure 1. As illustrated, VRU ITS-S cluster leader 141 generates cluster VAMs such as VAM 131 for transmitting items of information regarding VRU Cluster 195. It is set in the VRU-ACTIVE-CLUSTER-LEADER state. VRU ITS-S 151 and 171 are cluster members of VRU Cluster 195. They are set in the VRU-PASSIVE state, meaning they do not send individual VAMs. At time t1, CPM ITS-S 112 generates a CPM (e.g., CPM 130) containing items of information directed to VRU Group 195, re-grouping bicyclists 142, 152, and 162 detected by analysis module 111. At time t2, CPM ITS-S 112 receives a cluster VAM containing items of information directed to VRU Cluster 145. The cluster VAM includes the reporting of VRU ITS-S 141, 151, and 171 (considered as a set of VRU ITS-Ss and not as individual VRU ITS-Ss). Next, the VRU Group and Cluster Management module of CPM ITS-S 112 determines that VRU Cluster 145 can be associated with VRU Group 195, for example based on the intersection over the union as described with reference to FigurelO. At time t3, a CPM is generated, referencing the cluster identifier (clusterld) of VRU Cluster 145 in the data structure describing VRU Group 195. Compared to the state-of-the-art, the VRU Group and Management module may determine that the VRU cluster leader role is to be kept by the VRU ITS-S leading this cluster. Accordingly, the generated CPM may comprise the following items of information: CPM at time t1 CPM at time t3 Object Id: 195 Object Id: 195 Classification: groupSubClass Classification: groupSubClass clusterld: not set clusterld: 145 clusterLeaderRole: OFF clusterLeaderRole: OFF VruClusterOperation clusterTransferType: 3, corresponding to KeepVAMCIusterLeader Next, at time t4, VRU cluster leader 141, upon receiving a CPM referencing its own cluster (e.g., by checking the cluster identifier value) and comprising an indication to keep the VAM cluster leader active, stays in the VRU-ACTIVE-CLUSTER-LEADER state. As a consequence, it does not break up the VAM Cluster and continues to generate and to transmit cluster VAM, for example at time t5. As illustrated, the cluster members (e.g., VRU ITS-Ss 151 and 171) stay in the VRU-PASSIVE state. In the case of a set of VRUs such as VRUs forming a bicycle queue that may move quickly and that may have large dimensions, the presence of the corresponding VRU Cluster in the detection areas of the CPM ITS-S may be quite short and may not be entirely perceived by the CPM ITS-S sensors. Thanks to some embodiments of the disclosure, the transfer of the cluster leader role from a CAM to a CPM is not systematic and may be kept as complementary to ensure a good level of VRU protection. Moreover it simplifies the analysis process at the ITS-S receiver end by referring the clusterld of the VRU cluster in the VRU Group. Example of a hardware to carry out steps of the method of embodiments of the present disclosure Figure 13 is a schematic representation of an example of a communication ITS-S device configured to implement some embodiments of the present disclosure. It may be either an ITS-S embedded in a vehicle or in a road side unit, for example road side unit 110 in Figure 1. The communication device 1300 may preferably be a device such as a microcomputer, a workstation or a light portable device embedded in a vehicle or a RSU. The communication device 1300 comprises a communication bus 1313 to which there are preferably connected: • a central processing unit 1311, such as a microprocessor, denoted CPU, or a GPU (for graphical processing unit); • a read only memory 1307, denoted ROM, for storing computer programs for implementing (at least partially) the disclosure; • a random access memory 1312, denoted RAM, for storing the executable code of methods according to embodiments of the disclosure as well as the registers adapted to record variables and parameters necessary for implementing methods according to embodiments of the disclosure; and • at least one communication interface 1302 connected to the radio communication network over which ITS messages are transmitted. The ITS messages are written from a FIFO sending memory in RAM 1312 to the network interface for transmission or are read from the network interface for reception and writing into a FIFO receiving memory in RAM 1312 under the control of a software application running in the CPU 1311. Optionally, the communication device 1300 may also include the following components: • a data storage means 1304 such as a hard disk, for storing computer programs for implementing methods according to one or more embodiments of the disclosure; • a disk drive 1305 for a disk 1306, the disk drive being adapted to read data from the disk 1006 or to write data onto said disk; • a screen 1309 for serving as a graphical interface with the user, by means of a keyboard 1310 or any other pointing means. The communication device 1300 may be optionally connected to various peripherals including perception sensors 1308, such as for example a digital camera, each being connected to an input / output card (not shown) so as to supply data to the communication device 1300. Preferably the communication bus provides communication and interoperability between the various elements included in the communication device 1300 or connected to it. The representation of the bus is not limiting and in particular the central processing unit is operable to communicate instructions to any element of the communication device 1300 directly or by means of another element of the communication device 1300. The disk 1306 may optionally be replaced by any information medium such as for example a compact disk (CD-ROM), rewritable or not, a ZIP disk, a USB key or a memory card and, in general terms, by an information storage means that can be read by a microcomputer or by a microprocessor, integrated or not into the apparatus, possibly removable and adapted to store one or more programs whose execution enables a method according to the disclosure to be implemented. The executable code may optionally be stored either in read-only memory 1307, on the hard disk 1304 or on a removable digital medium such as for example a disk 1306 as described previously. According to an optional variant, the executable code of the programs can be received by means of the communication network, via the interface 1302, in order to be stored in one of the storage means of the communication device 1300, such as the hard disk 1304, before being executed. The central processing unit 1311 is preferably adapted to control and direct the execution of the instructions or portions of software code of the program or programs according to the disclosure, which instructions are stored in one of the aforementioned storage means. On powering up, the program or programs that are stored in a nonvolatile memory, for example on the hard disk 1304 or in the read only memory 1307, are transferred into the random access memory 1312, which then contains the executable code of the program or programs, as well as registers for storing the variables and parameters necessary for implementing the disclosure. In a preferred embodiment, the apparatus is a programmable apparatus which uses software to implement the disclosure. However, alternatively, the disclosure may be implemented in hardware (for example, in the form of an Application Specific Integrated Circuit or ASIC). Although the present disclosure has been described hereinabove with reference to specific embodiments, the disclosure is not limited to the specific embodiments, and modifications will be apparent to a skilled person in the art which lie within the scope of 5 the disclosure. Many further modifications and variations will suggest themselves to those versed in the art upon making reference to the foregoing illustrative embodiments, which are given by way of example only and which are not intended to limit the scope of the disclosure, that being determined solely by the appended claims. In particular, the 10 different features from different embodiments may be interchanged, where appropriate. Each of the embodiments of the disclosure described above can be implemented solely or as a combination of a plurality of the embodiments. Also, features from different embodiments can be combined where necessary or where the combination of elements or features from individual embodiments in a single embodiment is beneficial. 15 In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. The mere fact that different features are recited in mutually different dependent claims does not indicate that a combination of these features cannot be advantageously used.

