CN115278553B - A safe message broadcast method for Internet of Vehicles based on RSU collaboration - Google Patents

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

A safe message broadcast method for Internet of Vehicles based on RSU collaboration

Technical field

The present invention relates to the technical field of Internet of Vehicles broadcasting, and in particular to a method for broadcasting safety messages of Internet of Vehicles based on RSU collaboration.

Background technique

With the development of the information age and the rapid promotion of Internet of Things applications, the amount of data generated by in-vehicle applications is increasing, requiring massive computing power and storage resources. Smart vehicles will generate a large amount of data while driving, and various terminals need to realize data communication in order to share and utilize the above data information. As a key carrier of messages in the Intelligent Transportation System (ITS), the Internet of Vehicles (IoV) is a wireless communication network established between vehicle nodes and infrastructure, using vehicle-to-vehicle communication (Vehicle-to-Vehicle) -Vehicle (V2V) or vehicle-to-infrastructure (V2I) communication (Vehicle-to-Infrastructure, V2I) transmits vehicle status, traffic safety and other information to realize vehicle networking and intelligence.

While bringing travel convenience, cars also bring many traffic safety hazards, so driving safety has also become an important issue of social concern. In addition to directly endangering personal safety and property safety, traffic accidents also cause road congestion as a secondary disaster and waste a large amount of fossil fuels.

An important factor causing traffic accidents is the limited transmission of traffic information. Drivers have limited sight distance of the road conditions ahead. They can only observe the situation within a certain range and cannot know the road conditions ahead in advance and make correct decisions. Cooperative vehicle safety system (CVSS) is an important application in the Internet of Vehicles and can effectively solve the above problems. During driving, the vehicle periodically broadcasts (Periodic Broadcast, PBC) data packets containing vehicle direction, acceleration, geographical location and other status information to realize driving information sharing between vehicles. Based on the broadcast message received, the driver tracks the direction, speed and location of neighbor vehicles, detects traffic dangers and congestion messages and provides timely warnings, reminding the driver to adjust strategies to avoid traffic accidents.

However, due to the limited vehicle communication range and different vehicles traveling at different speeds on the road, the network topology is unstable and messages are prone to being unreachable during data communication. At the same time, safety messages are transmitted between vehicles through periodic broadcasts, and there are situations where multiple nodes repeatedly broadcast a safety event. A large number of redundant messages are generated in the network, and vehicle nodes compete with each other for channel resources. In severe cases, broadcast storms occur, resulting in significant packet loss during the sending of safety warning messages and excessive transmission delays, affecting the quality of safety warning services.

Not only that, the computing resources of vehicle nodes are limited and cannot meet the computing needs of a large number of secure message processing. Although cloud computing technology can solve the problem of insufficient computing resources of vehicle nodes, because the cloud server is far away from the vehicle node, the delay in transmitting safety messages to the remote server is large, which is not conducive to the transmission and processing of safety messages.

Contents of the invention

The purpose of the present invention is to provide a safety message broadcast method for Internet of Vehicles based on RSU collaboration. Traditional periodic broadcasts have too many packets when the vehicle density is high, which can easily lead to broadcast storm problems, delay or loss of safety warning messages, and seriously affect Traffic warning effect; In response to the above problems, the present invention proposes a safety message broadcast method for Internet of Vehicles based on RSU collaboration; first, a new broadcast grouping structure containing "event level" and "geographic location" is designed; secondly, vehicle edge computing technology is introduced , use the roadside unit RSU with rich computing and storage resources to filter redundant safety messages, and then the RSU assists in publishing broadcast messages, reducing the sending rate of vehicle source nodes and easing channel competition; ultimately improving broadcast packet delivery performance and reducing average transmission delay.

The purpose of this invention is to provide a safety message broadcast method for Internet of Vehicles based on RSU collaboration. Before proposing an improved broadcast method, mathematical modeling of the network packet delivery rate is first performed, and on this basis, performance optimization ideas are analyzed to improve the network. Delivery performance.

The CSMA/CA mechanism is the basic working method of the MAC layer in IEEE 802.11p. In order to avoid data conflicts during channel transmission, the binary exponential backoff method is used for message backoff and retransmission. The following is an analysis of the vehicle node channel competition process based on its principle. .

