WO2014046575A1 - V2v device and method for controlling rate of transmission - Google Patents

V2v device and method for controlling rate of transmission Download PDF

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Publication number
WO2014046575A1
WO2014046575A1 PCT/SE2012/050982 SE2012050982W WO2014046575A1 WO 2014046575 A1 WO2014046575 A1 WO 2014046575A1 SE 2012050982 W SE2012050982 W SE 2012050982W WO 2014046575 A1 WO2014046575 A1 WO 2014046575A1
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WO
WIPO (PCT)
Prior art keywords
message
rate
repeatedly transmitted
distance
competition
Prior art date
Application number
PCT/SE2012/050982
Other languages
French (fr)
Inventor
Robert Baldemair
Erik Dahlman
Stefan Parkvall
Original Assignee
Telefonaktiebolaget L M Ericsson (Publ)
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
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Publication date
Application filed by Telefonaktiebolaget L M Ericsson (Publ) filed Critical Telefonaktiebolaget L M Ericsson (Publ)
Priority to PCT/SE2012/050982 priority Critical patent/WO2014046575A1/en
Publication of WO2014046575A1 publication Critical patent/WO2014046575A1/en

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Classifications

    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L47/00—Traffic control in data switching networks
    • H04L47/10—Flow control; Congestion control
    • H04L47/25—Flow control; Congestion control with rate being modified by the source upon detecting a change of network conditions
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04W—WIRELESS COMMUNICATION NETWORKS
    • H04W4/00—Services specially adapted for wireless communication networks; Facilities therefor
    • H04W4/02—Services making use of location information
    • H04W4/023—Services making use of location information using mutual or relative location information between multiple location based services [LBS] targets or of distance thresholds
    • G—PHYSICS
    • G08—SIGNALLING
    • G08G—TRAFFIC CONTROL SYSTEMS
    • G08G1/00—Traffic control systems for road vehicles
    • G08G1/09—Arrangements for giving variable traffic instructions
    • G08G1/091—Traffic information broadcasting
    • G08G1/093—Data selection, e.g. prioritizing information, managing message queues, selecting the information to be output
    • G—PHYSICS
    • G08—SIGNALLING
    • G08G—TRAFFIC CONTROL SYSTEMS
    • G08G1/00—Traffic control systems for road vehicles
    • G08G1/16—Anti-collision systems
    • G08G1/161—Decentralised systems, e.g. inter-vehicle communication
    • G08G1/163—Decentralised systems, e.g. inter-vehicle communication involving continuous checking
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L51/00—User-to-user messaging in packet-switching networks, transmitted according to store-and-forward or real-time protocols, e.g. e-mail
    • H04L51/21—Monitoring or handling of messages
    • H04L51/234—Monitoring or handling of messages for tracking messages
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04W—WIRELESS COMMUNICATION NETWORKS
    • H04W28/00—Network traffic management; Network resource management
    • H04W28/02—Traffic management, e.g. flow control or congestion control
    • H04W28/0215—Traffic management, e.g. flow control or congestion control based on user or device properties, e.g. MTC-capable devices
    • H04W28/0221—Traffic management, e.g. flow control or congestion control based on user or device properties, e.g. MTC-capable devices power availability or consumption
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04W—WIRELESS COMMUNICATION NETWORKS
    • H04W28/00—Network traffic management; Network resource management
    • H04W28/02—Traffic management, e.g. flow control or congestion control
    • H04W28/0226—Traffic management, e.g. flow control or congestion control based on location or mobility
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04W—WIRELESS COMMUNICATION NETWORKS
    • H04W28/00—Network traffic management; Network resource management
    • H04W28/02—Traffic management, e.g. flow control or congestion control
    • H04W28/10—Flow control between communication endpoints
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04W—WIRELESS COMMUNICATION NETWORKS
    • H04W4/00—Services specially adapted for wireless communication networks; Facilities therefor
    • H04W4/30—Services specially adapted for particular environments, situations or purposes
    • H04W4/40—Services specially adapted for particular environments, situations or purposes for vehicles, e.g. vehicle-to-pedestrians [V2P]
    • H04W4/46—Services specially adapted for particular environments, situations or purposes for vehicles, e.g. vehicle-to-pedestrians [V2P] for vehicle-to-vehicle communication [V2V]
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L47/00—Traffic control in data switching networks
    • H04L47/10—Flow control; Congestion control
    • H04L47/15—Flow control; Congestion control in relation to multipoint traffic
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L51/00—User-to-user messaging in packet-switching networks, transmitted according to store-and-forward or real-time protocols, e.g. e-mail
    • H04L51/58—Message adaptation for wireless communication

Definitions

  • the disclosure relates to V2V communication systems, and more specifically to a device and a method for controlling a rate of a repeated transmission of a first device in a V2V communication system used in an ITS.
  • ITS Intelligent Transport Systems
  • ITS utilizes a variety of technologies including among others car navigation, traffic signal control systems, container management systems, variable message signs, automatic number plate recognition, and speed cameras, together with systems that provide traffic participants with relevant and updated information.
  • wireless technologies are applied, such as car- mounted RADAR systems, and cellular technologies to establish Internet connectivity required by several ITS technologies, but also direct Vehicle-to- Vehicle (V2V) communication.
  • V2V Vehicle-to- Vehicle
  • V2V communication is required for several traffic safety related applications where reliance on cellular infrastructure is not possible. Furthermore, data related to these applications is typically only of local interest. Therefore it makes sense not to load a cellular network with such data.
  • Some examples of traffic safety applications that require direct V2V communications are Emergency Electronic Brake Lights, Slow Vehicle Warning, Intersection Collision Warning, Hazardous Location Warning, Pre-Crash Sensing, Lane Change Warning, Cooperative Forward Collision Warning, and Emergency Vehicle Assistance.
  • a car that performs a hard brake communicates this to surrounding cars which can trigger a warning to the driver to increase his/her attention.
  • Cars equipped with Slow Vehicle Warning systems warn other cars about their slow speed or stop which in turn triggers a warning to the drivers of surrounding cars.
  • Intersection Collision Warning systems can be built upon RADAR technology, RADAR based systems have the disadvantage that potentially colliding cars become visible very late, often too late in urban areas.
  • V2V based Intersection Collision Warning systems cars periodically broadcast their position and speed and receiving surrounding cars can calculate potential collision risks and warn drivers if there is a collision risk.
  • the required relative positioning accuracy is very high in such a system, typically up to 0.5 meter.
  • a vehicle equipped with a Hazardous Location Warning system broadcasts the positions of a hazardous spot and other vehicles receiving this information use the information to warn their driver when the vehicle is approaching the hazardous spot. Vehicles receiving the warning may broadcast this information further within a certain geographic area.
  • a Pre-Crash Sensing system is triggered if an accident becomes unavoidable.
  • the Pre-Crash Sensing system prepares the vehicle to mitigate crash impact, e.g. by tightening seat belts. Better crash mitigation requires knowledge regarding other vehicles involved in the crash. Such knowledge can be exchanged via V2V communication.
  • the Lane Change Warning system provides information about vehicles in other lanes and assists the driver when changing lanes.
  • the vehicle intending to change a lane broadcasts this information to other traffic participants.
  • Vehicles moving closely together co-operate with each other in the Cooperative Forward Collision Warning system to avoid longitudinal collisions. Not only vehicles driving directly before or after each other are included in this co-operation but also vehicles separated by some vehicles.
  • the required relative positioning accuracy is at least one meter.
  • Table 1 lists some requirements and properties for the selected services described above.
  • the 802.1 1 p standard is based on the Physical (PHY) layer and the Media Access Control (MAC) layer of 802.1 1 , i.e., Orthogonal Frequency Division Multiplex (OFDM) and Carrier Sensing Multiple Access (CSMA), respectively.
  • OFDM Orthogonal Frequency Division Multiplex
  • CSMA Carrier Sensing Multiple Access
  • CSMA transmissions from two or more nodes may collide if they simultaneously have found the channel to be available followed by the message transmission phase. This in turns results in a serious drawback of 802.1 1 p namely that it cannot guarantee the above mentioned delay requirements if the load increases.
