CN108449720B - City VANET multi-hop broadcasting method based on competition and finite state machine - Google Patents

City VANET multi-hop broadcasting method based on competition and finite state machine Download PDF

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CN108449720B
CN108449720B CN201810202051.0A CN201810202051A CN108449720B CN 108449720 B CN108449720 B CN 108449720B CN 201810202051 A CN201810202051 A CN 201810202051A CN 108449720 B CN108449720 B CN 108449720B
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CN108449720A (en
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黄传河
李思政
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Wuhan University WHU
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04W—WIRELESS COMMUNICATION NETWORKS
    • H04W4/00—Services specially adapted for wireless communication networks; Facilities therefor
    • H04W4/06—Selective distribution of broadcast services, e.g. multimedia broadcast multicast service [MBMS]; Services to user groups; One-way selective calling services
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L1/00—Arrangements for detecting or preventing errors in the information received
    • H04L1/12—Arrangements for detecting or preventing errors in the information received by using return channel
    • H04L1/16—Arrangements for detecting or preventing errors in the information received by using return channel in which the return channel carries supervisory signals, e.g. repetition request signals
    • H04L1/1607—Details of the supervisory signal
    • H04L1/1671—Details of the supervisory signal the supervisory signal being transmitted together with control information
    • H04L1/1678—Details of the supervisory signal the supervisory signal being transmitted together with control information where the control information is for timing, e.g. time stamps
    • 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
    • H04W—WIRELESS COMMUNICATION NETWORKS
    • H04W4/00—Services specially adapted for wireless communication networks; Facilities therefor
    • H04W4/90—Services for handling of emergency or hazardous situations, e.g. earthquake and tsunami warning systems [ETWS]
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04W—WIRELESS COMMUNICATION NETWORKS
    • H04W40/00—Communication routing or communication path finding
    • H04W40/02—Communication route or path selection, e.g. power-based or shortest path routing
    • H04W40/12—Communication route or path selection, e.g. power-based or shortest path routing based on transmission quality or channel quality
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04W—WIRELESS COMMUNICATION NETWORKS
    • H04W40/00—Communication routing or communication path finding
    • H04W40/02—Communication route or path selection, e.g. power-based or shortest path routing
    • H04W40/20—Communication route or path selection, e.g. power-based or shortest path routing based on geographic position or location

Abstract

The invention discloses a competition and finite state machine based urban VANET multi-hop broadcasting method, wherein the state of each node comprises the following steps: n is a message which is not received; p is a possible received message; and R is the confirmed received message. A source node which learns the emergency constructs an emergency message with a sending node list and broadcasts the emergency message, and the state of the source node is set as R; if the state of the neighbor node is N, the state is changed to P; the node receiving the emergency message finds the message sending node and the nodes in the sending node list contained in the message in the local neighbor table, and if the nodes exist, the states of the nodes are set to be R. The receivers then calculate their own latencies based on the relative distances to the last hop node, the quality of the communication, etc. The node whose waiting time is first ended updates the urgent message and broadcasts it, and the new message can be regarded as an implicit ACK. If there is a neighbor with a state other than R, the sender will retransmit overtime. The invention is suitable for various network topologies, and ensures the real-time reliable transmission of the emergency message in the whole network.

Description

City VANET multi-hop broadcasting method based on competition and finite state machine
Technical Field
The invention belongs to the technical field of vehicle-mounted ad hoc network communication, and particularly relates to a multi-hop broadcasting method based on competition and a finite-state machine for realizing real-time reliable transmission of emergency messages in a city VANET.
Background
As an indispensable part of an intelligent traffic system, vanet (vehicular Ad Hoc network) is a special mobile Ad Hoc network, and has the distinct characteristics of flexible networking, capability of tracking the movement of communication nodes, rapid network topology change, more unstable wireless communication channel quality and the like. Applications of an on-board ad hoc network generally include driving safety applications and entertainment information service type applications. The application of driving safety is to quickly and reliably transmit an emergency message generated by an emergency accident occurring road section to a part of or all vehicles in a certain area, thereby ensuring the driving safety of the vehicles in the area and avoiding more serious traffic accidents. The method proposed by the present invention serves mainly for this type of applications.