Figure 9 shows the process of two vehicle nodes competing for the channel to transmit data under the CSMA/CA mechanism, where DIFS is the distributed inter-frame time slot, SIFS is the short inter-frame interval, and SlotTime represents a time slot; when vehicle A needs to When the wireless AP sends data, it first listens to the channel. When the channel idle time reaches a DIFS time, it determines that the channel is idle at this time and starts to send data. When the AP successfully receives the data, it sends an ACK confirmation frame after a SIFS time to indicate that this time For successful transmission of messages, as vehicle density increases, nodes compete with each other for channels to send data, and the channel is prone to be busy. When vehicle A and vehicle B send data at the same time, vehicle A occupies the channel to send data, and vehicle B listens. When the channel is busy, in order to avoid data conflicts, Vehicle B adds a backoff process after the DIFS time of the listening channel. Vehicle B randomly generates a BackoffTime as the backoff count value based on the competition window size, and continues to listen at the same time. channel, if the continuous idle time of the listening channel reaches one time slot, the backoff count value will be decremented by 1; if the channel is busy during the listening process, the backoff count value will be frozen. When the backoff count value is reduced to 0, vehicle B will occupy the channel to send data. ;

According to the definition of backoff time, it can be expressed as:

BackoffTime＝Random()·CW·SlotTime (1)

Among them, Random() takes a random number from 0 to 1, CW is the current competition window size of the node, SlotTime represents a time slot size, BackoffTime is calculated according to the above formula and rounded, let CW _min be the minimum competition window value of the vehicle node, CW _max is the maximum competition window size, and the initial value of CW is CW _min . When the vehicle node fails to send data, it is considered that the network data traffic is large at this time, and the vehicle node multiplies the CW competition window value by 2, that is

Among them, m represents the number of data retransmissions. When the CW value reaches CW _max , the CW value remains unchanged; when the node successfully sends data, the CW value is set to CW _min ;

Consider the situation where there are n vehicle nodes within the communication range of a certain RSU. Assume that the probability of any vehicle node sending data in a random time slot is τ. Then the probability of the data colliding with other node data is η. When the number of nodes n is one, Timing, η always remains constant and independent, and the following relationship can be obtained:

η＝1-(1-τ) ^n-1 (3)

According to the data collision probability, the message delivery rate p of the vehicle node can be expressed as:

p＝1-η (4)

By simplifying formula (3), we can get,

According to the modeling of Markov chain, let W=CW _min , the relationship between τ and η can be derived:

The numerical solution to the nonlinear equation can be obtained by combining formula (5) and formula (6). When the maximum number of retransmissions m and the number of nodes n remain unchanged, the value of the competition window value W affects the data collision probability η and message delivery. rate p, assuming that the maximum number of retransmissions m is 7, the number of nodes n is 50, the relationship between the contention window value W and the data collision probability eta is shown in Figure 3, and the relationship between the contention window value W and the message delivery rate p is shown in Figure 4 .

It can be seen from Figure 3 and Figure 4 that when the network load is large, selecting a larger W can improve the network delivery rate of early warning message packets. Since there are a large number of vehicle nodes in the network, optimizing the delivery performance of vehicle nodes can effectively improve the entire network. Delivery rate. Therefore, in the process of reducing network load, increase the minimum competition window value of vehicle source nodes, reduce message collisions, and ensure that more vehicle nodes within its communication range receive safe messages. Secondly, establish a network average delay model and analyze performance Optimize ideas.

Transmission delay is the time between sending and receiving a message in the network. In the Internet of Vehicles, different messages have different latency requirements. As a typical delay-sensitive application, the event warning service has strict requirements on message transmission time. Since the Internet of Vehicles adopts Data communication is carried out in broadcast mode without the need for an ACK confirmation mechanism. The sending time T of a broadcast frame can be expressed as:

T＝t _DIFS +t _BO +t _Data +t _SIFS (7)

Among them, t _DIFS is the distributed inter-frame gap time that needs to be waited before sending data, t _BO is the back-off time waiting for the channel to be idle, if the channel is idle when sending data, there is no back-off time, the value is 0, and t _Data is The broadcast data frame sending time is a key part of the broadcast delay. t _SIFS is the short inter-frame gap time, used to separate each frame belonging to a conversation.