  • STDMA Self-Organizing Time Division Multiple Access
  • AIS Automatic Identification System
  • International regulations mandate AIS on all ships larger than 300 gross tons.
  • STDMA organizes transmissions into a Time Division Multiple Access (TDMA) frame structure consisting of multiple slots where each node is assigned a timeslot of its own using a distributed algorithm.
  • Figure 1 schematically illustrates a TDMA frame 100 with some occupied time slots 101 .
  • the STDMA algorithm consists of four phases:
  • a node entering the network listens to other transmissions to form a picture of which slots that are occupied by other nodes, such as the picture of the frame 100 in Figure 1 , where the occupied time slots 101 are in black.
  • the node selects unoccupied slots, roughly equally spaced to match the desired reporting rate. If there are no free slots in a desired time interval the node selects the slot used by the node furthest away. Thus, in case the system is very loaded, delivery of the position message still occurs but the region in which the message can be heard shrinks due to interference from other, far away nodes, using the same slot(s).
  • SI Selection Interval
  • NTS Nominal Transmission Slot
  • TTL Time To Live
  • the node transmits the positioning information in the slots selected. After a certain time the node performs a reselection of slots in order to handle network changes, e.g. due to the nodes changing their relative position.
  • the result of this step is schematically illustrated in Figure 2b.
  • a. Transmit the information in NTS b. When TTL expires for a NTS select a new NTS and TTL within the SI as in steps 2 and 3. This is useful to handle network changes.
  • V2V communication used for ITS
  • the application area of V2V is expanded to include also pedestrians and bicycles
  • the system load may become very high. If pedestrians are included in the V2V system, they will also be transmitting their position regularly. Pedestrians in a crowd are an example of a situation which may cause load problems in the system.
  • a method for controlling a rate of a repeated transmission of a first device in a V2V communication system used in an ITS is provided.
  • the method is performed by the first device and comprises transmitting a first message comprising a current position of the first device.
  • the first message is transmitted repeatedly with a defined rate.
  • the method also comprises receiving from a second device a second message comprising a position of the second device, and determining a distance to the second device based on the received second message and the current position of the first device.
  • the method further comprises determining whether to alter the rate of the repeatedly transmitted first message based on a comparison of the determined distance and a threshold distance.
  • a first device for a vehicular- to-vehicular communication system used in an intelligent transport system is provided.
  • the first device is configured to control a rate of a repeated transmission of the first device.
  • the first device comprises a transmitter configured to transmit a first message comprising a current position of the first device.
  • the first message is transmitted repeatedly with a defined rate.
  • the first device also comprises a receiver configured to receive from a second device a second message comprising a position of the second device.
  • the first device further comprises a processing circuit configured to determine a distance to the second device based on the received second message and the current position of the first device.
  • the processing circuit is also configured to determine whether to alter the rate of the repeatedly transmitted first message based on a comparison of the determined distance and a threshold distance.
  • An advantage of embodiments of the invention is that the system load of V2V traffic safety wireless communication systems is reduced in areas where many V2V devices are closely located, as in a crowd of pedestrians. Due to the reduced load more devices can be served in a system with a fixed capacity.
  • Figure 1 is a schematic illustration of slot allocations in a TDMA frame.
  • Figure 2a-b are schematic illustrations of slot allocations in a TDMA frame.
  • Figure 3 is a schematic illustration of clustering of V2V devices.
  • Figure 4 is a schematic illustration of device competition based on candidate slot comparisons.
  • FIGS. 5a-e are flowcharts illustrating the method in the V2V device according to embodiments of the invention.
  • Figures 6 is a block diagram schematically illustrating two devices according to embodiments of the invention.
  • embodiment B devices recognizing that they are in a crowd and thus closely located to other devices would autonomously reduce their reporting rate, i.e. their rate of broadcast transmission comprising their position information. Also this embodiment would reduce the load on the system as fewer devices transmit their position during a specified time period.
  • a first device coming close to a second device determines if the two devices should form a cluster by comparing the first device's current position and the second device's broadcasted position. If the two devices are within a certain threshold distance from each other, the first device considers the second device to be within the first device's cluster.
  • the threshold distance may be either hardcoded, or it may be signaled e.g. by the second device, or by another network component if available. Furthermore, the threshold distance could be speed dependent.
  • Both systems based on 802.1 1 p and systems based on STDMA have in common that they broadcast messages related to their position. Also parameters as the speed of device, and/or a direction of movement of the device and the like may be broadcasted in the message.
  • the decision regarding if a first device should form a cluster with a second device and thus if it should alter its rate of broadcasted transmissions is based not only on the distance between the devices but also on a comparison of the speed of the first and the second device, and/or a comparison of the direction of movement of the devices. Devices moving slowly in the same direction may very well be in the same cluster, while two devices moving fast in opposite directions may not be in the same cluster. In this way frequent cluster updates are avoided.
  • clustering of closely located devices is network controlled to avoid capacity shortages in crowded areas.
  • a network can signal a maximum geographic cluster size.
  • FIG. 3 schematically illustrates a group of pedestrians carrying V2V devices 31 a-f.
  • Devices 31 a-e belong to a cluster 30 in which one device is the broadcasting device 31 a.
  • Devices 31 b-e are thus not broadcasting, but are within an "umbrella" created by the broadcasting device 31 a.
  • Devices 31 b-e are thus within the threshold distance from the broadcasting device 31 a, and belong to the broadcasting device's 31 a cluster.
  • Device 31 f is not within the threshold distance from device 31 a, and is not part of the cluster 30.
  • Device 31f is thus obliged to broadcast its position itself.
  • Devices are moving around, and clusters are therefore very dynamic. Devices will enter and leave the cluster all the time.
  • An important part of embodiments of the invention is therefore how to dynamically adapt a cluster to devices coming close to the cluster and thus becoming members, and to devices moving away from a cluster and thus either become a member of another cluster or become a stand- alone device
  • a device coming close to a first devices cluster may become a member and will therefore stop to transmit.
  • a joining member may compete within the devices of the cluster to become the broadcasting device and thus transmitting the broadcast message valid for the whole cluster.
  • a newly joining device and the currently broadcasting device start to compete for the broadcasting position.
  • all devices of the cluster could compete for becoming the broadcaster. In that case all devices would have to listen to all slots which may be a drawback in some cases. In a car mounted or even a bicycle mounted device this may not cause any problems, but for pedestrian devices the energy consumption may become quite high when listening to all slots as the receiver is always active.
  • the currently broadcasting device may broadcast a special signal stating that a new broadcasting device will be selected and that all or at least subset of devices should join the competition.
  • all competing devices calculate a candidate slot position they would use for the broadcast transmission, as well as the candidate slot positions which would be used by other competitors. This may be done by computing the candidate slot positions based on parameters known to all competing devices, e.g. the slot position currently used by each device for broadcasting. A frame number may also be used together with the current slot position to obtain randomization of the candidate slot positions. The slot position of other currently broadcasting devices may be obtained by listening to neighbor device transmissions. Based on these parameters each competing device calculates a metric for itself and for all other competing devices and, based on a selection criterion, a winning device is selected. Described in another way, the competing devices use a predefined function that depends on e.g. the currently used slot number, and a frame number. The output of the function is the metric. Each device thus calculates the metric of its own and of its competitors, and uses the metrics to determine the winner of the competition.
  • a simple exemplary embodiment of how to determine a winning device is schematically illustrated in Figure 4, illustrating the frames 41 , 42, 43, of three different competing devices.
  • the currently used slot of each device is used as input to the function that determines a new candidate slot.
  • the new candidate slot is thus the output of the function, also referred to as the metric.
  • Each device knows its own slot. By listening each device also knows which slots that are currently used by the other two competing devices.
  • Each competing device thus calculates its own candidate slot 44 or metric, as well as the candidate slots or metrics that would be used by the other two devices 45.