Unlike highways, urban traffic environments are more complex not only because of the various road topologies such as intersections, overpasses, etc., but also tall buildings that can produce severe multipath effects and shadowing effects on radio wave propagation. Under such an intricate and complex multidimensional scene, reliably transmitting the urgent message to more vehicles in the VANET in real time is a research point of great interest. Broadcasting is one of the most efficient methods for transmitting urgent messages in VANET.
Most of the existing VANET multi-hop broadcasting methods aim at end-to-end communication, namely a source node and a destination are known, messages are transmitted from the source node to the destination through multi-hop broadcasting, the message coverage effect is good generally on a straight road, and the transmission of the messages on one road section can be ensured at an intersection while other road sections are ignored. However, in an actual driving scene, not only the vehicle in the accident road section needs to obtain the accident scene through receiving the emergency message broadcast by the source node, so as to avoid secondary accidents; and vehicles having an intention to enter but not yet entering the accident section should also receive the message to detour in order to avoid traffic congestion. In selecting the relay node, most of the existing methods consider the relative distance between the nodes, but the farther the relay node is, the better the relay node is, the faster the relay node can transmit the message to the far side. In practice, however, in the case of low traffic density, a neighbor closer to the last sending node should be selected as the relay node to ensure reliable transmission of the message. Some broadcasting methods rely too much on roadside communication units (RSUs), resulting in a limited practical application scenario. Finally, most of the existing methods lack an effective message confirmation and retransmission mechanism, and cannot meet the requirements of real-time performance and reliability of message transmission.
Disclosure of Invention
Aiming at the defects of the prior art, the invention provides a competition and finite state machine-based urban VANET multi-hop broadcasting method so as to realize the real-time reliable transmission of emergency messages triggered by emergency accidents in the urban VANET.
The technical scheme adopted by the invention is as follows: a city VANET multi-hop broadcasting method based on competition and finite state machines is characterized in that: all nodes of the VANET have three states: the message which is not received is marked as N; possibly received messages, denoted as P; confirming that the message is received and recording as R; the initial state of each node is N; the switching of node states is triggered by different events and will be explained in detail in the main steps;
the method comprises the following steps:
step 1: each node collects self-motion and surrounding environment information by various sensors, and encapsulates the information into beacon messages to be periodically broadcast to surrounding nodes; any two nodes which receive the beacon message mutually are neighbor nodes; each node ViLocally maintaining a neighbor table N (i), wherein the content of each table entry comprises the ID of a neighbor node and the state value of the neighbor node; elapsed time Tout1Rear, node ViStill does not receive a neighbor node VjIn N (i), node V is removedjThe table entry of (2);
step 2, firstly, a vehicle which learns a certain traffic accident is used as a source node to broadcast an emergency message in a self communication range, wherein the emergency message comprises an emergency message ID, accident information, TT L and a transmitting node list, TT L refers to the maximum hop count which can be passed by the emergency message, an initial value is set by the source node, and TT L is reduced by 1 every time the emergency message is forwarded, a sender ID of the emergency message which is received by the current transmitting node is contained in the transmitting node list, and the transmitting node list in the message broadcasted by the source node is an empty list;
and step 3: the node receiving the urgent message firstly finds the list item of the sender of the message in the neighbor list of the node, and sets the state of the sender of the message as R; meanwhile, whether the IDs of other neighbor nodes are in a sending node list attached to the emergency message or not is checked, and if the IDs of other neighbor nodes are in the sending node list attached to the emergency message, the state of the neighbor node is set to be R in a neighbor list; then, the message is receivedThe receiver calculates the maximum competition window value CW of the receiver according to the relative distance between the receiver and the message sender, the obtained road traffic priority, the density of surrounding nodes, the communication channel quality and other factorsi,maxAnd in [0, CW ]i,max]By randomly selecting a value CWi,selectSetting the self waiting time and entering the waiting process;
step 4, the message receiver with the first ending of waiting time becomes a forwarding node, updates the ID, TT L and the sending node list of the emergency message, and other receivers in the waiting process can terminate the self waiting process and abandon the qualification of broadcasting the emergency message in a new round after receiving the emergency message containing the same accident information for the second time;
step 5, the forwarding node judges whether TT L of the emergency message is less than 1;
when TT L of the urgent message is not less than 1, the forwarding node broadcasts the message to the surrounding nodes, sets the self state as R after broadcasting, sets the neighbor node state with the node state as N in the neighbor table as P, then executes step 6;
when the TT L of the emergency message is less than 1, the forwarding node only needs to piggyback an implicit ACK to the sending node of the previous hop in the beacon message so as to enable the forwarding node to set the state value of the forwarding node to be R in the neighbor table of the forwarding node in time;
and 6, setting an overtime retransmission timer after the source node and the subsequent forwarding nodes broadcast the message, checking a neighbor table of the source node and the subsequent forwarding nodes when the timer expires, and if the states of all the nodes in the neighbor table are R, not executing the overtime retransmission, or continuously updating the ID, TT L and the sending node list of the emergency message and broadcasting the new emergency message.
