CN115278553B - Internet of vehicles safety message broadcasting method based on RSU cooperation - Google Patents
Internet of vehicles safety message broadcasting method based on RSU cooperation Download PDFInfo
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Abstract
The invention provides a vehicle networking safety message broadcasting method based on RSU cooperation, and belongs to the technical field of vehicle networking broadcasting. The problems of low delivery rate and high time delay when vehicles send safety messages in the traditional periodic broadcast are solved. The technical proposal is as follows: the method comprises three steps of filtering the same event, uniformly early warning the event and reducing the network load. The beneficial effects of the invention are as follows: the invention is to guarantee the delivery performance of the security message, first, a new broadcast packet structure comprising 'event class' and 'geographic position' fields is designed; secondly, filtering redundant safety information by using a roadside unit (RSU) with abundant computing and storage resources, and helping to release broadcast information by the RSU, so that the broadcast transmission rate of a vehicle source node is reduced, and channel competition is relieved; finally, through experiments, the novel method is verified to be capable of increasing the coverage range of the broadcast early warning, improving the message delivery rate, reducing the transmission delay and meeting the performance requirement of the safety early warning.
Description
Technical Field
The invention relates to the technical field of Internet of vehicles broadcasting, in particular to an Internet of vehicles safety message broadcasting method based on RSU cooperation.
Background
With the development of the information age and the rapid popularization of the application of the internet of things, the data volume generated by the vehicle-mounted application is increased increasingly, and massive computing capacity and storage resources are required. The intelligent vehicle can generate a large amount of data in the running process, and various terminals need to realize data communication so as to share and utilize the data information. As a key carrier for transmitting messages in the intelligent transportation system (Intelligent transportation system, ITS), the internet of vehicles (Internet of Vehicle, ioV) is a wireless communication network established between Vehicle nodes and Infrastructure, and uses Vehicle-to-Vehicle (V2V) or Vehicle-to-Infrastructure (V2I) to transmit information such as Vehicle conditions, traffic safety, etc., so as to realize Vehicle networking and intellectualization.
While bringing convenience to travel, automobiles bring a lot of traffic safety hazards, so that the traveling safety becomes an important problem of social concern. Traffic accidents not only directly endanger personal safety and property safety, but also cause a great deal of fossil fuel waste due to road congestion as a secondary disaster.
An important factor causing traffic accidents is that traffic information is limited, a driver has sight distance limitation on the condition of a road ahead, and can only observe the condition within a certain range, and can not know the condition of the road ahead in advance to make a correct decision. The cooperative vehicle security system (Cooperative vehicle safety system, CVSS) is an important application in the internet of vehicles, which can effectively solve the above-mentioned problems. The vehicles periodically broadcast (Periodic Broadcast, PBC) data packets containing state information such as vehicle direction, acceleration, geographic position and the like in the running process, so that driving information sharing among the vehicles is realized. And the driver tracks the direction, the speed and the position of the neighbor vehicles according to the received broadcast message, detects traffic danger and congestion messages and gives early warning in time, and prompts the driver to adjust the strategy so as to avoid traffic accidents.
However, since the communication range of the vehicle is limited, the running speeds of different vehicles in the road are different, so that the network topology is unstable, and the situation that the message is unreachable is easy to occur during data communication. Meanwhile, safety messages are transmitted between vehicles through periodic broadcasting, and a plurality of nodes can repeatedly broadcast one safety event. A large number of redundant messages are generated in the network, vehicle nodes compete for channel resources, and when the problem of broadcast storm is caused seriously, the packet loss is remarkable in the process of sending the safety early warning message, the transmission delay is overlarge, and the safety early warning service quality is influenced.
Furthermore, the computational resources of the vehicle nodes are limited and cannot meet the computational requirements of a large number of secure message processing. Although the cloud computing technology can solve the problem of insufficient computing resources of the vehicle nodes, as the cloud server is far away from the vehicle nodes, the delay of transmitting the safety message to the remote server is large, which is not beneficial to the transmission and processing of the safety message.
Disclosure of Invention
The invention aims to provide a vehicle networking safety message broadcasting method based on RSU cooperation, wherein the traditional periodic broadcasting has excessive number of packets when the vehicle density is high, so that the problem of broadcasting storm is easy to cause delay or loss of safety early warning messages, and the traffic early warning effect is seriously influenced; aiming at the problems, the invention provides an RSU collaboration-based Internet of vehicles safety message broadcasting method; firstly, designing a novel broadcast packet structure containing an event grade and a geographic position; secondly, introducing a vehicle-mounted edge computing technology, filtering redundant safety information by using a roadside unit (RSU) with abundant computing storage resources, and then assisting in issuing broadcast information by using the RSU, so that the sending rate of a vehicle source node is reduced, and channel competition is relieved; and finally, the delivery performance of the broadcast packet is improved, and the average transmission delay is reduced.
The invention aims to provide an RSU collaboration-based Internet of vehicles safety message broadcasting method, which is characterized in that before an improved broadcasting method is provided, firstly, mathematical modeling is carried out on the network grouping delivery rate, and on the basis, a performance optimization thought is analyzed so as to improve the network delivery performance.
The CSMA/CA mechanism is a basic working mode of the MAC layer in IEEE 802.11p, and in order to avoid collision of data in channel transmission, a binary exponential backoff method is used to perform message backoff and retransmission, and according to the principle thereof, the channel contention process of the vehicle node is analyzed.
