WO2007091219A1

WO2007091219A1 - Network and method for repetitively broadcasting messages in a wireless ad-hoc network

Info

Publication number: WO2007091219A1
Application number: PCT/IB2007/050410
Authority: WO
Inventors: Hans-Jurgen Reumerman; Guido R. Hiertz; Yunpeng Zang; Lothar Stibor
Original assignee: Philips Intellectual Property & Standards Gmbh; Koninklijke Philips Electronics N.V.
Priority date: 2006-02-10
Filing date: 2007-02-08
Publication date: 2007-08-16
Also published as: TW200745998A

Abstract

The invention relates to a method for wireless communicating between a plurality of nodes (A, B, C) in an ad-hoc network, in which a single wireless channel is used for broadcasting messages. To provide a network and a method performing a distribution of high priority messages with high reliability independently from the density of nodes in an ad-hoc network the method comprises the steps of: detecting an event by a first node (A); evaluating the event, whether to be broadcasted to all other nodes (B, C) in the receiving range of the first node (A); broadcasting a message including information of the event; periodically repeating the message broadcasting; listen to the channel between transmissions of the message during a listen interval; amending the repetition interval for broadcasting the message if the same message is received within a predetermined period. Thereby a solution is presented in which the message dissemination is distributed resulting in an increased reliability and bandwidth efficiency. The danger warning message will be broadcasted reliable in case of low density of nodes in the area of relevance. In case of high density of nodes the common shared channel is used efficiently due to the distributed broadcasting of the danger warning message.

Description

DESCRIPTION

NETWORK AND METHOD FOR REPETITIVELY BROADCASTING MESSAGES IN A WIRELESS AD-HOC NETWORK

The present invention relates to a wireless ad-hoc network and in particular to car-to- car communication, also called inter- vehicle network, in which sensor-equipped cars 5 communicate cooperatively to e.g. avoid collisions or exchange information. However the invention is not limited to the application within the inter- vehicle network. It is also applicable in the area of sensor networks, wherein a plurality of sensors communicates wireless with each other.

10 An ad-hoc network used as communication system provides a communication platform on top of which many different applications may run at the same time, each of them providing support to the user in different situations. In the particular application of car- to-car communication it is used to assist the driver for e.g. intersection assistance, rear- end collision avoidance, lane merging assistance.

15

Every type of communication equipment used for communicating is however limited in the amount of bandwidth it can supply in general and in particular to the applications running on the communication equipment. Every application requires a certain amount of bandwidth in order to work correctly, wherein the amount of bandwidth depends on

20 the application and may be variable in time for each application.

In vehicular ad-hoc networks an important class of applications deal with safety related warnings based on inter-vehicle communication. These applications require vehicles to rebroadcast messages in order to warn drivers that are heading towards a zone of 25 relevance where a hazard has been detected. A scalable solution both has to cope with a situation where (a) low vehicle density is the reason for a highly fragmented network, and (b) a traffic jam like scenario would lead to excessive broadcast repeats and a resulting network congestion and overload.

30 The present invention is relevant for car-to-car communication, where sensor-equipped cars interact cooperatively to avoid collisions on the common channel during broadcasting the message. Some examples are given in Fig 1, where cars are depicted entering an intersection to a highway, wherein at low distance to the intersection an accident has occurred.

Likewise car-to-car communication is considered crucial to avoid collisions during lane change/merge maneuvers and for reporting of invisible obstacles, possibly talking to obscured or shadowed objects. In these applications, danger warning message dissemination for wireless vehicular communication is applied where broadcasting is used to make sure the message is received by every vehicle, which it is concerned.

Moreover, the danger warning message should be kept alive by repeatedly broadcasting within its lifetime and passed on toward the direction where the innocent vehicles are coming from. A common used single wireless channel is used for all nodes to broadcast and receive the message.