  • the selection criteria used to determine the winner is that the highest candidate slot number wins. Therefore, the device that will use the candidate slot with the highest slot number, candidate slot 44, wins the competition and becomes the new broadcasting device.
  • the device with candidate slot number 8 wins the competition and starts using its own candidate slot for broadcasting its position.
  • This example implies that all competing devices are also currently broadcasting, since the currently used slot is one of the input parameters.
  • non-broadcasting cluster members may start to transmit from time to time. The time instance could be at regular intervals or preferable randomly selected.
  • the currently broadcasting device which just lost a competition will in one embodiment send the new slot number (slot number eight in the above example) in its last or few last broadcast messages transmitted with the old slot pattern, so that other devices get to know about the new slot position. This is needed since non-transmitting devices have to listen to the broadcast message from the new broadcasting device and compare their own position with the broadcasted position to determine if they still belong to the cluster or not.
  • a device type identifier or flag is comprised in the broadcasted message together with the position related information.
  • the device type identifier could for example indicate that the device is a stationary device.
  • Such a stationary type device could preferable be used by pedestrian carried devices in the vicinity as a cluster broadcaster.
  • a stationary device typically has an electric connection and thus battery life time is not a problem.
  • pedestrian carried V2V devices may suffer from short battery life times. Therefore, at crowded places such as subway exits, schools, pedestrian crossings, and even directly on school buses, fixed devices could be placed in order to always work as broadcasting devices of a cluster.
  • a stationary device thus indicates by the type identifier that it has a higher priority than other devices as it is a fixed device without battery life problems.
  • the advantage of a type identifier indicating e.g. that a device is stationary will thus result in increased battery life times of pedestrian carried V2V devices in traffic safety systems when using clustering.
  • such a flag could also be used to indicate a purpose, e.g. a high likelihood of pupils or pedestrians, which in turn could trigger a warning, an increased attention sign or an automatic response in a vehicle.
  • pedestrian carried devices could broadcast a "pedestrian” identifier
  • devices for pupils could broadcast a “pupil” identifier to enable other traffic participants in the ITS such as vehicles and their drivers to react accordingly.
  • Emergency cars could broadcast an "emergency vehicle” identifier which could again trigger a warning/increased attention sign/automatic response in a vehicle.
  • an identifier could be used to identify possible new cluster participants, as it may be advantageous to group only pedestrian devices in a cluster and not to mix pedestrian devices and vehicle devices.
  • a device leaves a cluster it will recognize that the distance to the currently broadcasting device increases and sooner or later it will leave the cluster to either join a new cluster or to become a stand-alone device. The same applies if the currently broadcasting device moves away from other devices of the cluster. The other devices of the cluster will recognize that the distance to the currently broadcasting device increases and will either become stand-alone devices or form a new cluster. The broadcasting device becomes either a stand-alone device or joins another cluster and competes there to become broadcaster of the new cluster.
  • a device could, after recognizing that the broadcasting device has left the cluster, wait a random time period, also called a back-off time, prior to starting to transmit again. If one of the devices starts to transmit first, other devices can hear this device and won't start to transmit themselves. However, such a random back-off time must be chosen short enough to still guarantee safety of the traffic participant.
  • devices recognize they are within a cluster based on their own and other devices position, they only reduce their rate of broadcasted messages. Each device is thus still broadcasting but at a reduced rate. The total system load is thereby reduced.
  • a device recognizes that it is moving away from the cluster, by determining that the distance to another device of the cluster is increasing, it may increase its broadcasting rate in order to keep the same service level and traffic security.
  • Figure 5a is a flowchart illustrating an embodiment of a method for controlling a rate of a repeated transmission of a first device in a V2V communication system used in an ITS. The method is performed by the first device and comprises:
  • - 510 Transmitting a first message comprising a current position of the first device.
  • the first message is transmitted repeatedly with a defined rate. This is thus the message broadcasted by the first device as described previously.
  • the message informs the surrounding devices of the first device's position.
  • - 520 Receiving from a second device a second message comprising a position of the second device.
  • the first device listens to broadcasted messages from surrounding devices such as the second device, and thus receives information related to their positions.
  • the second device's position received in the second message and the first devices current position are used to determine a distance between the two devices.
  • - 540 Determining whether to alter the rate of the repeatedly transmitted first message based on a comparison of the determined distance and a threshold distance.
  • the threshold distance may be predetermined. Alternatively the threshold may be received from the second device or from a node in the V2V communication system.
  • FIG. 5b is a flowchart illustrating embodiment B of the method.
  • the steps 510- 530 are the same as the one described above with reference to Figure 5a.
  • Step 540 of determining whether to alter the rate of the repeatedly transmitted first message comprises if the determined distance is smaller than the threshold distance:
  • FIG. 5c is a flowchart illustrating embodiment A of the method. The steps 510- 530 are the same as the one described above with reference to Figure 5a.
  • Step 540 of determining whether to alter the rate of the repeatedly transmitted first message comprises if the determined distance is smaller than the threshold distance: - 545: Competing with the second device for transmitting the first message, wherein the competition is based on a comparison of metrics calculated for the first and the second device.
  • the metrics are candidate slot numbers calculated based on a predefined function with currently used slot numbers as input. The competing device that gets the highest candidate slot number is the winning device.
  • the device will continue to transmit the first message comprising an updated current position of the first device, and to receive the second message comprising the position of the second device or other devices, in order to determine distances to other devices and determine whether to cluster and alter the rate of the repeatedly transmitted first message. If the competition is won by the first device, the rate of the repeatedly transmitted first message will also be left unchanged.
  • competing 545 with the second device comprises calculating a candidate time slot position for the repeated transmission of the first message. This is done based on a currently used time slot position and a predefined function for determining the candidate time slot position. It is done for both the first and the second device. The respective calculated candidate time slot position of the first and the second device are compared, and the outcome of the competition is determined based on the comparison of the respective calculated candidate time slot positions.
  • the criterion for determining which device that wins the competition may be the device with the highest candidate time slot number, as described previously. However, other criteria are possible.
  • competing 545 with the second device further comprises transmitting the calculated candidate time slot position of the second device when the competition is lost. This is done to inform other devices about the new time slot that will be used for the broadcast by the new winning device.
  • Figure 5d is a flowchart illustrating embodiment A of the method. The steps 510- 530 are the same as the one described above with reference to Figure 5a. However, the received second message also comprises a type identifier of the second device. Step 540 of determining whether to alter the rate of the repeatedly transmitted first message if the determined distance is smaller than the threshold distance comprises before the step of competing 545:
  • the rate of the repeatedly transmitted first message will be set to zero in step 544.
  • the stationary second device acts as a broadcasting device as it is not constrained by a limited battery life time.
  • the first device being e.g. a pedestrian carried device will thus save its battery. If it is determined that a competition will be performed, the same steps 545 and 546 of competing with the second device for transmitting the first message, and setting the rate of the repeatedly transmitted first message to zero when the competition is lost as described above with reference to Figure 5c will be performed.
  • the transmitted first message and the received second message also comprise a speed of device, and/or a direction of movement of device
  • the method further comprises comparing the respective speed of device and/or the respective direction of movement of device of the first and the second device.
  • the determining 540 of whether to alter the rate of the repeatedly transmitted first message is based also on the comparison of the speed of device and/or the direction of movement of device. Devices moving in the same direction should likely be clustered while devices moving in opposite directions should not.
  • Figure 5e is a flowchart illustrating another embodiment of the method, combinable with any of the above described embodiments.
  • the steps 510-540 are the same as the one described above with reference to Figure 5a.
  • the method further comprises when the rate of the repeatedly transmitted first message has been altered: - 550: Receiving from the second device an updated second message comprising the position of the second device.
  • the second device may have moved away from the first device or the first device may have moved away from the second device. In both cases, the distance between the two devices has augmented and should be compared with the threshold distance.
  • - 570 Increasing the rate of the repeatedly transmitted first message, when the determined updated distance is equal to or above the threshold distance. If the two devices are two far away from each other, they should both be transmitting at a higher rate again.