Fig. 9 shows a process in which two vehicle nodes contend for a channel to transmit data under the CSMA/CA scheme, wherein DIFS is a distributed inter-frame time slot, SIFS is a short inter-frame interval, and SlotTime represents one time slot; when the vehicle A needs to send data to the wireless AP, firstly, intercepting a channel, judging that the channel is idle at the moment when the idle time of the channel reaches a DIFS time, starting to send data, when the AP successfully receives the data, sending an ACK (acknowledgement) frame after a SIFS time, indicating the successful transmission of the message, along with the increase of the vehicle density, nodes mutually contend for the channel to send the data, the condition that the channel is busy easily occurs, when the vehicle A and the vehicle B send the data at the same time, the vehicle A occupies the channel to send the data, the condition that the vehicle B listens to the channel occurs the channel is caused, in order to avoid data collision, after the DIFS time of the interception channel, adding a backoff process, randomly generating a backoff time backofTime as a count value according to the size of a contention window, simultaneously continuing to intercept the channel, and continuously idle time of the interception channel reaches a time slot, and subtracting 1 from the backoff count value; if the channel is busy in the monitoring process, freezing the rollback count value, and when the rollback count value is reduced to 0, occupying the channel by the vehicle B to send data;
according to the back-off time definition, this can be expressed as:
BackoffTime＝Random()·CW·SlotTime (1)
wherein Random () takes a Random number of 0-1, CW is the current contention window size of the node, slotTime represents a time slot size, backoffTime is rounded up after being calculated according to the above formula, and CW is set min Minimum contention window value for vehicle node, CW max For maximum contention window size, CW is initially valued as CW min When the vehicle node fails to send data, the network data flow is considered to be large, and the vehicle node multiplies the CW contention window value by 2, namely
Wherein m represents the number of data retransmissions when the CW value reaches CW max When the CW value is unchanged; when the node successfully transmits data, the CW value is set to CW min ；
Considering the situation that n vehicle nodes exist in a certain RSU communication range, assuming that the probability of any vehicle node transmitting data in one random time slot is tau, the probability of collision of the data with other nodes is eta, and when the value of the node number n is constant, eta is always constant and independent, and the following relational expression can be obtained:
η＝1-(1-τ) n-1 (3)
from the probability of data collision, the message delivery rate p of the vehicle node can be expressed as:
p＝1-η (4)
is obtained by simplifying the formula (3),
let w=cw according to modeling of markov chain min The relationship between τ and η can be deduced:
the simultaneous equation (5) and the equation (6) can calculate the numerical solution of the nonlinear equation, when the maximum retransmission times m and the node number n are kept unchanged, the value of the contention window value W affects the data collision probability eta and the message delivery rate p, the value of the maximum retransmission times m is set to be 7, the node number n is set to be 50, the relationship between the contention window value W and the data collision probability eta is shown in fig. 3, and the relationship between the contention window value W and the message delivery rate p is shown in fig. 4.
As can be obtained from fig. 3 and fig. 4, when the network load is large, selecting a larger W can improve the delivery rate of the early warning message packet network, and because a large number of vehicle nodes exist in the network, optimizing the delivery performance of the vehicle nodes can effectively improve the delivery rate of the whole network. Therefore, in the process of reducing network load, the minimum competition window value of the vehicle source node is increased, message collision is reduced, more vehicle nodes in the communication range are guaranteed to receive safety messages, a network average time delay model is built, and the performance optimization thought is analyzed.
The transmission delay is the time from sending to receiving the message in the network, in the car networking, different message pair delay demands are different, wherein the event early warning service is used as a typical delay sensitive application, the requirement on the message transmission time is harsh, because the car networking adopts a broadcasting mode to carry out data communication, an ACK confirmation mechanism is not needed, and the sending time T of one broadcasting frame can be expressed as:
T＝t DIFS +t BO +t Data +t SIFS (7)
wherein t is DIFS To wait for a distributed interframe space time, t, before transmitting data BO For waiting the back-off time of the channel idle, if the channel idle is detected when the data is transmitted, the back-off time does not exist, the value is 0, t Data For the number of broadcastsThe time of transmission of the data frames is a key part in the broadcast delay. t is t SIFS For short inter-frame gap times, to separate frames belonging to a conversation.
When the busy channel is detected in the data transmission process, the node randomly selects a back-off count value according to the current contention window value for calculating the back-off time, so that message collision is avoided; the back-off count value is typically taken when there are a large number of broadcast messages in the networkThen the first time period of the first time period,
where slot represents a slot size, which is taken into equation (7):
according to t Data Definition of t Data Can be expressed as:
where L is the broadcast packet length in bytes; r is the node sending rate, unit Mb/s.
And (3) bringing the formula (10) into the formula (9) to obtain a simplified average time delay formula:
due to t DIFS 、slot、t SIFS Are all fixed parameters of the MAC layer and are not changed in the data transmission process. Therefore, the broadcast frame transmission time T is related only to the contention window value, the broadcast packet length, and the node transmission rate.
Therefore, for the safety message broadcasting service, under the condition that the broadcasting packet length is unchanged, the average transmission delay of the safety early warning message can be reduced by selecting smaller W and higher R. In terms of communication performance, RSUs are far superior to vehicle nodes. By utilizing the characteristic of high bandwidth of the RSU, the minimum competition window value is reduced, the safety early warning message is ensured to be sent in a shorter time, and the driving safety is improved.
For safety early warning messages of different grades, the RSU broadcasting mechanism sets different minimum competition window values, the minimum competition window value of the urgent message is set to be 1/8 of the initial value, the minimum competition window value of the urgent message is set to be 1/4 of the initial value, the minimum competition window value of the general message is set to be 1/2 of the initial value, and the transmission performance of the high-grade message is preferentially ensured.