In emergency cases, successful delivery of high priority messages, in particular danger warning messages is very important. Therefore, messages are broadcasted to neighboring nodes. To cover large areas with the danger warning message, repeater nodes pick up the message and rebroadcast them. When a single channel is shared by all nodes for receiving and rebroadcast the message, the coordination of the message transmission is important to avoid collisions, which prevent successful message reception and waste the energy.

The US 2005/0088318 Al describes a short range wireless communication based on danger warning message dissemination. It describes that a repeat cycle for rebroadcasting a danger warning message is defined, wherein a number of initial repetitions for the message is specified. The message is transmitted repeatedly by a leader vehicle, with a pause between each transmission, until the maximum number of repetitions has been reached.

It therefore is an object of the present invention to provide a network and a method performing a distribution of high priority messages with high reliability independently from the density of nodes in an ad-hoc network.

The object is solved by the features of the independent claims.

The invention describes a method for message dissemination in scenarios where the reliability and bandwidth efficiency are both required. The key idea is to achieve a message repetition interval around a zone of relevance, where each node rebroadcasts a danger warning message at a repetition interval that depends on the available number of equipped nodes.

According to a first aspect of the invention there is provided a method for communicating between a plurality of nodes in an ad-hoc network in which a single wireless channel is used for broadcasting messages comprising the steps of: detecting an event by a first node; evaluating the event, whether to be broadcasted to all other nodes in the receiving range of the first node; broadcasting a message including information of the event; periodically repeating the message broadcasting; listen to the channel between the repeated broadcastings during a listen interval; and amending the repetition interval of broadcasting the message, if the same message is received within a predetermined period.

Thus, it is possible to send out a message including information of the event with a first repetition interval. If the broadcasting node receives the danger warning message within a predetermined period the broadcasting first node will increase the periodicity. That means, the message will be broadcasted with a longer period or a larger repetition interval.

Since there is a constantly changing of the amount of nodes in a ad-hoc network, in particular in the application of inter-vehicle networks, it is possible that an adjusted repetition interval needs to be decreased, means the message is broadcasted more often, if the number of nodes available for broadcasting the danger warning message decreases.

Thus, it is ensured that in case of low density of nodes the broadcasting node will broadcast the danger warning message with a predetermined repetition interval, which depends on the number of nodes in the zone of relevance. However, in case of a traffic jam it is ensured that the common used single wireless channel is not overcrowded due to a lot of collisions since the broadcasting node will increase its repetition interval if it detects a further node which may also broadcast the danger warning message.

The danger warning message includes information for informing all nodes in the area of the event or the area of danger that this message is a danger warning message that needs to be broadcasted to all nodes within the area of the event. The area of dissemination will be controlled by a higher layer application. It is possible to include a parameter in the danger warning message defining the dimension of dissemination, e.g. along a predetermined road or circular distance from the point of danger. Depending on this parameter the receiving node will decide whether to broadcast the danger warning message.

In a preferred embodiment the message is received in a second node which is rebroadcasting the received message after a rebroadcasting interval, wherein the rebroadcasting interval is shorter than the listen interval. Since the rebroadcasting interval is shorter than the listen interval the first node will receive the broadcast message before starting its next transmitting of the broadcast message.

By receiving the same message the first nodes detects the existence of a second node. Thus, the first node increases the repetition interval for broadcasting the message. Further, by receiving the message in the second node, the second node recognizes the existence of the first node. Thus, its repetition interval for broadcasting the message is adjusted according to the existence of two nodes. Since the first node increases its repetition interval and the second nodes sets at the same time the same repetition interval it is secured that both nodes broadcast the message with a periodicity or repetition interval which is greater than the repetition interval in case of a single node only. The terms periodicity or repetition interval are used for describing the same period. In particular the terms describe the distance or period between broadcasting the danger warning message from a certain node.

For informing the nodes in the zone of relevance, which nodes reside in the receiving range, each transmitting/receiving node inserts an information in the message characterizing the node before transmitting/rebroadcasting the message. Thus, all participating nodes are informed, which nodes have rebroadcasted the message.