The multi-hop broadcasting method provided by the invention is completely distributed, does not depend on facilities such as RSU (remote subscriber Unit), considers factors such as vehicle movement and wireless communication quality when selecting the forwarding nodes, is not easily influenced by frequent changes of network topology due to the matching of a perfect message confirmation and retransmission mechanism, and can powerfully ensure the real-time reliable transmission of the emergency message in the urban VANET.
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FIG. 1 is a sender and receiver workflow diagram of an embodiment of the invention;
fig. 2 is a schematic diagram of a finite state machine according to an embodiment of the present invention.
Detailed Description
In order to facilitate the understanding and implementation of the present invention for those of ordinary skill in the art, the present invention is further described in detail with reference to the accompanying drawings and examples, it is to be understood that the embodiments described herein are merely illustrative and explanatory of the present invention and are not restrictive thereof.
A node in the urban VANET is a vehicle; according to whether the broadcasted emergency message is received, the nodes in the urban VANET have three different states, which are: state N indicates that the message has not been received; state P indicates that the message may have been received; state R indicates that the message was acknowledged as received. The initial states of the nodes are all N, and the states of the nodes per se and the neighbor nodes are changed according to specific events mentioned in the main steps of the invention. The source node broadcasting the urgent message and the subsequent forwarding nodes may be referred to as a sender, and the node receiving the urgent message may be referred to as a receiver.
As shown in fig. 1, the urban VANET multi-hop broadcasting method based on contention and finite state machines provided by the present invention includes the following steps:
step 1: each node collects information such as self position, speed and the like at regular time by using a sensor such as a GPS and the like, and encapsulates the information into a beacon message to be periodically broadcast to surrounding nodes. Any two nodes that receive the beacon message from each other are neighbors of each other. Each node being e.g. ViA neighbor table N (i) of the node is locally maintained, and the content of each table entry comprises the ID of a neighbor node and the state value of the neighbor node; elapsed time Tout1Rear, node ViStill does not receive a neighbor node VjIn N (i), node V is removedjThe table entry of (2);
step 2, firstly, a vehicle which learns a certain traffic accident is used as a source node to broadcast an emergency message in a self communication range, wherein the emergency message comprises a message ID, accident information, TT L and a sending node list, the emergency message is uniquely identified by the message ID, the accident information contained in the message refers to the time, the place, the reason and the like of the accident, the TT L contained in the message refers to the maximum hop number which can be passed by the message, the source node sets an initial value, and 1 is subtracted from the TT L every 1 time of forwarding, the sending node list contained in the message is an empty list in the emergency message which is obviously broadcasted by the source node and is received by the current sending node by a sender ID. of the emergency message, the source node sets the self state to R after broadcasting, and the neighbor node with the state of N in the neighbor list is set to P;
and step 3: the node receiving the urgent message finds the table item of the sender of the message in the neighbor table of the node, and sets the state of the node as R; and meanwhile, checking whether the IDs of other neighbors are in a sending node list attached to the emergency message or not, and if so, setting the state of the neighbor to be R in a neighbor list. Then, the message receiver will calculate its maximum contention window value CW according to the relative distance between itself and the message sender, the obtained road traffic priority, the density of the surrounding nodes, the quality of the communication channel, and other factorsi,maxAnd in [0, CW ]i,max]By randomly selecting a value CWi,selectSetting the waiting time of the device and entering the waiting process. In particular, the relative distance between the message receiver and the message sender, the density of surrounding nodes, and the like can be obtained directly or indirectly through the content contained in the mutual beacon messages;
maximum contention window value CWi,maxThe calculation process of (2) is as follows:
suppose that the current transmitting node is VsPosition coordinates of (x)s,ys) Node ViAs one of the recipients, the location coordinate is (x)i,yi) Then their relative distance DiThe calculation formula is as follows:
Figure BDA0001594775560000041
when driving on a non-intersection road section, the node ViRoad traffic priority
Figure BDA0001594775560000051
As with the other nodes, 1 may be assumed. At the crossing, node V is limited by traffic lights or other converged trafficiRoad traffic priority
Figure BDA0001594775560000052
Possibly different from other nodes. According to the order of green light release,
Figure BDA0001594775560000053
1, 2, 3 … … in sequence, namely ViCan pass through the intersection first, then
Figure BDA0001594775560000054
Is 1.