In order to achieve the aim of the invention, the invention adopts the technical scheme that: the Internet of vehicles safety message broadcasting method based on RSU cooperation is characterized by comprising the following steps:
step one, filtering the same event, when the RSU receives a safety early warning message sent by a vehicle source node, reading the level and geographic position information of the event in the packet, judging the emergency safety type of the event, and filtering repeated broadcasting of the same traffic event.
And step two, uniformly early warning the event, and after the same event is filtered by the RSU, adaptively improving the message sending rate and reducing the minimum competition window value according to the emergency type of the event, and carrying out safety early warning in the communication range of the event. During the regional transmission, after receiving the early warning broadcast of the vehicle source node and the RSU, the neighbor vehicle node does not carry out packet forwarding, so that the broadcasting performance of the RSU in the whole network is ensured. When the information is transmitted to the rear area, the safety information is sent to the rear RSU through a wired technology such as optical fiber and the like, so that the information is broadcasted in a communication range, and the early warning information is ensured to be received by a rear vehicle node in advance.
And thirdly, reducing network load, reading a grouping geographic position information judgment event when the vehicle source node receives a safety early warning message broadcast by the RSU, and if the grouping geographic position information judgment event is judged to be the safety message broadcast by the vehicle before, reducing the message sending rate in a self-adaptive manner according to the emergency type of the event, improving the minimum competition window value and continuing to broadcast in the communication range.
Further, the filtering of the same event in the first step specifically includes the following steps:
(1) Vehicle source node V that detects abnormal traffic events such as accidents or congestion i 、V j Each generates a safety precaution message P i And P j ；
(2) Source node V i 、V j Will secure the early warning message P i And P j Broadcasting to roadside units RSU1 and neighboring vehicles V within communication range k ；
(3) In the sampling time t, the RSU1 receives the safety precaution message P i And P j After (assuming P is received first i Message), respectively reading the "geographical location" field value P in the message packet i Address and P j Filtering repeated messages of the same event;
step1: the RSU1 sets the same event distance judging threshold D;
step2: RSU1 will P i Address and P j Subtracting the addresses, and calculating the distance d between the events;
step3: if D is less than or equal to D, judging P i And P j The content of the message is the same traffic event, and the RSU1 receives the first safety precaution message P i Joining transmit buffer queue Q inte Discard other safety precaution messages P j ；
Step4: if D > D, then determine P i And P j The content of the messages are different traffic events, and the RSU1 sends the two safety precaution messages P i And P j Joining transmit buffer queue Q inte 。
Further, the event unified early warning in the second step specifically includes the following steps:
(1) The neighbor vehicle node receives the safety precaution message P i And P j Broadcast forwarding is not performed;
(2) RSU1 is in broadcast transmission buffer queue Q inte In the safety precaution message P inte Reading the "event level" field value P in the message packet inte_level It can be known that the safety precaution message P inte Is a part of the emergency level. Let R beThe minimum contention window initial value of the wireless MAC layer of SU is W R0 ；
Step1: for emergency events, RSU1 sets the MAC layer minimum contention window CW min ＝a 1 *W R0 Wherein a is 1 ＝1/8；
Step2: for emergency events, RSU1 sets the MAC layer minimum contention window CW min ＝a 2 *W R0 Wherein a is 2 ＝1/4；
Step3: for general events, RSU1 sets the MAC layer minimum contention window CW min ＝a 3 *W R0 Wherein a is 3 ＝1/2；
(3) After the minimum contention window value of the MAC layer is reset according to the message grade, the RSU monitors the PBC sending rate R of the vehicle node V0 Setting own PBC sending rate as 2*R V0 Sending a safety precaution message in the communication range;
step1: during early warning in the area, the RSU1 sends a safety early warning message P to the vehicle nodes in the communication range in a broadcasting mode inte ；
Step2: during the early warning of the rear area, the RSU1 sends the early warning message to the rear RSU2 through the optical fiber wired communication mode, and then the RSU2 sends the safety message P inte Broadcast to vehicles within range thereof.
Further, the reducing the network load in the third step specifically includes the following steps:
(1) Source node V i 、V j Receiving RSU1 broadcast early warning message P inte If the safety message broadcast by the vehicle is judged, the sending rate is reduced, the minimum contention window value is increased, the broadcasting in the communication range is continued, and the initial value of the wireless MAC layer minimum contention window of the vehicle source node is set as W V0 The initial value of the message sending rate is R V0 ；
Step1: for emergency events, vehicle nodes set the MAC layer minimum contention window CW min ＝b 1 *W V0 Wherein b 1 =2, set the transmission rate R send ＝(1-c 1 )*R V0 Wherein c 1 ＝0.3；
Step2: for emergency events, vehicle nodes set the MAC layer minimum contention window CW min ＝b 2 *W V0 Wherein b 2 =4, set the transmission rate R send ＝(1-c 2 )*R V0 Wherein c 2 ＝0.5；
Step3: for general events, the vehicle node sets the MAC layer minimum contention window CW min ＝b 3 *W V0 Wherein b 3 =8, set the transmission rate R send ＝(1-c 3 )*R V0 Wherein c 3 ＝0.8；
(2) After the minimum contention window value and the sending rate of the MAC layer are set, the source node V i 、V j Continue broadcasting within its communication range.