  • the first device 610 is configured to control a rate of a repeated transmission of the first device.
  • the first device comprises a transmitter 61 1 configured to transmit a first message comprising a current position of the first device, the first message being transmitted repeatedly with a defined rate.
  • the first device 610 also comprises a receiver 612 configured to receive from the second device 620 a second message comprising a position of the second device.
  • the first device 610 further comprises a processing circuit 613 configured to determine a distance to the second device based on the received second message and the current position of the first device.
  • the processing circuit 613 is also configured to determine whether to alter the rate of the repeatedly transmitted first message based on a comparison of the determined distance and a threshold distance.
  • the threshold distance may be predetermined. Alternatively the threshold may be received from the second device or from a node in the V2V communication system.
  • the processing circuit 613 is configured to determine whether to alter the rate of the repeatedly transmitted first message by being configured to decrease the rate of the repeatedly transmitted first message if the determined distance is smaller than the threshold distance.
  • the processing circuit 613 is configured to determine whether to alter the rate of the repeatedly transmitted first message if the determined distance is smaller than the threshold distance, by being configured to let the first device compete with the second device for transmitting the first message, and by being further configured to set the rate of the repeatedly transmitted first message to zero when the competition is lost.
  • the competition is based on a comparison of metrics calculated for the first and the second device. As described above, the metrics may e.g. be candidate slot numbers, and the criteria for selecting the winning device is the highest candidate slot number.
  • the processing circuit 613 is further configured to compete with the second device by being configured to calculate a candidate time slot position for the repeated transmission of the first message based on a currently used time slot position and a predefined function for determining the candidate time slot position. This is done for both the first and the second device.
  • the processing circuit 613 is also configured to compare the respective calculated candidate time slot position of the first and the second device, and to determine the outcome of the competition based on the comparison of the respective calculated candidate time slot positions.
  • the transmitter 61 1 may be further configured to transmit the calculated candidate time slot position of the second device when the competition is lost.
  • the receiver 612 is further configured to receive the second message also comprising a type identifier of the second device.
  • the processing circuit 613 is configured to determine whether to alter the rate of the repeatedly transmitted first message by being configured to determine whether to perform the competition with the second device, based on the type identifier, and set the rate of the repeatedly transmitted first message to zero if it is determined not to perform the competition with the second device.
  • the transmitter 61 1 and the receiver 612 are further configured to respectively transmit the first message and receive the second message also comprising a speed of device, and/or a direction of movement of device.
  • the processing circuit 613 is further configured to compare the respective speed of device and/or the respective direction of movement of device of the first and the second device, and determine whether to alter the rate of the repeatedly transmitted first message based also on the comparison of the speed of device and/or the direction of movement of device.
  • the receiver 612 is further configured to receive from the second device an updated second message comprising the position of the second device.
  • the processing circuit 613 is further configured to determine an updated distance to the second device based on the received updated second message and the current position of the first device, and to increase the rate of the repeatedly transmitted first message, when the determined updated distance is equal to or above the threshold distance.
  • the second device 620 also comprises a transmitter 621 , a receiver 622, and a processing circuit 623, configured according to the above described embodiments.
  • the first device comprises a Central Processing Unit (CPU) which may be a single unit or a plurality of units. Furthermore, the first device comprises at least one computer program product (CPP) in the form of a non-volatile memory, e.g. an EEPROM (Electrically Erasable Programmable Read-Only Memory), a flash memory or a disk drive.
  • the CPP comprises a computer program, which comprises code means which when run on the device causes the CPU to perform steps of the procedure described earlier in conjunction with Figures 5a-e. In other words, when said code means are run on the CPU, they correspond to the processing circuit 613 in the device 610 of Figure 6.

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Abstract

The present invention relates to a method for controlling a rate of a repeated transmission of a first device in a V2V communication system used in an ITS. The method is performed by the first device and comprises transmitting (510) a first message comprising a current position of the first device. The first message is transmitted repeatedly with a defined rate. The method also comprises receiving (520) from a second device a second message comprising a position of the second device, and determining (530) a distance to the second device based on the received second message and the current position of the first device. The method further comprises determining (540) whether to alter the rate of the repeatedly transmitted first message based on a comparison of the determined distance and a threshold distance.

Description

V2V DEVICE AND METHOD FOR CONTROLLING RATE OF TRANSMISSION TECHNICAL FIELD
The disclosure relates to V2V communication systems, and more specifically to a device and a method for controlling a rate of a repeated transmission of a first device in a V2V communication system used in an ITS.
BACKGROUND
The development of Intelligent Transport Systems (ITS) is an effort to integrate information and communication technology into transportation systems and vehicles with the goal to improve traffic safety and transportation efficiency. To achieve this goal ITS utilizes a variety of technologies including among others car navigation, traffic signal control systems, container management systems, variable message signs, automatic number plate recognition, and speed cameras, together with systems that provide traffic participants with relevant and updated information. Furthermore, several wireless technologies are applied, such as car- mounted RADAR systems, and cellular technologies to establish Internet connectivity required by several ITS technologies, but also direct Vehicle-to- Vehicle (V2V) communication.
V2V communication is required for several traffic safety related applications where reliance on cellular infrastructure is not possible. Furthermore, data related to these applications is typically only of local interest. Therefore it makes sense not to load a cellular network with such data. Some examples of traffic safety applications that require direct V2V communications are Emergency Electronic Brake Lights, Slow Vehicle Warning, Intersection Collision Warning, Hazardous Location Warning, Pre-Crash Sensing, Lane Change Warning, Cooperative Forward Collision Warning, and Emergency Vehicle Assistance.
In the Emergency Electronic Brake Lights system a car that performs a hard brake communicates this to surrounding cars which can trigger a warning to the driver to increase his/her attention. Cars equipped with Slow Vehicle Warning systems warn other cars about their slow speed or stop which in turn triggers a warning to the drivers of surrounding cars.
Even though Intersection Collision Warning systems can be built upon RADAR technology, RADAR based systems have the disadvantage that potentially colliding cars become visible very late, often too late in urban areas. In V2V based Intersection Collision Warning systems, cars periodically broadcast their position and speed and receiving surrounding cars can calculate potential collision risks and warn drivers if there is a collision risk. The required relative positioning accuracy is very high in such a system, typically up to 0.5 meter.
A vehicle equipped with a Hazardous Location Warning system broadcasts the positions of a hazardous spot and other vehicles receiving this information use the information to warn their driver when the vehicle is approaching the hazardous spot. Vehicles receiving the warning may broadcast this information further within a certain geographic area.
A Pre-Crash Sensing system is triggered if an accident becomes unavoidable. The Pre-Crash Sensing system prepares the vehicle to mitigate crash impact, e.g. by tightening seat belts. Better crash mitigation requires knowledge regarding other vehicles involved in the crash. Such knowledge can be exchanged via V2V communication.
The Lane Change Warning system provides information about vehicles in other lanes and assists the driver when changing lanes. The vehicle intending to change a lane broadcasts this information to other traffic participants.
Vehicles moving closely together co-operate with each other in the Cooperative Forward Collision Warning system to avoid longitudinal collisions. Not only vehicles driving directly before or after each other are included in this co-operation but also vehicles separated by some vehicles. The required relative positioning accuracy is at least one meter.
In the Emergency Vehicle Assistance system emergency vehicles broadcast their presence, including lane information, to other traffic participants in order to clear an emergency corridor.
Table 1 lists some requirements and properties for the selected services described above.
Figure imgf000004_0001
Table 1 : Requirements and properties of selected traffic safety applications
Above mentioned traffic safety applications have in addition to the requirements of Table 1 rather stringent and sometimes even extreme positioning requirements which cannot be fulfilled with today's Global Navigation Satellite Systems (GNSS) positioning systems. Therefore additional positioning solutions are sometimes required to achieve the required accuracy.