Claims

1. A method of communication in an intelligent transport system, ITS, comprising a plurality of ITS stations, ITS-Ss, at least one ITS-S being associated with a vulnerable road user, VRU, belonging to a cluster of VRUs managed by a first ITS-S and belonging to a group of VRUs managed by a second ITS-S, the method, to be carried out at the second ITS-S, comprising:determining that the VRU cluster and the VRU group may be managed jointly by a single ITS-S; anddetermining whether the second ITS-S is to manage the VRU cluster.

2. The method of claim 1, further comprising generating and transmitting an ITS message, the ITS message comprising an identifier of the VRU cluster and a managing indicator for management of the VRU cluster, the managing indicator being determined as a function of the determining whether the second ITS-S is to manage the VRU cluster.

3. The method of claim 2, wherein the managing indicator is an indication to inform ITS-Ss about an ITS-S in charge of managing the VRU cluster.

4. The method of claim 3, wherein the managing indicator is a request to an ITS-S different from the second ITS-S to manage the VRU cluster.

5. The method of claim 4, wherein the ITS-S requested to manage the VRU cluster is the first ITS-S, the method further comprisingin response to transmitting the generated ITS message, receiving an ITS message from the first ITS-S, the received ITS message comprising an indicator to indicate that the first ITS-S accepts to manage the VRU cluster andassociating a group state to the VRU group to indicate that the VRU cluster is managed by an ITS-S different from the second ITS-S.