When the channel is busy during data transmission, the node randomly selects a backoff count value based on the current competition window value to calculate the backoff time to avoid message collisions; when there are a large number of broadcast messages in the network, the backoff count value is generally but,

Among them, slot represents a time slot size, which is brought into formula (7):

According to the definition of t _Data , t _Data can be expressed as:

Among them, L is the broadcast packet length, in bytes; R is the node sending rate, in Mb/s.

Putting formula (10) into formula (9), the simplified average delay formula can be obtained:

Since t _DIFS , slot, and t _SIFS are all fixed parameters of the MAC layer, they do not change during the data transmission process. Therefore, the broadcast frame sending time T is only related to the contention window value, broadcast packet length and node sending rate.

Therefore, for the safety message broadcast service, when the broadcast packet length remains unchanged, choosing a smaller W and a higher R can reduce the average transmission delay of the safety warning message. In terms of communication performance, RSU is far superior to vehicle nodes. Utilize the high-bandwidth characteristics of RSU to reduce its minimum competition window value to ensure that safety warning messages are sent in a shorter time and improve driving safety.

For different levels of security warning messages, the RSU broadcast mechanism sets different minimum competition window values. The minimum competition window value for urgent messages is set to 1/8 of the initial value, and the minimum competition window value for emergency messages is set to 1/1 of the initial value. 4. The minimum competition window value for general messages is set to 1/2 of the initial value, giving priority to ensuring the transmission performance of high-level messages.

In order to achieve the above-mentioned object of the invention, the technical solution adopted by the present invention is specifically: an Internet of Vehicles safety message broadcast method based on RSU collaboration, which is characterized by including the following steps:

Step 1: Same event filtering. When the RSU receives the safety warning message sent by the vehicle source node, it reads the level and geographical location information of the event in the group, judges its emergency safety type, and filters repeated broadcasts of the same traffic event.

Step two, unified event warning. After RSU filters the same event, it adaptively increases the message sending rate and reduces the minimum competition window value according to the emergency type of the event, and performs security warning within its communication range. When transmitting in this area, the neighbor vehicle node does not forward the packet after receiving the early warning broadcast from the vehicle source node and RSU to ensure the broadcast performance of the RSU in the entire network. When transmitted to the rear area, the safety message is sent to the rear RSU through wired technologies such as optical fiber, allowing it to broadcast within the communication range to ensure that the rear vehicle nodes receive early warning information in advance.

Step 3: Reduce the network load. When the vehicle source node receives the safety warning message broadcast by the RSU, it reads the grouped geographical location information to determine the event. If it is judged to be a safety message previously broadcast by the vehicle, it will adaptively reduce the message according to the emergency type of the event. The sending rate is increased, and the minimum contention window value is increased to continue broadcasting within its communication range.

Further, the same event filtering in step one specifically includes the following steps:

(1) Vehicle source nodes V _i and V _j that detect abnormal traffic events such as accidents or congestion generate safety warning messages Pi and _{P j respectively} ;

(2) The source nodes Vi _and V _j broadcast the safety warning messages Pi and _{P j to} the roadside unit RSU1 and neighbor vehicles V _k within the communication range;

(3) Within the sampling time t, after RSU1 receives the security warning messages Pi _and P _j (assuming that the _Pi message is received first), RSU1 reads the "geographic location" field values _{Pi_address} and P _j in the message group respectively. _address, filters duplicate messages for the same event;

Step1: RSU1 sets the same event distance judgment threshold D;

Step2: RSU1 subtracts P _i _address and P _j _address to calculate the distance d between events;

Step3: If d≤D, it is determined that the contents of the messages P _i and P _j are the same traffic event. RSU1 adds the first received safety warning message _Pi to the sending buffer queue Q _inte and discards other safety warning messages P _j ;

Step 4: If d>D, it is determined that the contents of the messages Pi and _{P j} are different traffic events, and RSU1 adds the _{two safety warning messages Pi and P j to} the sending buffer queue Q _inte .