Node VsTransmission power P when broadcasting messagestMaximum transmission distance of DmaxThen node ViSNR ofiThe calculation formula is as follows:
Figure BDA0001594775560000055
where α is a signal attenuation exponent, N0Refers to node ViThe power of the surrounding noise, both constant α is typically 3 or 4, and N can be adjusted according to the actual scene0The value of (c).
ρ represents a node VsThe density of surrounding nodes, ρ ∈ [0,1 ]](ii) a Then node ViMaximum contention window value CWi,maxThe calculation formula is as follows:
Figure BDA0001594775560000056
wherein k and β are scale factors, and the value is adjusted according to actual scene to ensure CWi,maxIs a reasonable positive integer value, CWbaseThe value is a constant, and the value is 2 when the general traffic is normal, and the value is an integer not less than 3 when the traffic is congested. SNRthreshRepresentsThe SNR at least reaches a value when the message is reliably transmitted, which is a positive integer that can be adjusted according to the actual scene.
Step 4, the message receiver with the first ending of waiting time becomes a forwarding node, updates the ID, TT L and the sending node list of the emergency message, and other receivers in the waiting process can terminate the self waiting process and abandon the qualification of broadcasting the emergency message in a new round after receiving the emergency message containing the same accident information for the second time;
step 5, the forwarding node judges whether TT L of the emergency message is less than 1;
when TT L of the urgent message is not less than 1, the forwarding node broadcasts the message to the surrounding nodes, sets the self state as R after broadcasting, sets the neighbor node state with the node state as N in the neighbor table as P, then executes step 6;
when the TT L of the emergency message is less than 1, the forwarding node only needs to piggyback an implicit ACK to the sending node of the previous hop in the beacon message so as to enable the forwarding node to set the state of the forwarding node to be R in the neighbor table of the forwarding node in time;
it should be noted that the emergency message newly broadcast by the forwarding node can be regarded as an implicit ACK signal, and if the previous-hop sending node receives the message, the state of the forwarding node should be set as R in its own neighbor table in time;
step 6, the source node and the subsequent forwarding nodes set an overtime retransmission timer after the information is broadcasted, the self neighbor list is checked when the timer expires, if the states of all the nodes in the list are R, the overtime retransmission is not executed, otherwise, the ID, TT L and the sending node list of the emergency information are continuously updated and the new emergency information is broadcasted, in order to reduce unnecessary retransmission, the invention introduces an indirect confirmation mechanism, namely the current sending node ViOf neighbor node VkIf it can confirm that the node V just leavesiNode V of the communication rangejHaving received the message, node VkNode V may be informed by beacon message piggybacking an implicit ACKiLet it terminate due to node VjRetransmission timeout due to unchanged state. Furthermore, if the node ViAlways can notAcknowledging node VjSuch retransmission can also be terminated directly if the message was received and therefore timed out for more than 3 retransmissions.
The multi-hop broadcast of the emergency message of the embodiment is actually repeating the steps 3 to 6 until any one of the conditions is satisfied, wherein ① the emergency message contains TT L smaller than 1, ② cannot find the forwarding node, and ③ a specified proportion of or all nodes in the area receive the emergency message.