With the standard 802.1 1 p there is already a solution that targets above mentioned V2V applications. The 802.1 1 p standard is based on the Physical (PHY) layer and the Media Access Control (MAC) layer of 802.1 1 , i.e., Orthogonal Frequency Division Multiplex (OFDM) and Carrier Sensing Multiple Access (CSMA), respectively. In a CSMA scheme, a node intending to transmit first listens to the channel in order to determine whether the channel is occupied or not. If the channel has been free for a certain time, the node transmits the message; otherwise it backs off for a period of time before it repeats the attempt to access the channel. Therefore, CSMA transmissions from two or more nodes may collide if they simultaneously have found the channel to be available followed by the message transmission phase. This in turns results in a serious drawback of 802.1 1 p namely that it cannot guarantee the above mentioned delay requirements if the load increases.
Self-Organizing Time Division Multiple Access (STDMA) is an alternative MAC scheme, capable of guaranteeing an upper bound on the channel access delay. It is used e.g. in the Automatic Identification System (AIS), a system allowing ships to identify other ships in the vicinity and avoid collisions. International regulations mandate AIS on all ships larger than 300 gross tons. Each ship, as well as other objects of interest, periodically transmits its position, its speed, and other related parameters. Based on the broadcasted data, ships can build a picture of surrounding activities.
STDMA organizes transmissions into a Time Division Multiple Access (TDMA) frame structure consisting of multiple slots where each node is assigned a timeslot of its own using a distributed algorithm. Figure 1 schematically illustrates a TDMA frame 100 with some occupied time slots 101 . The STDMA algorithm consists of four phases:
1 ) Initialization
2) Network entry
3) First frame
4) Continuous operation. The four steps are briefly described hereinafter.
1 . Network entry
A node entering the network listens to other transmissions to form a picture of which slots that are occupied by other nodes, such as the picture of the frame 100 in Figure 1 , where the occupied time slots 101 are in black.
Network entry and first frame
In essence the node selects unoccupied slots, roughly equally spaced to match the desired reporting rate. If there are no free slots in a desired time interval the node selects the slot used by the node furthest away. Thus, in case the system is very loaded, delivery of the position message still occurs but the region in which the message can be heard shrinks due to interference from other, far away nodes, using the same slot(s). These steps are explained with reference to Figure 2a, schematically illustrating a TDMA frame. a. Determine which slots to use for transmission using: i. Nominal Increment (Nl) 24 = (number of slots) / (report rate). ii. Nominal Start Slot (NSS) 23 e [current slot, Nl], randomly selected.
This is thus the frame start for this node.
iii. Selection Interval (SI) 21 determined as 20% of Nl 24, centered
around NSS 233.
iv. Nominal Transmission Slot (NTS) 22, randomly selected among free slots in SI 21 .
v. If all slots in SI 21 are occupied select slot used by the node furthest away, which is possible as the nodes transmit their position. b. Upon reaching first NTS 22 enter first frame phase.
I. Select NTS 22 in remaining SI 21 within a frame.
II Determine Time To Live (TTL) independently for each NTS 22 within a frame. TTL is 3...8 frames in AIS. 4. Continuous operation
The node transmits the positioning information in the slots selected. After a certain time the node performs a reselection of slots in order to handle network changes, e.g. due to the nodes changing their relative position. The result of this step is schematically illustrated in Figure 2b. a. Transmit the information in NTS b. When TTL expires for a NTS select a new NTS and TTL within the SI as in steps 2 and 3. This is useful to handle network changes.
However, if V2V communication, used for ITS, becomes very popular, and the application area of V2V is expanded to include also pedestrians and bicycles, the system load may become very high. If pedestrians are included in the V2V system, they will also be transmitting their position regularly. Pedestrians in a crowd are an example of a situation which may cause load problems in the system. SUMMARY
It is therefore an object to address some of the problems outlined above, and to provide a solution making it possible to handle the load on the system when many V2V devices are grouped together, as in a crowd of pedestrians carrying V2V devices. This object and others are achieved by the method and the device in a V2V communication system according to the independent claims, and by the embodiments according to the dependent claims.
In accordance with a first aspect of the invention, a method for controlling a rate of a repeated transmission of a first device in a V2V communication system used in an ITS is provided. The method is performed by the first device and comprises transmitting a first message comprising a current position of the first device. The first message is transmitted repeatedly with a defined rate. The method also comprises receiving from a second device a second message comprising a position of the second device, and determining a distance to the second device based on the received second message and the current position of the first device. The method further comprises determining whether to alter the rate of the repeatedly transmitted first message based on a comparison of the determined distance and a threshold distance.
In accordance with a second aspect of the invention, a first device for a vehicular- to-vehicular communication system used in an intelligent transport system is provided. The first device is configured to control a rate of a repeated transmission of the first device. The first device comprises a transmitter configured to transmit a first message comprising a current position of the first device. The first message is transmitted repeatedly with a defined rate. The first device also comprises a receiver configured to receive from a second device a second message comprising a position of the second device. The first device further comprises a processing circuit configured to determine a distance to the second device based on the received second message and the current position of the first device. The processing circuit is also configured to determine whether to alter the rate of the repeatedly transmitted first message based on a comparison of the determined distance and a threshold distance.
An advantage of embodiments of the invention is that the system load of V2V traffic safety wireless communication systems is reduced in areas where many V2V devices are closely located, as in a crowd of pedestrians. Due to the reduced load more devices can be served in a system with a fixed capacity.
Other objects, advantages and features of embodiments will be explained in the following detailed description when considered in conjunction with the accompanying drawings and claims.
BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic illustration of slot allocations in a TDMA frame. Figure 2a-b are schematic illustrations of slot allocations in a TDMA frame. Figure 3 is a schematic illustration of clustering of V2V devices. Figure 4 is a schematic illustration of device competition based on candidate slot comparisons.
Figures 5a-e are flowcharts illustrating the method in the V2V device according to embodiments of the invention.
Figures 6 is a block diagram schematically illustrating two devices according to embodiments of the invention.
DETAILED DESCRIPTION
In the following, different aspects will be described in more detail with reference to certain embodiments of the invention and to accompanying drawings. For purposes of explanation and not limitation, specific details are set forth, such as particular scenarios and techniques, in order to provide a thorough understanding of the different embodiments. However, other embodiments that depart from these specific details may also exist.
Moreover, those skilled in the art will appreciate that the functions and means explained herein below may be implemented using software functioning in conjunction with a programmed microprocessor or general purpose computer, and/or using an application specific integrated circuit (ASIC). It will also be appreciated that while embodiments of the invention are primarily described in the form of methods and nodes, they may also be embodied in a computer program product as well as in a system comprising a computer processor and a memory coupled to the processor, wherein the memory is encoded with one or more programs that may perform the functions disclosed herein.
The problem of the increased load in a system when V2V devices are used not just for ordinary vehicles but also for pedestrians and bicycles is addressed by a solution where devices closely located and which thus have a similar location, are grouped in a cluster. The device that recognizes that it is in the vicinity of another device will decide, based on the distance to the other device, if it should cluster with the other device and therefore alter its rate of broadcast transmissions comprising its current position. In a first embodiment of the invention, referred to as embodiment A, only one or a subset of devices in a cluster of devices are sending broadcast messages, thus meaning that the rate of transmitting broadcast messages is reduced to zero for the rest of the devices. The devices broadcasting their position create a so called umbrella and other devices not broadcasting their own position are "protected" as long as they are under the umbrella. The result is that system load decreases, since only one ore a few devices are broadcasting their position.
In an alternative embodiment of the invention, referred to as embodiment B, devices recognizing that they are in a crowd and thus closely located to other devices would autonomously reduce their reporting rate, i.e. their rate of broadcast transmission comprising their position information. Also this embodiment would reduce the load on the system as fewer devices transmit their position during a specified time period.
In embodiments of the invention, a first device coming close to a second device determines if the two devices should form a cluster by comparing the first device's current position and the second device's broadcasted position. If the two devices are within a certain threshold distance from each other, the first device considers the second device to be within the first device's cluster. The threshold distance may be either hardcoded, or it may be signaled e.g. by the second device, or by another network component if available. Furthermore, the threshold distance could be speed dependent.