Further, the unified early warning of events in step two specifically includes the following steps:

(1) Neighbor vehicle nodes receive safety warning messages Pi and P _{j and} do not broadcast and forward them;

(2) When RSU1 broadcasts the security warning message P _inte in the buffer queue Q _inte , it reads the "event level" field value P _{inte_level} in the message grouping to know the urgency of the security warning message P _inte . Assume that the initial value of the minimum contention window of the wireless MAC layer of RSU is W _R0 ;

Step1: For urgent events, RSU1 sets the minimum contention window of MAC layer CW _min =a ₁ *W _R0 , where a ₁ =1/8;

Step2: For emergencies, RSU1 sets the MAC layer minimum contention window CW _min =a ₂ *W _R0 , where a ₂ =1/4;

Step3: For general events, RSU1 sets the MAC layer minimum contention window CW _min =a ₃ *W _R0 , where a ₃ =1/2;

(3) After resetting the minimum contention window value of the MAC layer according to the message level, the RSU monitors the PBC sending rate R _V0 of the vehicle node, sets its own PBC sending rate to 2*R _V0 , and sends safety warning messages within its communication range;

Step1: When warning in this area, RSU1 sends the safety warning message P _inte to the vehicle nodes within its communication range through broadcast;

Step2: During the rear area warning, RSU1 sends the warning message to the rear RSU2 through optical fiber wired communication, and then RSU2 broadcasts the safety message _Pinte to the vehicles within its range.

Further, reducing the network load in step three specifically includes the following steps:

(1) The source nodes V _i and V _j receive the RSU1 broadcast warning message _Pinte . If it is determined to be a safety message previously broadcast by the vehicle, they will reduce the sending rate and increase the minimum competition window value, and continue to broadcast within their communication range. Suppose The initial value of the minimum contention window of the wireless MAC layer of the vehicle source node is W _V0 , and the initial value of the message sending rate is R _V0 ;

Step1: For urgent events, the vehicle node sets the MAC layer minimum competition window CW _min =b ₁ *W _V0 , where b ₁ =2, and sets the sending rate R _send =(1-c ₁ )*R _V0 , where c ₁ =0.3 ;

Step2: For emergency events, the vehicle node sets the MAC layer minimum competition window CW _min =b ₂ *W _V0 , where b ₂ =4, and sets the sending rate R _send =(1-c ₂ )*R _V0 , where c ₂ =0.5 ;

Step3: For general events, the vehicle node sets the MAC layer minimum competition window CW _min =b ₃ *W _V0 , where b ₃ =8, and sets the sending rate R _send =(1-c ₃ )*R _V0 , where c ₃ =0.8 ;

(2) After setting the minimum contention window value and transmission rate of the MAC layer, the source nodes V _i and V _j continue to broadcast within their communication range.

Compared with the prior art, the beneficial effects of the present invention are:

(1) Vehicular Edge Computing (VEC) of the present invention utilizes the edge infrastructure close to the Internet of Vehicles to provide computing, storage, transmission and other services nearby, which better solves the business needs of vehicle-mounted equipment for delay-sensitive applications. As a typical infrastructure, road side units (RSU) have rich computing resources and a wider transmission range, assisting vehicles in meeting application requirements and improving task execution efficiency; at the same time, optical fiber interconnected RSUs can serve as wired forwarding nodes. , expand the network connectivity area, stabilize the link quality, and ensure the successful transmission of early warning messages.

(2) The present invention is aimed at the business needs of safety warning messages in collaborative vehicle safety systems. First, the present invention designs a new broadcast grouping structure that adds event level and geographical location fields to the broadcast message to record the specific details of traffic safety events. information; secondly, a method of Internet of Vehicles safety message broadcasting based on RSU collaboration is proposed. In the same event filtering stage, the RAIB method uses RSU devices with rich computing and storage resources to filter redundant messages for the same event in the network, reducing the risk of The impact of redundant message collisions on transmission performance. In the event unified early warning stage, neighbor vehicles do not broadcast and forward the safety messages sent by the vehicle source node after receiving them. The RSU device filters the same events and then uniformly broadcasts the early warning, giving full play to the advantages of RSU communication performance. , in the network load reduction stage, the RAIB method reduces the transmission rate of vehicle nodes and increases the minimum competition window value, allowing it to broadcast early warnings within the communication range at a lower rate, reducing channel resource overhead and the broadcast load of the entire network; finally, through experiments It shows that this method can not only increase the scope of broadcast warning, improve the message delivery rate, but also reduce the transmission delay and meet the business needs of security warning.