Compared with the prior art, the invention has the beneficial effects that:
(1) According to the vehicle-mounted edge computing (Vehicular Edge Computing, VEC), the edge infrastructure close to the Internet of vehicles is utilized to provide services such as computing, storage and transmission nearby, so that the service requirements of time-sensitive application of vehicle-mounted equipment are well met, and a Road Side Units (RSU) is taken as a typical infrastructure, so that abundant computing resources and a wider transmission range are provided, the application requirements of vehicles are assisted, and the task execution efficiency is improved; meanwhile, the RSU interconnected with the optical fibers can be used as a wired forwarding node, a network communication area is enlarged, link quality is stabilized, and early warning information is ensured to be successfully transmitted.
(2) Aiming at the service requirement of a safety early warning message in a cooperative vehicle safety system, firstly, the novel broadcast packet structure designed by the invention adds event grade and geographic position fields in the broadcast message, and records the specific information of traffic safety events; secondly, providing an RSU cooperation-based Internet of vehicles safety message broadcasting method, in the same event filtering stage, the RAIB method utilizes RSU equipment with abundant computing storage resources to screen and filter redundant messages of the same event in a network, so that the influence of collision of the redundant messages on transmission performance is reduced, in the event unified early warning stage, a neighbor vehicle does not broadcast and forward after receiving the safety message sent by a vehicle source node, the RSU equipment filters the safety message of the same event and then uniformly broadcasts and early warns, the RSU communication performance advantage is fully exerted, and in the network load reduction stage, the RAIB method broadcasts early warning in a communication range at a lower speed by reducing the sending speed of the vehicle node and improving the minimum competition window value, and the channel resource overhead and the whole network broadcasting load are reduced; finally, experiments show that the method can not only increase the range of broadcast early warning and improve the message delivery rate, but also reduce the transmission delay and meet the service requirement of safety early warning.
Drawings
The accompanying drawings are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate the invention and together with the embodiments of the invention, serve to explain the invention.
Fig. 1 is a schematic overall flow diagram of an RSU collaboration-based internet of vehicles safety message broadcasting method in the present invention.
Fig. 2 is a schematic diagram of a broadcast packet structure designed in the present invention.
Fig. 3 is a graph showing the relationship between the contention window value W and the data collision probability η when the maximum retransmission number and the node number are fixed in the present invention.
Fig. 4 is a graph showing the relationship between the contention window value W and the message delivery rate p when the maximum retransmission number and the node number are fixed in the present invention.
Fig. 5 is a graph showing comparison of maximum notification vehicle number performance before and after the RAIB method is adopted in experiment 1 according to an embodiment of the present invention.
Fig. 6 is a diagram showing the performance comparison of network delivery rates before and after the RAIB method is adopted in experiment 2 according to the embodiment of the present invention.
Fig. 7 is a diagram showing comparison of transmission average delay performance before and after the RAIB method is adopted in experiment 3 according to an embodiment of the present invention.
Fig. 8 is a topology diagram of a vehicle transmission broadcast in the CVSS system of the present invention.
Fig. 9 is a schematic diagram of preemptive channel transfer data using the CSMA/CA mechanism in the present invention.
Detailed Description
The present invention will be described in further detail with reference to the drawings and examples, in order to make the objects, technical solutions and advantages of the present invention more apparent. Of course, the specific embodiments described herein are for purposes of illustration only and are not intended to limit the invention.
Example 1
Referring to fig. 1 to 9, the invention provides a vehicle networking safety message broadcasting method based on RSU cooperation, which comprises the steps of firstly, establishing a network delivery rate model and analyzing a performance optimization thought;
the CSMA/CA mechanism is a basic working mode of the MAC layer in IEEE 802.11p, and in order to avoid collision of data in channel transmission, a binary exponential backoff method is used to perform message backoff and retransmission, and then a vehicle node channel contention process is analyzed according to its principle.
Fig. 9 shows a process in which two vehicle nodes contend for a channel to transmit data under the CSMA/CA scheme, where DIFS is a distributed inter-frame time slot, SIFS is a short inter-frame interval, slotTime represents a time slot, and when vehicle a needs to transmit data to a wireless AP, first listens to the channel. When the idle time of the channel reaches a DIFS time, the idle time of the channel is judged to be idle at the moment, and data transmission is started. And when the AP successfully receives the data, an ACK confirmation frame is sent after a SIFS time, and the ACK confirmation frame indicates successful transmission of the message. As the density of vehicles increases, nodes compete with each other for channels to transmit data, and a channel busy condition is likely to occur. When the vehicle A and the vehicle B transmit data at the same time, the vehicle A occupies a channel to transmit data, and the vehicle B listens to the channel, so that the channel is busy. To avoid data collision, vehicle B adds a backoff procedure after listening to the DIFS time of the channel. The vehicle B randomly generates a back off time as a back off count value according to the size of the contention window, and continuously monitors a channel, and if the continuous idle time of the monitoring channel reaches a time slot, the back off count value is decremented by 1; and if the channel is busy in the monitoring process, freezing the rollback count value. When the back-off count value is reduced to 0, the vehicle B occupies the channel to transmit data.