The present invention defines a finite state machine mechanism for ensuring reliable transmission of urgent messages. The mechanism comprises the following steps: initializing the states of all nodes of the VANET to be N before the step 1 is executed; step 2 and step 5, the source node or the forwarding node sets the self state as R after broadcasting the emergency message, and sets the neighbor node with the original state of N as R in the self neighbor table; after the previous-hop sending node receives the beacon message carrying the implicit ACK of the forwarding node, the state of the forwarding node is set to be R in a neighbor table of the previous-hop sending node; in step 3, the emergency message receiver sets the state of the message sending node as R in the neighbor table of the emergency message receiver, sets the state of the node contained in the message sending node list as R, and the like.
Suppose node ViFor the current transmitting node, node VjIs node ViA certain neighbor node of VkAre their common neighbor node, node VjThe finite state machine of (2) is shown in fig. 2;
the initial states of all nodes in the vehicle-mounted ad hoc network are N, namely the emergency message is not received. Whether node ViOr node VjIf the urgent message is broadcasted, the state of the user can be changed from N to R, which indicates that the user has received the urgent message, i.e. Event 1 in fig. 2.
Node ViAfter broadcasting one round, the node V is sent to the neighbor tablejIs changed from N to P, i.e. node VjPossibly receiving the urgent message but not to the node ViFeeding back an ACK signal; this is Event 2 in FIG. 2.
Thereafter, if the node VjBecome a forwarding node, node V, in a competitive manneriAt the backTo receive node VjThe broadcast emergency message, then the node VjChanges state from P to R. This is a direct implicit ACK mechanism, also Event 3 in fig. 2. If node VjBecomes a forwarding node, but cannot broadcast the urgent message directly because TT L of the urgent message is less than 1, node VjWill piggyback a direct implicit ACK to the node V in the beacon messagei. Node ViAfter receiving, the node V is also transmittedjChanges state from P to R, which is Event 4 in fig. 2. If node VjBecomes a forwarding node, but the emergency message broadcast by it cannot be received by node ViReceived, and node ViAnd node VjOf a common neighbor node VkUpon receipt of this message, node VkWill inform node V in the beacon messagei. Node ViAfter receiving, the node V is also transmittedjChanges state from P to R. This is an indirect implicit ACK mechanism, also Event 5 in fig. 2. If node ViLater, the emergency message broadcasted from other nodes is received, and the node V is found in the sending node list in the messagejNode V will also be connectedjThe state of (2) is changed to R, which is Event 6 in fig. 2. After all broadcasts are terminated, the state of the node is restored to N, i.e., Event 7 in fig. 2.
The finite state machine mechanism is matched with the direct and indirect implicit confirmation mechanism and the overtime retransmission mechanism mentioned in the steps of the invention, thereby effectively ensuring the real-time reliable transmission of the emergency message triggered by the emergency in the urban VANET.
It should be understood that parts of the specification not set forth in detail are well within the prior art.
It should be understood that the above description of the preferred embodiments is given for clarity and not for any purpose of limitation, and that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims.

Claims (4)

1. A city VANET multi-hop broadcasting method based on competition and finite state machines is characterized in that: all nodes of the VANET have three states depending on whether or not an emergency message is received: the message which is not received is marked as N; possibly received messages, denoted as P; confirming that the message is received and recording as R; the switching of the node states is triggered by different events; the initial state of each node is N;
the method comprises the following steps:
step 1: each node collects self-motion and surrounding environment information by various sensors, and encapsulates the information into beacon messages to be periodically broadcast to surrounding nodes; any two nodes which receive the beacon message mutually are neighbor nodes; each node ViLocally maintaining a neighbor table N (i), wherein the content of each table entry comprises the ID of a neighbor node and the state value of the neighbor node; elapsed time Tout1Rear, node ViStill does not receive a neighbor node VjIn N (i), node V is removedjThe table entry of (2);
step 2, firstly, a vehicle which learns a certain traffic accident is used as a source node to broadcast an emergency message in a self communication range, wherein the emergency message comprises an emergency message ID, accident information, TT L and a transmitting node list, TT L refers to the maximum hop count which can be passed by the emergency message, an initial value is set by the source node, and TT L is reduced by 1 every time the emergency message is forwarded, a sender ID of the emergency message which is received by the current transmitting node is contained in the transmitting node list, and the transmitting node list in the message broadcasted by the source node is an empty list;