Description of drawings

The drawings are used to provide a further understanding of the present invention and constitute a part of the specification. They are used to explain the present invention together with the embodiments of the present invention and do not constitute a limitation of the present invention.

Figure 1 is a schematic flowchart of the overall flow of the Internet of Vehicles safety message broadcast method based on RSU collaboration in the present invention.

Figure 2 is a schematic diagram of the broadcast packet structure designed in the present invention.

Figure 3 is a diagram showing the relationship between the contention window value W and the data collision probability eta when the maximum number of retransmissions and the number of nodes are fixed in the present invention.

Figure 4 is a diagram showing the relationship between the contention window value W and the message delivery rate p when the maximum number of retransmissions and the number of nodes are fixed in the present invention.

Figure 5 is a schematic diagram showing the performance comparison of the maximum number of notified vehicles before and after using the RAIB method in Experiment 1 in the embodiment of the present invention.

Figure 6 is a schematic diagram comparing the network delivery rate performance before and after using the RAIB method in Experiment 2 in the embodiment of the present invention.

Figure 7 is a schematic diagram showing the comparison of average transmission delay performance before and after using the RAIB method in Experiment 3 in the embodiment of the present invention.

Figure 8 is a topology diagram of vehicle broadcasts in the CVSS system of the present invention.

Figure 9 is a schematic diagram of the CSMA/CA mechanism used in the present invention to seize the channel and transmit data.

Detailed ways

In order to make the purpose, technical solutions and advantages of the present invention more clear, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Of course, the specific embodiments described here are only used to explain the present invention and are not intended to limit the present invention.

Example 1

Referring to Figures 1 to 9, the technical solution provided by the present invention is a method of broadcasting safety messages for Internet of Vehicles based on RSU collaboration. First, a network delivery rate model is established and performance optimization ideas are analyzed;

The CSMA/CA mechanism is the basic working method of the MAC layer in IEEE 802.11p. In order to avoid data conflicts during channel transmission, the binary exponential backoff method is used for message backoff and retransmission. Based on its principle, the vehicle node channel competition process is analyzed below. .

Figure 9 shows the process of two vehicle nodes competing for the channel to transmit data under the CSMA/CA mechanism, where DIFS is the distributed inter-frame time slot, SIFS is the short inter-frame space, and SlotTime represents a time slot. When vehicle A needs to When the wireless AP sends data, it first listens to the channel. When the channel idle time reaches a DIFS time, it is determined that the channel is idle at this time and data transmission begins. When the AP successfully receives data, it sends an ACK confirmation frame after a SIFS time to indicate the successful transmission of this message. As vehicle density increases, nodes compete with each other for channels to send data, and channels are prone to being busy. When vehicle A and vehicle B send data at the same time, because vehicle A occupies the channel to send data, the channel is busy when vehicle B listens to the channel. To avoid data conflicts, vehicle B adds a backoff process after the DIFS time of the listening channel. Vehicle B randomly generates a backoff time as the backoff count value based on the competition window size, and continues to monitor the channel. When the continuous idle time of the listening channel reaches one time slot, the backoff count value is decremented by 1; if the channel is busy during the monitoring process , freeze the rollback count value. When the backoff count decreases to 0, vehicle B occupies the channel to send data.

According to the definition of backoff time, it can be expressed as:

BackoffTime＝Random()·CW·SlotTime (1)

Among them, Random() takes a random number from 0 to 1, CW is the current competition window size of the node, SlotTime represents the size of a time slot, and BackoffTime is calculated according to the above formula and rounded. Let CW _min be the minimum competition window value of the vehicle node, CW _max be the maximum competition window size, and the initial value of CW is CW _min . When the vehicle node fails to send data, it is considered that the network data traffic is large at this time, and the vehicle node multiplies the CW competition window value by 2, that is,

Among them, m represents the number of data retransmissions. When the CW value reaches CW _max , the CW value remains unchanged. When the node successfully sends data, set the CW value to CW _min .