According to the back-off time definition, this can be expressed as:
BackoffTime＝Random()·CW·SlotTime (1)
wherein Random () takes a Random number of 0-1, CW is the current contention window size of the node, slotTime represents a time slot size, and backoffTime is rounded up after calculation according to the above formula. Set CW min Minimum contention window value for vehicle node, CW max For maximum contention window size, CW is initially valued as CW min . When the vehicle node fails to send data, the network data flow is considered to be large, and the vehicle node multiplies the CW contention window value by 2, namely
Where m represents the number of data retransmissions. When the CW value reaches CW max While the CW value is kept unchanged. When the node successfully transmits data, the CW value is set to CW min 。
Considering the situation that n vehicle nodes exist in a certain RSU communication range, assuming that the probability of any vehicle node transmitting data in one random time slot is tau, the probability of collision of the data with other nodes is eta, and when the value of the node number n is constant, eta is always constant and independent, and the following relational expression can be obtained:
η＝1-(1-τ) n-1 (3)
from the probability of data collision, the message delivery rate p of the vehicle node can be expressed as:
p＝1-η (4)
is obtained by simplifying the formula (3),
let w=cw according to modeling of markov chain min The relationship between τ and η can be deduced:
the simultaneous equations (5) and (6) can solve the numerical solution of the nonlinear equation. When the maximum retransmission times m and the node number n are kept unchanged, the value of the competition window value W influences the data collision probability eta and the message delivery rate p. The maximum retransmission number m is set to be 7, the node number n is 50, the relationship between the contention window value W and the data collision probability eta is shown in fig. 3, and the relationship between the contention window value W and the message delivery rate p is shown in fig. 4.
As can be seen from fig. 3 and fig. 4, when the network load is large, selecting a larger W can improve the early warning message packet network delivery rate. Because a large number of vehicle nodes exist in the network, the delivery performance of the vehicle nodes is optimized, and the delivery rate of the whole network can be effectively improved. Therefore, in the process of reducing the network load, the minimum competition window value of the vehicle source node is increased, the message collision is reduced, and more vehicle nodes in the communication range are ensured to receive the safety message.
And secondly, establishing a network average time delay model and analyzing a performance optimization thought.
Transmission delay is the time a message is sent to reception in the network. In the internet of vehicles, different messages have different time delay requirements, wherein the event early warning service is applied as typical time delay sensitivity, and the requirements on the message transmission time are harsh. Because the internet of vehicles adopts a broadcasting mode to carry out data communication, an ACK confirmation mechanism is not needed, and the sending time T of one broadcasting frame can be expressed as:
T＝t DIFS +t BO +t Data +t SIFS (7)
wherein t is DIFS A distributed interframe space time to wait before transmitting data. t is t BO In order to wait for the back-off time of the channel idle, if the channel idle is detected when data is transmitted, the back-off time does not exist and takes a value of 0.t is t Data The transmission time of the broadcast data frame is a key part in the broadcast time delay. t is t SIFS For short inter-frame gap times, to separate frames belonging to a conversation.
When the busy channel is detected in the data transmission process, the node randomly selects a back-off count value according to the current contention window value for calculating the back-off time, so that message collision is avoided. The back-off count value is typically taken when there are a large number of broadcast messages in the networkThen
Where slot represents a slot size, which is taken into equation (7):
according to t Data Definition of t Data Can be expressed as:
where L is the broadcast packet length in bytes; r is the node sending rate, unit Mb/s.
And (3) bringing the formula (10) into the formula (9) to obtain a simplified average time delay formula:
due to t DIFS 、slot、t SIFS Are all fixed parameters of the MAC layer and are not changed in the data transmission process. Therefore, the broadcast frame transmission time T is related only to the contention window value, the broadcast packet length, and the node transmission rate.
Therefore, for the safety message broadcasting service, under the condition that the broadcasting packet length is unchanged, the average transmission delay of the safety early warning message can be reduced by selecting smaller W and higher R. In terms of communication performance, RSUs are far superior to vehicle nodes. By utilizing the characteristic of high bandwidth of the RSU, the minimum competition window value is reduced, the safety early warning message is ensured to be sent in a shorter time, and the driving safety is improved.
For safety early warning messages of different grades, the RSU broadcasting mechanism sets different minimum competition window values, the minimum competition window value of the urgent message is set to be 1/8 of the initial value, the minimum competition window value of the urgent message is set to be 1/4 of the initial value, the minimum competition window value of the general message is set to be 1/2 of the initial value, and the transmission performance of the high-grade message is preferentially ensured.
The invention is realized by the following measures: an Internet of vehicles safety message broadcasting method based on RSU cooperation comprises the following steps:
step one, filtering the same event, when the RSU receives a safety early warning message sent by a vehicle source node, reading the level and geographic position information of the event in the packet, judging the emergency safety type of the event, and filtering repeated broadcasting of the same traffic event.
And step two, uniformly early warning the event, and after the same event is filtered by the RSU, adaptively improving the message sending rate and reducing the minimum competition window value according to the emergency type of the event, and carrying out safety early warning in the communication range of the event. During the regional transmission, after receiving the early warning broadcast of the vehicle source node and the RSU, the neighbor vehicle node does not carry out packet forwarding, so that the broadcasting performance of the RSU in the whole network is ensured. When the information is transmitted to the rear area, the safety information is sent to the rear RSU through a wired technology such as optical fiber and the like, so that the information is broadcasted in a communication range, and the early warning information is ensured to be received by a rear vehicle node in advance.
And thirdly, reducing network load, reading a grouping geographic position information judgment event when the vehicle source node receives a safety early warning message broadcast by the RSU, and if the grouping geographic position information judgment event is judged to be the safety message broadcast by the vehicle before, reducing the message sending rate in a self-adaptive manner according to the emergency type of the event, improving the minimum competition window value and continuing to broadcast in the communication range.
Further, the filtering specific content of the same event in the first step is as follows:
there are a large number of emergency events in the traffic running process, and different types of events cause traffic hazards of different degrees. These information can be classified into three levels according to the event safety factor: emergency events, and general events. The emergency events comprise events such as traffic accidents and the like which seriously damage personal safety, the emergency events comprise events such as severe weather environments and the like which cause travel hazards, and the general events comprise events such as traffic congestion and the like which influence driving experience.