Both systems based on 802.1 1 p and systems based on STDMA have in common that they broadcast messages related to their position. Also parameters as the speed of device, and/or a direction of movement of the device and the like may be broadcasted in the message. In one embodiment, the decision regarding if a first device should form a cluster with a second device and thus if it should alter its rate of broadcasted transmissions is based not only on the distance between the devices but also on a comparison of the speed of the first and the second device, and/or a comparison of the direction of movement of the devices. Devices moving slowly in the same direction may very well be in the same cluster, while two devices moving fast in opposite directions may not be in the same cluster. In this way frequent cluster updates are avoided.
In one embodiment of the invention which may be combined with either embodiment A or B, clustering of closely located devices is network controlled to avoid capacity shortages in crowded areas. For example, a network can signal a maximum geographic cluster size.
Embodiment A
Figure 3 schematically illustrates a group of pedestrians carrying V2V devices 31 a-f. Devices 31 a-e belong to a cluster 30 in which one device is the broadcasting device 31 a. Devices 31 b-e are thus not broadcasting, but are within an "umbrella" created by the broadcasting device 31 a. Devices 31 b-e are thus within the threshold distance from the broadcasting device 31 a, and belong to the broadcasting device's 31 a cluster. Device 31 f is not within the threshold distance from device 31 a, and is not part of the cluster 30. Device 31f is thus obliged to broadcast its position itself. Devices are moving around, and clusters are therefore very dynamic. Devices will enter and leave the cluster all the time. An important part of embodiments of the invention is therefore how to dynamically adapt a cluster to devices coming close to the cluster and thus becoming members, and to devices moving away from a cluster and thus either become a member of another cluster or become a stand- alone device.
In one embodiment, a device coming close to a first devices cluster may become a member and will therefore stop to transmit. In an alternative embodiment, a joining member may compete within the devices of the cluster to become the broadcasting device and thus transmitting the broadcast message valid for the whole cluster.
In one possible embodiment, a newly joining device and the currently broadcasting device start to compete for the broadcasting position. However, in principle all devices of the cluster could compete for becoming the broadcaster. In that case all devices would have to listen to all slots which may be a drawback in some cases. In a car mounted or even a bicycle mounted device this may not cause any problems, but for pedestrian devices the energy consumption may become quite high when listening to all slots as the receiver is always active.
For a device not listening to all slots, a car quickly approaching the device will not be recognized. However, for a pedestrian device the transmitting functionality making the device visible to other traffic participants is more important than the receiver functionality making it possible to recognize threats from quickly approaching devices, due to the limited reaction time for a pedestrian.
To enable a competition for the broadcast transmission between all devices of a cluster without forcing each device to listen in all slots, the currently broadcasting device may broadcast a special signal stating that a new broadcasting device will be selected and that all or at least subset of devices should join the competition.
In one embodiment, all competing devices calculate a candidate slot position they would use for the broadcast transmission, as well as the candidate slot positions which would be used by other competitors. This may be done by computing the candidate slot positions based on parameters known to all competing devices, e.g. the slot position currently used by each device for broadcasting. A frame number may also be used together with the current slot position to obtain randomization of the candidate slot positions. The slot position of other currently broadcasting devices may be obtained by listening to neighbor device transmissions. Based on these parameters each competing device calculates a metric for itself and for all other competing devices and, based on a selection criterion, a winning device is selected. Described in another way, the competing devices use a predefined function that depends on e.g. the currently used slot number, and a frame number. The output of the function is the metric. Each device thus calculates the metric of its own and of its competitors, and uses the metrics to determine the winner of the competition.
A simple exemplary embodiment of how to determine a winning device is schematically illustrated in Figure 4, illustrating the frames 41 , 42, 43, of three different competing devices. The currently used slot of each device is used as input to the function that determines a new candidate slot. The new candidate slot is thus the output of the function, also referred to as the metric. Each device knows its own slot. By listening each device also knows which slots that are currently used by the other two competing devices. Each competing device thus calculates its own candidate slot 44 or metric, as well as the candidate slots or metrics that would be used by the other two devices 45. The selection criteria used to determine the winner is that the highest candidate slot number wins. Therefore, the device that will use the candidate slot with the highest slot number, candidate slot 44, wins the competition and becomes the new broadcasting device. In Figure 4, the device with candidate slot number 8 (frame 43) wins the competition and starts using its own candidate slot for broadcasting its position. This example implies that all competing devices are also currently broadcasting, since the currently used slot is one of the input parameters. To enable other currently silent devices to compete for becoming broadcasters, non-broadcasting cluster members may start to transmit from time to time. The time instance could be at regular intervals or preferable randomly selected.
The currently broadcasting device which just lost a competition will in one embodiment send the new slot number (slot number eight in the above example) in its last or few last broadcast messages transmitted with the old slot pattern, so that other devices get to know about the new slot position. This is needed since non-transmitting devices have to listen to the broadcast message from the new broadcasting device and compare their own position with the broadcasted position to determine if they still belong to the cluster or not.
In one embodiment of the invention a device type identifier or flag is comprised in the broadcasted message together with the position related information. The device type identifier could for example indicate that the device is a stationary device. Such a stationary type device could preferable be used by pedestrian carried devices in the vicinity as a cluster broadcaster. A stationary device typically has an electric connection and thus battery life time is not a problem. On the contrary, pedestrian carried V2V devices may suffer from short battery life times. Therefore, at crowded places such as subway exits, schools, pedestrian crossings, and even directly on school buses, fixed devices could be placed in order to always work as broadcasting devices of a cluster. The broadcast of special flag or identifier indicating that it is a stationary device, could thereby be used to avoid that other pedestrian carried V2V devices compete to become new cluster broadcasters. A stationary device thus indicates by the type identifier that it has a higher priority than other devices as it is a fixed device without battery life problems. The advantage of a type identifier indicating e.g. that a device is stationary, will thus result in increased battery life times of pedestrian carried V2V devices in traffic safety systems when using clustering. In addition to the prolonged battery life time benefit, such a flag could also be used to indicate a purpose, e.g. a high likelihood of pupils or pedestrians, which in turn could trigger a warning, an increased attention sign or an automatic response in a vehicle. In a similar way, pedestrian carried devices could broadcast a "pedestrian" identifier, and devices for pupils could broadcast a "pupil" identifier to enable other traffic participants in the ITS such as vehicles and their drivers to react accordingly. Emergency cars could broadcast an "emergency vehicle" identifier which could again trigger a warning/increased attention sign/automatic response in a vehicle. Furthermore, such an identifier could be used to identify possible new cluster participants, as it may be advantageous to group only pedestrian devices in a cluster and not to mix pedestrian devices and vehicle devices.
If a device leaves a cluster it will recognize that the distance to the currently broadcasting device increases and sooner or later it will leave the cluster to either join a new cluster or to become a stand-alone device. The same applies if the currently broadcasting device moves away from other devices of the cluster. The other devices of the cluster will recognize that the distance to the currently broadcasting device increases and will either become stand-alone devices or form a new cluster. The broadcasting device becomes either a stand-alone device or joins another cluster and competes there to become broadcaster of the new cluster. To avoid that all remaining devices of a cluster where the broadcaster has moved away immediately start to transmit, a device could, after recognizing that the broadcasting device has left the cluster, wait a random time period, also called a back-off time, prior to starting to transmit again. If one of the devices starts to transmit first, other devices can hear this device and won't start to transmit themselves. However, such a random back-off time must be chosen short enough to still guarantee safety of the traffic participant.
Embodiment B
If devices recognize they are within a cluster based on their own and other devices position, they only reduce their rate of broadcasted messages. Each device is thus still broadcasting but at a reduced rate. The total system load is thereby reduced. When a device recognizes that it is moving away from the cluster, by determining that the distance to another device of the cluster is increasing, it may increase its broadcasting rate in order to keep the same service level and traffic security.