and step 3: the node receiving the urgent message firstly finds the list item of the sender of the message in the neighbor list of the node, and sets the state of the sender of the message as R; meanwhile, whether the IDs of other neighbor nodes are in a sending node list attached to the emergency message or not is checked, and if the IDs of other neighbor nodes are in the sending node list attached to the emergency message, the state of the neighbor node is set to be R in a neighbor list; the message recipient then calculates fromMaximum contention window value CWi,maxAnd in [0, CW ]i,max]By randomly selecting a value CWi,selectSetting the self waiting time and entering the waiting process;
step 4, the message receiver with the first ending of waiting time becomes a forwarding node, updates the ID, TT L and the sending node list of the emergency message, and other receivers in the waiting process can terminate the self waiting process and abandon the qualification of broadcasting the emergency message in a new round after receiving the emergency message containing the same accident information for the second time;
step 5, the forwarding node judges whether TT L of the emergency message is less than 1;
when TT L of the urgent message is not less than 1, the forwarding node broadcasts the message to the surrounding nodes, sets the self state as R after broadcasting, sets the neighbor node state with the node state as N in the neighbor table as P, then executes step 6;
when the TT L of the emergency message is less than 1, the forwarding node only needs to piggyback an implicit ACK to the sending node of the previous hop in the beacon message so as to enable the forwarding node to set the state value of the forwarding node to be R in the neighbor table of the forwarding node in time;
and 6, setting an overtime retransmission timer after the source node and the subsequent forwarding nodes broadcast the message, checking a neighbor table of the source node and the subsequent forwarding nodes when the timer expires, and if the states of all the nodes in the neighbor table are R, not executing the overtime retransmission, or continuously updating the ID, TT L and the sending node list of the emergency message and broadcasting the new emergency message.
2. The contention and finite state machine based urban VANET multi-hop broadcasting method according to claim 1, characterized in that the own maximum contention window value CW in step 3i,maxThe calculation process is as follows:
suppose that the current transmitting node is VsPosition coordinates of (x)s,ys) Node ViAs one of the recipients, the location coordinate is (x)i,yi) Then their relative distance DiThe calculation formula is as follows:
Figure FDA0001594775550000021
when driving on a non-intersection road section, the node ViRoad traffic priority
Figure FDA0001594775550000022
As with the other nodes, 1 can be assumed; at the intersection, node ViRoad traffic priority
Figure FDA0001594775550000023
According to the order of green light release,
Figure FDA0001594775550000024
1, 2 and 3 … … in sequence,
Figure FDA0001594775550000025
1 denotes a node ViCan firstly pass through the intersection;
node VsTransmission power P when broadcasting messagestMaximum transmission distance of DmaxThen node ViSNR ofiThe calculation formula is as follows:
Figure FDA0001594775550000026
where α is a signal attenuation exponent, N0Refers to node ViAmbient noise power, both constant;
ρ represents a node VsThe density of surrounding nodes, ρ ∈ [0,1 ]](ii) a Then node ViMaximum contention window value CWi,maxThe calculation formula is as follows:
Figure FDA0001594775550000027
wherein k and β are scale factors, and the value is adjusted according to actual scene to ensure CWi,maxIs a reasonable positive integer value; CWbaseThe traffic congestion detection method is a constant, wherein 2 is taken when traffic is normal, and an integer value not less than 3 is taken when traffic is congested; SNRthreshThe value representing at least the SNR achieved when the message is reliably transmitted is a positive integer adjusted according to the actual scene.
3. The contention and finite state machine based urban VANET multi-hop broadcasting method according to claim 1, characterized in that: step 5, the newly broadcast emergency message of the forwarding node is regarded as an implicit ACK signal, and if the previous hop sending node receives the message, the state of the forwarding node is set as R in a neighbor table of the previous hop sending node in time; if the previous hop sending node finds the node in the sending node list contained in the message in the neighbor table of the previous hop sending node, the previous hop sending node sets the state of the previous hop sending node to be R.
4. The contention and finite state machine based urban VANET multi-hop broadcasting method according to claim 1, characterized in that: in step 6, the current transmitting node ViOf neighbor node VkIf it can confirm that the node V just leavesiNode V of the communication rangejHaving received the message, node VkNotification of node V with beacon message piggybacking an implicit ACKiLet it terminate due to node VjRetransmission timeout due to unchanged state.
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