The safety early warning service provided by the Internet of vehicles can notify traffic safety events in the whole network, and vehicle nodes know road conditions in advance according to the received safety early warning message and adjust driving strategies. Thus, the present invention is directed to redesigning the broadcast packet structure by adding the "event level" (1 byte) and "geographic location" (4 bytes) fields in the original data payload area (load), and placing them in the "PBC header" after which the packet structure is designed as shown in fig. 2. Wherein "event rating" records the degree of emergency of an event, including three levels of urgency, and general; the geographic position is determined according to a GPS system in the vehicle, and the lane distance has little influence on the distance between vehicles due to the high running speed of the expressway vehicle, so that only one-dimensional coordinate information when a source node detects a traffic event is recorded.
(1) Vehicle source node V that detects abnormal traffic events such as accidents or congestion i 、V j Respectively generating safety precaution messages P i And P j ；
(2) Source node V i 、V j Will secure the early warning message P i And P j Broadcasting to roadside units RSU1 and neighboring vehicles V within communication range k ；
(3) In the sampling time t, the RSU1 receives the safety precaution message P i And P j After (assuming P is received first i Message), respectively reading the "geographical location" field value P in the message packet i Address and P j Filtering repeated messages of the same event;
step1: the RSU1 sets the same event distance judging threshold D;
step2: RSU1 will P i Address and P j Subtracting the addresses, and calculating the distance d between the events;
step3: if D is less than or equal to D, judging P i And P j The content of the message is the same traffic event, and the RSU1 receives the first safety precaution message P i Joining transmit buffer queue Q inte Discard other safety precaution messages P j ；
Step4: if D > D, then determine P i And P j The content of the message isDifferent traffic events, the RSU1 sends the two safety precaution messages P i And P j Joining transmit buffer queue Q inte 。
Further, the unified early warning specific content of the event in the second step is as follows:
the broadcasting performance of the traditional periodic broadcasting mode is low, and the message packet loss is caused by the fact that the vehicle nodes compete with each other for channel resources due to the fact that a large amount of redundant data exists in the network. After the same safety early warning message is filtered, the message forwarding at the vehicle node is reduced in the event unified early warning stage, the RSU carries out unified broadcast early warning, a large amount of redundant data in the network is reduced, the channel intense competition is relieved, and the packet delivery rate is improved.
The multi-hop assisted transmission in the internet of vehicles can improve the message transmission range, but the multiple forwarding of intermediate vehicles generates a large amount of redundant data, and in order to avoid the large amount of redundant data generated by vehicle forwarding, the internet of vehicles safety message broadcasting method based on RSU cooperation adopts RSU with wide communication range to assist forwarding during safety early warning, and the intermediate vehicles do not forward the message.
Although the communication range of RSUs is far greater than that of vehicle nodes, the communication range of RSUs is still limited, and for the later remote early warning messages, high-speed wired communication transmission between RSUs can be used.
(1) The neighbor vehicle node receives the safety precaution message P i And P j Broadcast forwarding is not performed;
(2) RSU1 is in broadcast transmission buffer queue Q inte In the safety precaution message P inte Reading the "event level" field value P in the message packet inte_level It can be known that the safety precaution message P inte Is a part of the emergency level. Let the minimum contention window initial value of wireless MAC layer of RSU be W R0 ；
Step1: for emergency events, RSU1 sets the MAC layer minimum contention window CW min ＝a 1 *W R0 Wherein a is 1 ＝1/8；
Step2：For emergency events, RSU1 sets the MAC layer minimum contention window CW min ＝a 2 *W R0 Wherein a is 2 ＝1/4；
Step3: for general events, RSU1 sets the MAC layer minimum contention window CW min ＝a 3 *W R0 Wherein a is 3 ＝1/2；
(3) After the minimum contention window value of the MAC layer is reset according to the message grade, the RSU monitors the PBC sending rate R of the vehicle node V0 Setting own PBC sending rate as 2*R V0 Sending a safety precaution message in the communication range;
step1: during early warning in the area, the RSU1 sends a safety early warning message P to the vehicle nodes in the communication range in a broadcasting mode inte ；
Step2: during the early warning of the rear area, the RSU1 sends the early warning message to the rear RSU2 through the optical fiber wired communication mode, and then the RSU2 sends the safety message P inte Broadcast to vehicles within range thereof.
Further, the specific content of reducing the network load in the third step is as follows:
the event unified early warning stage is mainly optimized for the safety early warning message in the transmission process, and the network load reduction stage is optimized for the safety early warning message at the source node. After the source node monitors the security event, the source node periodically broadcasts early warning in the communication range, and as the RSU assists in broadcasting early warning in the event unified early warning stage, the sending rate of the source node can be reduced, the minimum competition window value can be improved, the channel communication resource is reserved, and the broadcasting performance of the RSU is ensured.
(1) Source node V i 、V j Receiving RSU1 broadcast early warning message P inte If the safety message broadcast by the vehicle is judged, the sending rate is reduced, the minimum contention window value is increased, the broadcasting in the communication range is continued, and the initial value of the wireless MAC layer minimum contention window of the vehicle source node is set as W V0 The initial value of the message sending rate is R V0 ；
Step1: for emergency events, vehicle nodes set the MAC layer minimum contention window CW min ＝b 1 *W V0 Wherein b 1 =2, set the transmission rate R send ＝(1-c 1 )*R V0 Wherein c 1 ＝0.3；
Step2: for emergency events, vehicle nodes set the MAC layer minimum contention window CW min ＝b 2 *W V0 Wherein b 2 =4, set the transmission rate R send ＝(1-c 2 )*R V0 Wherein c 2 ＝0.5；
Step3: for general events, the vehicle node sets the MAC layer minimum contention window CW min ＝b 3 *W V0 Wherein b 3 =8, set the transmission rate R send ＝(1-c 3 )*R V0 Wherein c 3 ＝0.8；
(2) After the minimum contention window value and the sending rate of the MAC layer are set, the source node V i 、V j Continue broadcasting within its communication range.