Consider the situation where there are n vehicle nodes within the communication range of a certain RSU. Assume that the probability of any vehicle node sending data in a random time slot is τ. Then the probability of the data colliding with other node data is η. When the number of nodes n is one, Timing, η always remains constant and independent, and the following relationship can be obtained:

η＝1-(1-τ) ^n-1 (3)

According to the data collision probability, the message delivery rate p of the vehicle node can be expressed as:

p＝1-η (4)

By simplifying formula (3), we can get,

According to the modeling of Markov chain, let W=CW _min , the relationship between τ and η can be derived:

The numerical solution of the nonlinear equation can be obtained by combining formula (5) and formula (6). When the maximum number of retransmissions m and the number of nodes n remain unchanged, the value of the competition window value W affects the data collision probability η and the message delivery rate p. Assume that the maximum number of retransmissions m is 7, the number of nodes n is 50, the relationship between the contention window value W and the data collision probability eta is shown in Figure 3, and the relationship between the contention window value W and the message delivery rate p is shown in Figure 4.

It can be seen from Figure 3 and Figure 4 that when the network load is large, selecting a larger W can improve the network delivery rate of early warning message packets. Since there are a large number of vehicle nodes in the network, optimizing the delivery performance of vehicle nodes can effectively improve the delivery rate of the entire network. Therefore, in the process of reducing the network load, the minimum competition window value of the vehicle source node is increased to reduce message collisions and ensure that more vehicle nodes within its communication range receive safety messages.

Secondly, a network average delay model is established to analyze performance optimization ideas.

Transmission delay is the time from when a message is sent on the network to when it is received. In the Internet of Vehicles, different messages have different latency requirements. As a typical latency-sensitive application, the event warning service has strict requirements on message transmission time. Since the Internet of Vehicles uses broadcast mode for data communication, there is no need for an ACK confirmation mechanism. The sending time T of a broadcast frame can be expressed as:

T＝t _DIFS +t _BO +t _Data +t _SIFS (7)

Among them, t _DIFS is the distributed inter-frame gap time to wait before sending data. t _BO is the backoff time for waiting for the channel to be idle. If the channel is idle when sending data, there is no backoff time and the value is 0. t _Data is the broadcast data frame sending time, which is a key part of the broadcast delay. t _SIFS is the short inter-frame gap time, used to separate each frame belonging to a conversation.

When the channel is detected to be busy during data transmission, the node randomly selects a backoff count value based on the current competition window value to calculate the backoff time to avoid message collisions. When there are a large number of broadcast messages in the network, the backoff count value is generally but

Among them, slot represents a time slot size, which is brought into formula (7):

According to the definition of t _Data , t _Data can be expressed as:

Among them, L is the broadcast packet length, in bytes; R is the node sending rate, in Mb/s.

Putting formula (10) into formula (9), the simplified average delay formula can be obtained:

Since t _DIFS , slot, and t _SIFS are all fixed parameters of the MAC layer, they do not change during the data transmission process. Therefore, the broadcast frame sending time T is only related to the contention window value, broadcast packet length and node sending rate.

Therefore, for the safety message broadcast service, when the broadcast packet length remains unchanged, choosing a smaller W and a higher R can reduce the average transmission delay of the safety warning message. In terms of communication performance, RSU is far superior to vehicle nodes. Utilize the high-bandwidth characteristics of RSU to reduce its minimum competition window value to ensure that safety warning messages are sent in a shorter time and improve driving safety.

For different levels of security warning messages, the RSU broadcast mechanism sets different minimum competition window values. The minimum competition window value for urgent messages is set to 1/8 of the initial value, and the minimum competition window value for emergency messages is set to 1/1 of the initial value. 4. The minimum competition window value for general messages is set to 1/2 of the initial value, giving priority to ensuring the transmission performance of high-level messages.

The present invention is realized through the following measures: an Internet of Vehicles safety message broadcast method based on RSU collaboration, including the following steps:

Step 1: Same event filtering. When the RSU receives the safety warning message sent by the vehicle source node, it reads the level and geographical location information of the event in the group, judges its emergency safety type, and filters repeated broadcasts of the same traffic event.