Flowcharts of method and block diagram of device
Figure 5a is a flowchart illustrating an embodiment of a method for controlling a rate of a repeated transmission of a first device in a V2V communication system used in an ITS. The method is performed by the first device and comprises:
- 510: Transmitting a first message comprising a current position of the first device. The first message is transmitted repeatedly with a defined rate. This is thus the message broadcasted by the first device as described previously. The message informs the surrounding devices of the first device's position.
- 520: Receiving from a second device a second message comprising a position of the second device. Correspondingly, the first device listens to broadcasted messages from surrounding devices such as the second device, and thus receives information related to their positions.
- 530: Determining a distance to the second device based on the received second message and the current position of the first device. The second device's position received in the second message and the first devices current position are used to determine a distance between the two devices.
- 540: Determining whether to alter the rate of the repeatedly transmitted first message based on a comparison of the determined distance and a threshold distance. The threshold distance may be predetermined. Alternatively the threshold may be received from the second device or from a node in the V2V communication system.
Figure 5b is a flowchart illustrating embodiment B of the method. The steps 510- 530 are the same as the one described above with reference to Figure 5a. Step 540 of determining whether to alter the rate of the repeatedly transmitted first message comprises if the determined distance is smaller than the threshold distance:
- 541 : Decreasing the rate of the repeatedly transmitted first message.
If the determined distance is larger or equal to the threshold distance, the rate of the repeatedly transmitted first message is left unchanged. In this case, no cluster has been formed and the device will continue to transmit the first message comprising an updated current position of the first device, and to receive the second message comprising the position of the second device or other devices, in order to determine distances to other devices and determine whether to cluster and alter the rate of the repeatedly transmitted first message. Figure 5c is a flowchart illustrating embodiment A of the method. The steps 510- 530 are the same as the one described above with reference to Figure 5a. Step 540 of determining whether to alter the rate of the repeatedly transmitted first message comprises if the determined distance is smaller than the threshold distance: - 545: Competing with the second device for transmitting the first message, wherein the competition is based on a comparison of metrics calculated for the first and the second device. In the above described example, the metrics are candidate slot numbers calculated based on a predefined function with currently used slot numbers as input. The competing device that gets the highest candidate slot number is the winning device.
- 546: Setting the rate of the repeatedly transmitted first message to zero when the competition is lost. When the competition is lost, in the above described example embodiment when the candidate slot number is the lowest one, the first device stops transmitting the first message. The second device will then take on the broadcasting role.
If the determined distance is larger or equal to the threshold distance, no competition is performed and the rate of the repeatedly transmitted first message is left unchanged. In this case, no cluster has been formed and the device will continue to transmit the first message comprising an updated current position of the first device, and to receive the second message comprising the position of the second device or other devices, in order to determine distances to other devices and determine whether to cluster and alter the rate of the repeatedly transmitted first message. If the competition is won by the first device, the rate of the repeatedly transmitted first message will also be left unchanged.
In one embodiment, competing 545 with the second device comprises calculating a candidate time slot position for the repeated transmission of the first message. This is done based on a currently used time slot position and a predefined function for determining the candidate time slot position. It is done for both the first and the second device. The respective calculated candidate time slot position of the first and the second device are compared, and the outcome of the competition is determined based on the comparison of the respective calculated candidate time slot positions. The criterion for determining which device that wins the competition may be the device with the highest candidate time slot number, as described previously. However, other criteria are possible.
In another embodiment, competing 545 with the second device further comprises transmitting the calculated candidate time slot position of the second device when the competition is lost. This is done to inform other devices about the new time slot that will be used for the broadcast by the new winning device. Figure 5d is a flowchart illustrating embodiment A of the method. The steps 510- 530 are the same as the one described above with reference to Figure 5a. However, the received second message also comprises a type identifier of the second device. Step 540 of determining whether to alter the rate of the repeatedly transmitted first message if the determined distance is smaller than the threshold distance comprises before the step of competing 545:
- 543: Determining whether to perform the competition with the second device, based on the type identifier.
If it is determined that no competition will be performed, e.g. because the type identifier indicates that the second device is a stationary device, the rate of the repeatedly transmitted first message will be set to zero in step 544. The reason is that it is better if the stationary second device acts as a broadcasting device as it is not constrained by a limited battery life time. The first device being e.g. a pedestrian carried device will thus save its battery. If it is determined that a competition will be performed, the same steps 545 and 546 of competing with the second device for transmitting the first message, and setting the rate of the repeatedly transmitted first message to zero when the competition is lost as described above with reference to Figure 5c will be performed. In one embodiment, combinable with any of the above described embodiments, the transmitted first message and the received second message also comprise a speed of device, and/or a direction of movement of device, and the method further comprises comparing the respective speed of device and/or the respective direction of movement of device of the first and the second device. Furthermore, the determining 540 of whether to alter the rate of the repeatedly transmitted first message is based also on the comparison of the speed of device and/or the direction of movement of device. Devices moving in the same direction should likely be clustered while devices moving in opposite directions should not.
Figure 5e is a flowchart illustrating another embodiment of the method, combinable with any of the above described embodiments. The steps 510-540 are the same as the one described above with reference to Figure 5a. The method further comprises when the rate of the repeatedly transmitted first message has been altered: - 550: Receiving from the second device an updated second message comprising the position of the second device.
- 560: Determining an updated distance to the second device based on the received updated second message and the current position of the first device. The second device may have moved away from the first device or the first device may have moved away from the second device. In both cases, the distance between the two devices has augmented and should be compared with the threshold distance.
- 570: Increasing the rate of the repeatedly transmitted first message, when the determined updated distance is equal to or above the threshold distance. If the two devices are two far away from each other, they should both be transmitting at a higher rate again.
An embodiment of the first device 610 for a V2V communication system used in an ITS, and the second device 620, are schematically illustrated in the block diagram in Figure 6. The first device 610 is configured to control a rate of a repeated transmission of the first device. The first device comprises a transmitter 61 1 configured to transmit a first message comprising a current position of the first device, the first message being transmitted repeatedly with a defined rate. The first device 610 also comprises a receiver 612 configured to receive from the second device 620 a second message comprising a position of the second device. The first device 610 further comprises a processing circuit 613 configured to determine a distance to the second device based on the received second message and the current position of the first device. The processing circuit 613 is also configured to determine whether to alter the rate of the repeatedly transmitted first message based on a comparison of the determined distance and a threshold distance. The threshold distance may be predetermined. Alternatively the threshold may be received from the second device or from a node in the V2V communication system.
In embodiment B, the processing circuit 613 is configured to determine whether to alter the rate of the repeatedly transmitted first message by being configured to decrease the rate of the repeatedly transmitted first message if the determined distance is smaller than the threshold distance.
In embodiment A, the processing circuit 613 is configured to determine whether to alter the rate of the repeatedly transmitted first message if the determined distance is smaller than the threshold distance, by being configured to let the first device compete with the second device for transmitting the first message, and by being further configured to set the rate of the repeatedly transmitted first message to zero when the competition is lost. The competition is based on a comparison of metrics calculated for the first and the second device. As described above, the metrics may e.g. be candidate slot numbers, and the criteria for selecting the winning device is the highest candidate slot number.
In one embodiment, the processing circuit 613 is further configured to compete with the second device by being configured to calculate a candidate time slot position for the repeated transmission of the first message based on a currently used time slot position and a predefined function for determining the candidate time slot position. This is done for both the first and the second device. The processing circuit 613 is also configured to compare the respective calculated candidate time slot position of the first and the second device, and to determine the outcome of the competition based on the comparison of the respective calculated candidate time slot positions. The transmitter 61 1 may be further configured to transmit the calculated candidate time slot position of the second device when the competition is lost.
In a further embodiment, the receiver 612 is further configured to receive the second message also comprising a type identifier of the second device. The processing circuit 613 is configured to determine whether to alter the rate of the repeatedly transmitted first message by being configured to determine whether to perform the competition with the second device, based on the type identifier, and set the rate of the repeatedly transmitted first message to zero if it is determined not to perform the competition with the second device.