After receiving the rebroadcasted message in the first node during the first listen interval the first node will wait for a further or subsequent listen interval. After this subsequent listen interval the danger warning message will be transmitted or broadcasted again by the first node in a next transmission interval. Thus, the repetition interval is increased. In particular, the period of transmitting the message is duplicated. In case of a first and a second node only in the receiving range of the first node the message is broadcasted by the first node in the first transmission interval, by the second node in the second transmission interval and in the third transmissions interval by the first node again and so on.

In case of more than two nodes altogether, which receive the broadcasted message of the first node, the second broadcasting of the message from the first node after the subsequent listen interval depends on the number of nodes within the transmission range.

In case that the broadcast message is received in a plurality of nodes each node rebroadcasts the message within the rebroadcasting interval, which follows the first transmission interval. In most cases this will result in a collision on the channel. Thus, no node will receive the broadcast message. The amount of collisions may be reduced by using as so called "listen before talk" functionality. That means each node must check whether the channel is free for transmitting its message. If the channel is busy the requesting node will back off and will try to access the channel after its back off time.

However, the nodes will detect that there a collision has occurred on the channel.

After detecting the collision on the channel the first, the second and third nodes will wait a further listen interval. If they do not receive a danger warning message they will randomly decide which one of the second or third node will broadcast the message.

Thus, the transmitting of the danger warning message will be distributed to the nodes within the receiving range of the first node.

The invention is further solved by a network having a plurality of nodes communicating wireless with each other, wherein a node includes a transmitting and a receiving unit, means for detecting an event, means for evaluating the event, wherein after evaluating an event a message is broadcasted by the transmitting unit and after broadcasting the message the communication channel is listen whether the same message is received within a listen interval, wherein, when the message is received by the broadcasting node again, the repetition interval for broadcasting the message from the node is increased.

The object is further solved by a node within a network.

The invention is described in detail below with reference to the accompanying schematic drawings, wherein

Fig. 1 shows a traffic situation having a plurality of nodes;

Fig. 2 shows a flowchart according to the present invention;

Fig. 3 shows a flowchart illustrating the operation of the present invention in case of two nodes; Fig. 4 shows a flowchart illustrating the operation of the invention in case of three nodes; Fig. 5 shows a schematic illustrating the transmission of the danger warning messages as shown for the flowchart of Figs. 2 and 4.

The drawings are provided for illustrative purpose only and do not necessarily represent practical examples of the present invention to scale.

In the following various examplary embodiments of the invention are described.

However, the present invention is applicable in a broad variety of applications which will be described with a focus put on car-to-car communication in an inter- vehicle network. A further field for applying the invention might be a sensor network having nodes communicating asynchronous, wherein they are not controlled by an access point.

Fig. 1 illustrates a couple of cars A-C, wherein the car A detects an event in front of the car A. It broadcasts a danger warning message to all nodes B, C, etc within its receiving range. As could be easily seen there is a plurality of cars on the highway, thus, it may be possible that the common used single wireless channel may be easily overcrowded due to the collisions during broadcasting a danger warning message.

According to the invention the repetition interval for broadcasting of the danger warning message will be adjusted in dependency on the number of the nodes or cars within the receiving range of the first node. In fig. 1 the car B on the outer lane will broadcast the message also to the car C which is approaching the highway in the near future.

Reference is made to Fig. 2 showing a flowchart according to the present invention. In step 20 the node A detects an event. The event may be an accident or a traffic jam etc. A further possibility is that the crashed car will send out a danger warning by itself.

A further example for an event for allotting a danger warning message may be ice on the road. In this case node A receives e.g. a message from a traffic control station informing about ice on the road. After receiving this message node A evaluates this message as an event which needs to be broadcasted to all other nodes within the receiving range of node A in step 21. In step 22, node A broadcasts a danger warning message using a predetermined repetition interval. This predetermined repetition interval is adjusted to the number of one node within the zone of relevance at this moment. In case that no further nodes are within the receiving range node A will broadcast the danger warning message by using the unchanged predetermined repetition interval.