Step two, unified event warning. After RSU filters the same event, it adaptively increases the message sending rate and reduces the minimum competition window value according to the emergency type of the event, and performs security warning within its communication range. When transmitting in this area, the neighbor vehicle node does not forward the packet after receiving the early warning broadcast from the vehicle source node and RSU to ensure the broadcast performance of the RSU in the entire network. When transmitted to the rear area, the safety message is sent to the rear RSU through wired technologies such as optical fiber, allowing it to broadcast within the communication range to ensure that the rear vehicle nodes receive early warning information in advance.

Step 3: Reduce the network load. When the vehicle source node receives the safety warning message broadcast by the RSU, it reads the grouped geographical location information to determine the event. If it is judged to be a safety message previously broadcast by the vehicle, it will adaptively reduce the message according to the emergency type of the event. The sending rate is increased, and the minimum contention window value is increased to continue broadcasting within its communication range.

Further, the specific content of filtering the same events in step one is:

There are a large number of emergencies during traffic driving, and different types of events cause varying degrees of traffic hazards. According to the safety factor of the event, this information can be divided into three levels: critical events, emergency events and general events. Emergency events include traffic accidents and other events that seriously endanger personal safety. Emergency events include severe weather conditions and other events that cause travel danger. General events include traffic congestion and other events that affect the driving experience.

The safety warning service provided by the Internet of Vehicles can report traffic safety incidents throughout the entire network. Vehicle nodes can understand road conditions in advance and adjust driving strategies based on the safety warning messages received. Therefore, the present invention redesigns the broadcast packet structure, adds "event level" (1 byte) and "geographic location" (4 bytes) fields to the original data load area (load), and places them in the "PBC header" "After that, the designed grouping structure is shown in Figure 2. Among them, "event level" records the emergency level of the event, including three levels: urgent, emergency and general; "geographic location" is determined based on the GPS system in the vehicle. Due to the high speed of highway vehicles, lane distance has a greater impact on the distance between vehicles. Small, only the one-dimensional coordinate information of the source node when detecting traffic events is recorded here.

(1) Vehicle source nodes V _i and V _j that detect abnormal traffic events such as accidents or congestion generate safety warning messages Pi and _{P j respectively} ;

(2) The source nodes Vi _and V _j broadcast the safety warning messages Pi and _{P j to} the roadside unit RSU1 and neighbor vehicles V _k within the communication range;

(3) Within the sampling time t, after RSU1 receives the security warning messages Pi _and P _j (assuming that the _Pi message is received first), RSU1 reads the "geographic location" field values _{Pi_address} and P _j in the message group respectively. _address, filters duplicate messages for the same event;

Step1: RSU1 sets the same event distance judgment threshold D;

Step2: RSU1 subtracts P _i _address and P _j _address to calculate the distance d between events;

Step3: If d≤D, it is determined that the contents of the messages P _i and P _j are the same traffic event. RSU1 adds the first received safety warning message _Pi to the sending buffer queue Q _inte and discards other safety warning messages P _j ;

Step 4: If d>D, it is determined that the contents of the messages Pi and _{P j} are different traffic events, and RSU1 adds the two safety warning messages Pi and _{P j to} the sending buffer queue Q _inte .

Further, the specific content of the unified early warning of events in step two is:

The broadcast performance of the traditional periodic broadcast method is low, largely due to the large amount of redundant data in the network, and vehicle nodes competing with each other for channel resources, resulting in the loss of message packets. After filtering the same security warning messages, the event unified warning stage reduces message forwarding at vehicle nodes and the RSU performs unified broadcast warnings, reducing a large amount of redundant data in the network, easing fierce channel competition, and improving packet delivery rate.

Although multi-hop assisted transmission in the Internet of Vehicles can improve the message transmission range, multiple forwardings of intermediate vehicles generate a large amount of redundant data. The safe message broadcast method of the Internet of Vehicles based on RSU collaboration avoids the large amount of redundant data generated by vehicle forwarding, and in safety During early warning, RSUs with wide communication range are used to assist in forwarding, and intermediate vehicles do not forward messages. Due to the reduction of a large number of data packets in the network, the new method improves communication resource utilization while ensuring message reliability.