In any of the above described embodiments of the first device, the transmitter 61 1 and the receiver 612 are further configured to respectively transmit the first message and receive the second message also comprising a speed of device, and/or a direction of movement of device. The processing circuit 613 is further configured to compare the respective speed of device and/or the respective direction of movement of device of the first and the second device, and determine whether to alter the rate of the repeatedly transmitted first message based also on the comparison of the speed of device and/or the direction of movement of device.
In one embodiment, the receiver 612 is further configured to receive from the second device an updated second message comprising the position of the second device. The processing circuit 613 is further configured to determine an updated distance to the second device based on the received updated second message and the current position of the first device, and to increase the rate of the repeatedly transmitted first message, when the determined updated distance is equal to or above the threshold distance.
Correspondingly, the second device 620 also comprises a transmitter 621 , a receiver 622, and a processing circuit 623, configured according to the above described embodiments.
In an alternative way to describe the embodiments in Figure 6, the first device comprises a Central Processing Unit (CPU) which may be a single unit or a plurality of units. Furthermore, the first device comprises at least one computer program product (CPP) in the form of a non-volatile memory, e.g. an EEPROM (Electrically Erasable Programmable Read-Only Memory), a flash memory or a disk drive. The CPP comprises a computer program, which comprises code means which when run on the device causes the CPU to perform steps of the procedure described earlier in conjunction with Figures 5a-e. In other words, when said code means are run on the CPU, they correspond to the processing circuit 613 in the device 610 of Figure 6.
The above mentioned and described embodiments are only given as examples and should not be limiting. Other solutions, uses, objectives, and functions within the scope of the accompanying patent claims may be possible.

Claims

1 . A method for controlling a rate of a repeated transmission of a first device in a vehicular-to-vehicular communication system used in an intelligent transport system, the method being performed by the first device and comprising:
- transmitting (510) a first message comprising a current position of the first device, the first message being transmitted repeatedly with a defined rate,
- receiving (520) from a second device a second message comprising a position of the second device,
- determining (530) a distance to the second device based on the received second message and the current position of the first device, and
- determining (540) whether to alter the rate of the repeatedly transmitted first message based on a comparison of the determined distance and a threshold distance.
2. The method according to claim 1 , wherein determining (540) whether to alter the rate of the repeatedly transmitted first message comprises, if the determined distance is smaller than the threshold distance:
- decreasing (541 ) the rate of the repeatedly transmitted first message.
3. The method according to claim 1 , wherein determining (540) whether to alter the rate of the repeatedly transmitted first message comprises, if the determined distance is smaller than the threshold distance:
- competing (545) with the second device for transmitting the first message, wherein the competition is based on a comparison of metrics calculated for the first and the second device, and
- setting (546) the rate of the repeatedly transmitted first message to zero when the competition is lost.
4. The method according to claim 3, wherein competing (545) with the second device comprises:
- calculating a candidate time slot position for the repeated transmission of the first message based on a currently used time slot position and a predefined function for determining the candidate time slot position, for both the first and the second device,
- comparing the respective calculated candidate time slot position of the first and the second device, and
- determining the outcome of the competition based on the comparison of the respective calculated candidate time slot positions.
The method according to claim 3-4, wherein competing (545) with the second device further comprises:
- transmitting the calculated candidate time slot position of the second device when the competition is lost.
The method according to any of claims 3-5, wherein the received second message also comprises a type identifier of the second device, and wherein determining (540) whether to alter the rate of the repeatedly transmitted first message if the determined distance is smaller than the threshold distance comprises before the step of competing (545):
- determining (543) whether to perform the competition with the second device, based on the type identifier, and
- setting (544) the rate of the repeatedly transmitted first message to zero if it is determined not to perform the competition with the second device.
7. The method according to any of the preceding claims, wherein the transmitted first message and the received second message also comprise a speed of device, and/or a direction of movement of device, the method further comprising:
- comparing the respective speed of device and/or the respective direction of movement of device of the first and the second device,
and wherein the determining (540) of whether to alter the rate of the repeatedly transmitted first message is based also on the comparison of the speed of device and/or the direction of movement of device.
8. The method according to any of the preceding claims, further comprising when the rate of the repeatedly transmitted first message has been altered:
- receiving (550) from the second device an updated second message comprising the position of the second device,
5 - determining (560) an updated distance to the second device based on the received updated second message and the current position of the first device, and
- increasing (570) the rate of the repeatedly transmitted first message, when the determined updated distance is equal to or above the threshold
10 distance.
9. The method according to any of the preceding claims, wherein the threshold distance is predetermined, or is received from the second device or from a node in the vehicular-to-vehicular communication system.
15
10. A first device (610) for a vehicular-to-vehicular communication system used in an intelligent transport system, configured to control a rate of a repeated transmission of the first device, the first device comprising:
- a transmitter (61 1 ) configured to transmit a first message comprising a 20 current position of the first device, the first message being transmitted repeatedly with a defined rate,
- a receiver (612) configured to receive from a second device (620) a second message comprising a position of the second device,
- a processing circuit (613) configured to determine a distance to the second 25 device based on the received second message and the current position of the first device, and configured to determine whether to alter the rate of the repeatedly transmitted first message based on a comparison of the determined distance and a threshold distance.
30 1 1 . The first device (610) according to claim 10, wherein the processing circuit (613) is configured to determine whether to alter the rate of the repeatedly transmitted first message by being configured to decrease the rate of the repeatedly transmitted first message if the determined distance is smaller than the threshold distance.
12. The first device (610) according to claim 10, wherein the processing circuit (613) is configured to determine whether to alter the rate of the repeatedly transmitted first message if the determined distance is smaller than the threshold distance by being configured to let the first device compete with the second device for transmitting the first message, wherein the competition is based on a comparison of metrics calculated for the first and the second device, and by being further configured to set the rate of the repeatedly transmitted first message to zero when the competition is lost.
13. The first device (610) according to claim 12, wherein the processing circuit (613) is further configured to compete with the second device by being configured to:
- calculate a candidate time slot position for the repeated transmission of the first message based on a currently used time slot position and a predefined function for determining the candidate time slot position, for both the first and the second device,
- compare the respective calculated candidate time slot position of the first and the second device, and
- determine the outcome of the competition based on the comparison of the respective calculated candidate time slot positions.
14. The first device (610) according to claim 12-13, wherein the transmitter (61 1 ) is further configured to:
- transmit the calculated candidate time slot position of the second device when the competition is lost.
15. The first device (610) according to any of claims 12-14, wherein the receiver (612) is further configured to receive the second message also comprising a type identifier of the second device, and wherein the processing circuit (613) is configured to determine whether to alter the rate of the repeatedly transmitted first message by being configured to:
- determine whether to perform the competition with the second device, based on the type identifier, and
- set the rate of the repeatedly transmitted first message to zero if it is determined not to perform the competition with the second device.
16. The first device (610) according to any of claims 10-15, wherein the transmitter (61 1 ) and the receiver (612) are further configured to respectively transmit the first message and receive the second message also comprising a speed of device, and/or a direction of movement of device, and the processing circuit (613) is further configured to:
- compare the respective speed of device and/or the respective direction of movement of device of the first and the second device, and
- determine whether to alter the rate of the repeatedly transmitted first message based also on the comparison of the speed of device and/or the direction of movement of device.
17. The first device (610) according to any of claims 10-16, wherein the receiver (612) is further configured to receive from the second device an updated second message comprising the position of the second device, and wherein the processing circuit (613) is further configured to determine an updated distance to the second device based on the received updated second message and the current position of the first device, and to increase the rate of the repeatedly transmitted first message, when the determined updated distance is equal to or above the threshold distance.
18. The first device (610) according to any of claims 10-17, wherein the threshold distance is predetermined, or wherein the receiver (612) is configured to receive the threshold distance from the second device (620) or from a node in the vehicular-to-vehicular communication system.
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