In order to analyze and evaluate the effectiveness of the internet of vehicles safety message broadcasting method based on RSU cooperation, the method is compared with a periodical broadcasting method, and performance comparison is carried out on three indexes of maximum notification vehicle number, broadcasting delivery rate and average time delay.
Experiment 1: maximum notification vehicle count performance comparison
Experimental environment: setting a two-lane traffic environment, and placing 250 vehicle nodes on each lane. Different vehicle density conditions are simulated by changing the distances between the vehicle nodes, and the distances between the vehicle nodes are respectively set to be 10m, 15m, 20m, 30m and 40m.
As shown in fig. 5, the number of vehicles to be notified maximally by the periodic broadcast method and the RAIB method increases as the average distance between vehicles increases, and the average number of vehicles in the communication range decreases, and the maximum number of vehicles that can be notified by one broadcast by the periodic broadcast method and the RAIB method also gradually decreases. In contrast to the specific data of the RAIB method and the periodic broadcast method at different vehicle densities, the RAIB method broadcasts a maximum of 402 vehicles at a time at a distance of 10m between vehicles, whereas the periodic broadcast method can only notify 119 vehicles. At a distance of 35m between vehicles, the RAIB method broadcasts a maximum of 116 vehicles at a time, whereas the periodic broadcast method can only notify 33 vehicles. The RAIB method proves that the message early warning range is larger, and the number of vehicles is more.
Experiment 2: broadcast delivery rate performance comparison
Experimental environment: and constructing a traffic scene with 76 vehicles uniformly placed in four lanes, namely 19 vehicles in each lane, wherein the distance between the vehicles is 15m. By varying the number of packets sent per second by the vehicle nodes, the communication pressure is set to different degrees, the greater the number of packets sent per second, the greater the amount of data in the network, and the greater the communication pressure.
The delivery rate performance of the periodic broadcast method and the RAIB method are shown in FIG. 6.
With conventional periodic broadcast pre-warning, the delivery rate in the network is 88.68% when the vehicle node sends 15 packets per second. As the number of vehicle node transmissions increases, the network load increases. When a vehicle node sends 20 packets per second, the network delivery rate drops to 66.40%. A large number of early warning messages are lost in the transmission process, so that emergency events cannot be accurately early warned, and traffic safety is threatened.
When broadcasting early warning by using RAIB method, RSU monitors PBC sending rate R of vehicle node V0 Setting own PBC sending rate as 2*R V0 I.e. when the vehicle node transmits 10 packets per second, the RSU transmits 20 packets per second. The RAIB method judges the pre-warning event of repeated broadcasting through the RSU, filters redundant packets in a network link, and improves broadcasting efficiency on the basis of guaranteeing message transmission. When the vehicle node transmits 15 packets per second, the early warning message delivery rate in the network is 97.52%. When the vehicle node sends 20 packets per second, the delivery rate drops only to 92.06%. In comparison to the traditional periodic broadcast method, delivery performance is optimized by 8.84% and 25.66% when the vehicle node transmits 15 and 20 packets per second, respectively. The emergency early warning message broadcast by the RAIB method has higher message receiving rate, and more vehicles in the communication range can avoid risks according to the message early warning, so that the traffic is safer.
Experiment 3: broadcast average delay performance comparison
Experimental environment: and constructing a traffic scene with 76 vehicles uniformly placed in four lanes, namely 19 vehicles in each lane, wherein the distance between the vehicles is 15m.
The notification speed of the traffic warning message is particularly important in safe driving, and the earlier the driver receives the message warning, the more time is needed to adjust the driving strategy. In the RAIB method, the RSU adaptively adjusts the sending rate and the competition window value of the RSU and the vehicle node according to the emergency degree of the event, so that the transmission time delay of the early warning message is reduced. The pair of periodic broadcast method and RAIB method delays is shown in FIG. 7.
With conventional periodic broadcast precautions, when a vehicle node transmits 15 packets per second, the average delay of the vehicle node transmitting a message is 7.905ms. As the number of vehicle node transmissions increases, the network load increases. When the vehicle node transmits 20 packets per second, the average delay of the vehicle node transmitting the message is 10.905ms.
When the RAIB method is used for broadcasting early warning, when the vehicle node transmits 15 packets per second, the average time delay of the vehicle node is 6.671ms. As the number of vehicle node transmissions increases, the network load increases. When the vehicle node transmits 20 packets per second, the average latency of the vehicle node is 7.255ms. Compared with the vehicle node in the traditional periodic broadcast method, the RAIB method optimizes the average delay performance by 15.6% and 33.47% when the vehicle node transmits 15 and 20 packets per second. When the RAIB method is used for broadcasting early warning, when the vehicle node transmits 15 packets per second, the average time delay of the RSU device is 5.767ms. As the number of vehicle node transmissions increases, the network load increases. When the vehicle node transmits 20 packets per second, the average latency of the RSU device is 6.021ms. The RAIB method optimizes the average latency performance by 27.05% and 44.79% when the vehicle node transmits 15 and 20 packets per second, respectively, as compared to the vehicle node in the conventional periodic broadcast method. From this, the RAIB method optimizes the transmission delay broadcast at both the RSU device and the vehicle node. The time delay optimization of the RSU equipment is more obvious, and the early warning message is ensured to be transmitted successfully in a shorter time.