6. The method of claim 1 or claim 2, further comprising receiving an ITS message from the first ITS-S, the received ITS message comprising an indicator to request that the first ITS-S manages the VRU cluster.

7. The method of claim 1 or claim 2, further comprising receiving an ITS message from the first ITS-S, the received ITS message comprising an indicator to indicate whether the first ITS-S manages the VRU cluster.

8. The method of claim 2, wherein the managing indicator indicates that the second ITS-S manages the VRU cluster, the method further comprising associating a cluster leader state to the VRU group.

9. The method of claim 8, further comprisingreceiving an ITS message from an ITS-S, the received ITS message comprising a request to join the VRU cluster or a request to leave the VRU cluster andupdating characteristics of the VRU cluster further to processing the received request.

10. The method of claim 8, wherein the managing indicator is indicative of a break up of the VRU cluster, the method further comprising associating a group state to the VRU group to indicate that the VRU cluster and the VRU group are not managed jointly by a single ITS-S.

11. The method of claim 10, wherein the generated ITS message further comprises an indicator of a reason of the break up.

12. The method of any one of claims 2 to 11, wherein claims 3 to 11 depend directly or indirectly on claim 2, wherein the generated ITS message is a Collective Perception Message, CPM.

13. The method of any one of claims 1 to 12, depending directly or indirectly on any one of claims 5 to 7 and 9, wherein the received ITS message is a VRU Awareness Message, VAM.

14. The method of any one of claims 1 to 13, further comprising detecting, using sensors of the second ITS-S, a plurality of VRUs, the plurality of VRUs comprising the VRU associated with the at least one ITS-S.

15. A method of communication in an intelligent transport system, ITS, comprising a plurality of ITS stations, ITS-Ss, at least one ITS-S being associated with a vulnerable road user, VRU, belonging to a cluster of VRUs managed by a first ITS-S and to a group of VRUs managed by a second ITS-S, the method, to be carried out at the first ITS-S, comprising:receiving an ITS message from the second ITS-S, the ITS message comprising characteristics of the VRU group, the characteristics of the VRU group comprising a reference to the VRU cluster,determining, from the received ITS message, that the VRU cluster and the VRU group may be managed jointly by a single ITS-S,generating and transmitting an ITS message, the ITS message comprising a managing indicator for management of the VRU cluster.

16. The method of claim 15, wherein the received ITS message further comprises a second managing indicator for management of the VRU cluster, the managing indicator of the generated ITS message being referred to as the first managing indicator, the second managing indicator requesting the first ITS-S to manage the VRU cluster, the method further comprisingsetting an operating mode of the first ITS-S to a cluster leader mode comprising managing the VRU cluster andsetting the first managing indicator to a value indicating that the first ITS-S manages the VRU cluster.

17. The method of claim 15, wherein the received ITS message further comprises a second managing indicator for management of the VRU cluster, the managing indicator of the generated ITS message being referred to as the first managing indicator, the second managing indicator indicating that the second ITS-S manages the VRU cluster.

18. The method of claim 17, further comprising setting an operating mode of the first ITS-S to a VRU passive mode comprising refraining the first ITS-S from broadcasting VRU awareness messages, VAMs.

19. The method of claim 17 or claim 18, further comprising generating and transmitting an ITS message to indicate a break up of the VRU cluster.

20. The method of any one of claims 15 to 19, wherein the received ITS message is a Collective Perception Message, CPM.

21. The method of any one of claims 15 to 20, wherein the generated ITS message is a VRU Awareness Message, VAM.

22. A computer program product for a programmable apparatus, the computer program product comprising a sequence of instructions for implementing each of the steps of the method according to any one of claims 1 to 21 when loaded into and executed by the programmable apparatus.

23. A non-transitory computer-readable storage medium storing instructions of a computer program for implementing each of the steps of the method according to any one of claims 1 to 21.

24. An Intelligent Transport System, ITS, station, ITS-S, comprising a processing unit configured for carrying out each of the steps of the method according to any one of claims 1 to 21.

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