In case of entering the receiving range of node A an entering node or car will be informed about the event by receiving the danger warning message. The entering node will itself rebroadcast the danger warning message. In this case node A will receive the same danger warning message from the other node in step 23. If node A detects the existence of a further node it increases its predetermined repetition interval in step 24. Due to the increased repetition interval, node A will broadcast the danger warning message using the increased repetition interval depending on the number of nodes in the transmission interval, step 25.

Thus, it is secured that the common used single wireless channel is not overcrowded with a plurality of broadcast attempts, which results in collisions. Thereby, it is achieved that the danger warning message due to the event will be distributed in an assured way.

The procedure of the present invention will be explained in more detail with respect to

Fig. 3. In Fig. 3 there are two columns. The left column represents the actions taken by node A, wherein the right column represents the actions of node B. It is implied that node A has detected an event and has evaluated this event to be broadcasted as a danger warning message.

The flowchart starts with step 31 on the left side in node A which transmits or broadcasts the danger warning message. If node B is in the receiving range of node A it receives the danger warning message in step 32. After broadcasting the danger warning message from node A, node A will wait the listen interval in step 33. The receiving node B will check whether the received danger warning message includes a parameter indicating the zone of relevance. Depending on the zone of relevance node B will decide whether to broadcast the danger warning message. In the illustrated flowchart it is assumed that node B decides to broadcast the danger warning message. Since the predetermined rebroadcasting interval is shorter than the listen interval node B broadcasts in step 34 the danger warning message. After having broadcasted node B will increase its repetition interval in step 34a. Node A will receive the danger warning message in step 35. Since node A receives the danger warning message it recognizes the existence of two nodes, thus it will increase its repetition interval for broadcasting the danger warning message in step 37 since there are two nodes only which are capable of broadcasting the danger warning message in the area of the event. Coming back to the right column in step 36, node B will wait the listen interval after broadcasting the danger warning message. Since node A has increased its repetition interval in step 37 it will broadcast the danger warning message in step 38 with the increased repetition interval. After receiving the danger warning message in step 39, node B will transmit the danger warning message again by use of the increased repetition interval.

Reference is made at this point to Fig. 5 which shows in the second row the situation which is explained in Fig. 3. There it could be recognized that after broadcasting the danger warning message by node A, node B will send out its danger warning message during the listen interval. Thus, after a repetition interval for two nodes, node A and B will alternate in sending out the danger warning message. On the right side of Fig. 5 the repetition frequency is illustrated for sending out the danger warning message by node A. As could be easily seen in the first row the repetition frequency is very high since there is only one node in the area of the event. After detecting the second node B the repetition frequency of node A is decreased, which is the same as increasing the repetition interval.

Now the situation will be explained in respect to Fig. 4, which is illustrating the situation in case of three nodes A, B, C. In step 31, node A broadcasts a danger warning message after detecting the event and evaluating the event to be broadcasted by use of a danger warning message to all other nodes within the area of the event. In steps 32B and 32C node B and node C will receive the danger warning message. Node A will wait in its listen interval, step 33. After receiving the danger warning message in nodes B, C, both nodes will broadcast the danger warning message in step 34B, 34C. This will result in a collision on the common shared channel, step 43. Node A will recognize this collision and will follow from this collision that there are further nodes in its receiving range and will increase the repetition interval in step 44. Since neither Node B nor Node C will receive a danger warning message from the other nodes both nodes B, C will wait the listen interval in steps 45 and 46. If they receive no danger warning message in steps 47, 48 a random decision is made in nodes B and C which one of them will broadcast the danger warning message. Fig. 4 illustrates the case in which node B will transmit the danger warning message in step 51. This danger warning message will be received in nodes A and B in steps 52, 53. After broadcasting the danger warning message node B will wait the listen interval in step 54. Node C will increase the repetition interval in step 55 after receiving the danger warning message. Then node C will transmit the danger warning message in the next transmission interval in step 58. This message is received in nodes A and C in steps 56, 57. After receiving the danger warning message in node B, node B will also increase the repetition interval. This will result in an order of broadcasting the danger warning message as shown in the fourth row of Fig. 5. Thus, the three nodes A, B, C will alternate in broadcasting the danger warning message. Compared with a second row of fig. 5 in case of two nodes only the repetition interval of node A is increased. The same repetition interval is adjusted for the nodes B and C. As could be recognized the repetition frequency for the nodes is decreased when compared with the second or first row of Fig. 5.