The foregoing description of the preferred embodiments of the invention is not intended to limit the invention to the precise form disclosed, and any such modifications, equivalents, and alternatives falling within the spirit and scope of the invention are intended to be included within the scope of the invention.
Claims (1)
1. The Internet of vehicles safety message broadcasting method based on roadside unit (RSU) cooperation is characterized by comprising the following steps:
step one, filtering the same event, reading the level and geographic position information of the event in the packet when the RSU receives the safety early warning message sent by the vehicle source node, judging the emergency safety type of the event, and filtering repeated broadcasting of the same traffic event;
the same event filtering in the first step specifically comprises the following steps:
(1) Vehicle source node V that detects an accident or a congestion abnormal traffic event i 、V j Each generates a safety precaution message P i And P j ；
(2) Source node V i 、V j Will secure the early warning message P i And P j Broadcasting to roadside units RSU1 and neighboring vehicles V within communication range k ；
(3) In the sampling time t, the RSU1 receives the safety precaution message P i And P j After that, assume that P is received first i Messages, respectively reading the 'geographic position' field value P in the message packet i Address and P j Filtering repeated messages of the same event;
step1: the RSU1 sets the same event distance judging threshold D;
step2: RSU1 will P i Address and P j Subtracting the addresses, and calculating the distance d between the events;
step3: if D is less than or equal to D, judging P i And P j The content of the message is the same traffic event, and the RSU1 receives the first safety precaution message P i Joining transmit buffer queue Q inte Discard other safety precaution messages P j ；
Step4: if D > D, then determine P i And P j The content of the messages are different traffic events, and the RSU1 sends the two safety precaution messages P i And P j Joining transmit buffer queue Q inte ；
Step two, after the same event is filtered by the RSU, the sending rate of the message is adaptively improved and the minimum competition window value is reduced according to the emergency type of the event, safety precaution is carried out in the communication range of the RSU, when the information is transmitted in the local area, the neighbor vehicle nodes do not carry out packet forwarding after receiving the precaution broadcast of the vehicle source nodes and the RSU, the broadcasting performance of the RSU in the whole network is ensured, when the information is transmitted to the rear area, the safety information is sent to the rear RSU through an optical fiber cable technology, the information is broadcasted in the communication range, and the early warning information is ensured to be received by the rear vehicle nodes in advance;
the event unified early warning in the second step comprises the following steps:
(1) The neighbor vehicle node receives the safety precaution message P i And P j Broadcast forwarding is not performed;
(2) RSU1 is in broadcast transmission buffer queue Q inte In the safety precaution message P inte Reading the "event level" field value P in the message packet inte_level It can be known that the safety precaution message P inte Setting the minimum contention window initial value of the wireless MAC layer of the RSU as W R0 ；
Step1: for emergency events, RSU1 sets the MAC layer minimum contention window CW min ＝a 1 *W R0 Wherein a is 1 ＝1/8；
Step2: for emergency events, RSU1 sets the MAC layer minimum contention window CW min ＝a 2 *W R0 Wherein a is 2 ＝1/4；
Step3: for general events, RSU1 sets the MAC layer minimum contention window CW min ＝a 3 *W R0 Wherein a is 3 ＝1/2；
(3) After the minimum contention window value of the MAC layer is reset according to the message grade, the RSU monitors the PBC sending rate R of the vehicle node V0 Setting own PBC sending rate as 2*R V0 Sending a safety precaution message in the communication range;
step1: during early warning in the area, the RSU1 sends a safety early warning message P to the vehicle nodes in the communication range in a broadcasting mode inte ；
Step2: during the early warning of the rear area, the RSU1 sends the early warning message to the rear RSU2 through the optical fiber wired communication mode, and then the RSU2 sends the safety message P inte Broadcasting to vehicles within range thereof;
step three, reducing network load, when the vehicle source node receives the safety precaution message broadcast by the RSU, reading a grouping geographic position information judgment event, if the judgment event is that the vehicle broadcasts the safety message before, reducing the message sending rate in a self-adaptive way according to the emergency type of the event, improving the minimum competition window value, and continuing broadcasting in the communication range;
the step three of reducing the network load comprises the following steps:
(1) Source node V i 、V j Receiving RSU1 broadcast early warning message P inte If the safety message broadcast by the vehicle is judged, the sending rate is reduced, the minimum contention window value is increased, the broadcasting in the communication range is continued, and the initial value of the wireless MAC layer minimum contention window of the vehicle source node is set as W V0 The initial value of the message sending rate is R V0 ；
Step1: for emergency events, vehicle nodes set the MAC layer minimum contention window CW min ＝b 1 *W V0 Wherein b 1 =2, set the transmission rate R send ＝(1-c 1 )*R V0 Wherein c 1 ＝0.3；
Step2: for emergency events, vehicle nodes set the MAC layer minimum contention window CW min ＝b 2 *W V0 Wherein b 2 =4, set the transmission rate R send ＝(1-c 2 )*R V0 Wherein c 2 ＝0.5；
Step3: for general events, the vehicle node sets the MAC layer minimum contention window CW min ＝b 3 *W V0 Wherein b 3 =8, set the transmission rate R send ＝(1-c 3 )*R V0 Wherein c 3 ＝0.8；
(2) After the minimum contention window value and the sending rate of the MAC layer are set, the source node V i 、V j Continue to be communicated withBroadcasting in the range.
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