The present invention provides a solution in which the message dissemination is distributed resulting in an increased reliability and bandwidth efficiency. Thus a danger warning message will be broadcasted reliable in case of low density of nodes in the area of relevance. In case of high density of nodes the common shared channel is used efficiently due to the distributed broadcasting of the danger warning message.

Claims

1. Method for wireless communicating between a plurality of nodes (A, B, C) in an ad-hoc network, in which a single wireless channel is used for broadcasting messages comprising the steps of: detecting (20) an event by a first node (A); - evaluating (21) the event, whether to be broadcasted to all other nodes (B, C) in the receiving range of the first node (A); broadcasting (22) a message including information of the event; periodically repeating the message broadcasting; listen to the channel between transmissions of the message during a listen interval; amending (24) the repetition interval for broadcasting the message if the same message is received (23) within a predetermined period.

2. Method according to claim 1, wherein the repetition interval is amended in dependency on the number of nodes (A, B, C) detected in the receiving range of the first node (A).

3. Method according to claim 1 or 2, wherein if the number of nodes in the receiving range of the first node (A) increases, the repetition interval for broadcasting the message is increased.

4. Method according to one of the claims 1 to 3, wherein if the number of nodes in the receiving range decreases, the repetition interval for broadcasting the message is decreased.

5. Method according to one of the claims 1 to 4, comprising further: receiving the broadcast message in a second node (B, C); rebroadcasting the message by the second node (B, C) after a rebroadcasting interval, wherein the rebroadcasting interval is shorter than the listen interval.

6. Method according to one of the claims 1 to 5, comprising further: inserting an information into the message at each receiving node (B, C) indicating the receiving node (B, C) before rebroadcasting the message.

7. Method according to one of the claims 1 to 6, further comprising: receiving the rebroadcasted message in the first node (A) during the listen interval; wait for a subsequent listen interval; broadcasting the message after the subsequent listen interval.

8. Method according to claim 7, wherein the broadcasting of the message after the subsequent listen interval dependents on the number of nodes (B, C) within the transmission range of the first node (A).

9. Method according to one of the claims 1 to 8, further comprising: receiving the message of the first node (A) in aplurality of nodes (B, C), - rebroadcasting the message by each node (B, C), wherein in case of collisions on the channel, detecting empty transmission intervals by the broadcasting nodes (B,

C); broadcasting the message in an empty transmission interval.

10. Method according to one of the claims 1 to 9, wherein the event represents a danger for the nodes (A, B, C) in the transmission range of the node (A) detecting the event.

11. Network having a plurality of nodes (A, B, C) communicating wireless with each other, a node (A, B, C) includes a transmitting and a receiving unit, means for detecting an event; means for evaluating the event, wherein after evaluating an event a message is broadcasted by the transmitting unit in dependency on the evaluating and after broadcasting the message the communication channel is listen whether the same message is received within a listen interval, wherein when the broadcasted message is received by the broadcasting node again, the repetition interval of broadcasting the message from the node is increased.

12. Node within an ad-hoc network, having a transmitting and a receiving unit, means for detecting an event; means for evaluating the event, wherein after evaluating an event a message is broadcasted by the transmitting unit in dependency on the evaluating and after broadcasting the message the communication channel is listen whether the same message is received within a listen interval, wherein when the broadcasted message is received by the broadcasting node again, the repetition interval of broadcasting the message from the node is increased.