Although the communication range of RSU is much larger than that of vehicle nodes, the communication range of RSU is still limited. For the long-distance early warning messages, high-speed wired communication transmission between RSUs can be used.

(1) Neighbor vehicle nodes receive safety warning messages Pi and P _{j and} do not broadcast and forward them;

(2) When RSU1 broadcasts the security warning message P _inte in the buffer queue Q _inte , it reads the "event level" field value P _{inte_level} in the message grouping to know the urgency of the security warning message P _inte . Assume that the initial value of the minimum contention window of the wireless MAC layer of RSU is W _R0 ;

Step1: For urgent events, RSU1 sets the minimum contention window of MAC layer CW _min =a ₁ *W _R0 , where a ₁ =1/8;

Step2: For emergencies, RSU1 sets the MAC layer minimum contention window CW _min =a ₂ *W _R0 , where a ₂ =1/4;

Step3: For general events, RSU1 sets the MAC layer minimum contention window CW _min =a ₃ *W _R0 , where a ₃ =1/2;

(3) After resetting the minimum contention window value of the MAC layer according to the message level, the RSU monitors the PBC sending rate R _V0 of the vehicle node, sets its own PBC sending rate to 2*R _V0 , and sends safety warning messages within its communication range;

Step1: When warning in this area, RSU1 sends the safety warning message P _inte to the vehicle nodes within its communication range through broadcast;

Step2: During the rear area warning, RSU1 sends the warning message to the rear RSU2 through optical fiber wired communication, and then RSU2 broadcasts the safety message _Pinte to the vehicles within its range.

Further, the specific content of reducing the network load in step three is:

The unified event warning stage mainly optimizes the security warning messages during the transmission process, while the network load reduction stage optimizes the security warning messages at the source node. After detecting a security event, the source node periodically broadcasts warnings within its communication range. Since the RSU has assisted in broadcasting warnings during the unified warning phase of the event, the source node's sending rate can be reduced and the minimum competition window value can be increased to reserve channel communication. resources to ensure the broadcast performance of RSU.

(1) The source nodes V _i and V _j receive the RSU1 broadcast warning message _Pinte . If it is determined to be a safety message previously broadcast by the vehicle, they will reduce the sending rate and increase the minimum competition window value, and continue to broadcast within their communication range. Suppose The initial value of the minimum contention window of the wireless MAC layer of the vehicle source node is W _V0 , and the initial value of the message sending rate is R _V0 ;

Step1: For urgent events, the vehicle node sets the MAC layer minimum competition window CW _min =b ₁ *W _V0 , where b ₁ =2, and sets the sending rate R _send =(1-c ₁ )*R _V0 , where c ₁ =0.3 ;

Step2: For emergency events, the vehicle node sets the MAC layer minimum competition window CW _min =b ₂ *W _V0 , where b ₂ =4, and sets the sending rate R _send =(1-c ₂ )*R _V0 , where c ₂ =0.5 ;

Step3: For general events, the vehicle node sets the MAC layer minimum competition window CW _min =b ₃ *W _V0 , where b ₃ =8, and sets the sending rate R _send =(1-c ₃ )*R _V0 , where c ₃ =0.8 ;

(2) After setting the minimum contention window value and transmission rate of the MAC layer, the source nodes V _i and V _j continue to broadcast within their communication range.

In order to analyze and evaluate the effectiveness of the Internet of Vehicles safety message broadcast method based on RSU collaboration, it is compared with the periodic broadcast method, and the performance is compared from three indicators: the maximum number of notified vehicles, broadcast delivery rate and average delay.

Experiment 1: Performance comparison of maximum number of notified vehicles

Experimental environment: Set up a two-lane traffic environment, with 250 vehicle nodes placed in each lane. Different vehicle density situations are simulated by changing the distance between vehicle nodes, and the distances between vehicle nodes are set to 10m, 15m, 20m, 30m and 40m respectively.

The comparison of the maximum number of notified vehicles between the periodic broadcast method and the RAIB method is shown in Figure 5. As the vehicle density decreases, the average distance between vehicles increases, and the average number of vehicles within the communication range decreases. The periodic broadcast method and the RAIB method broadcast once The maximum number of vehicles that can be notified is also gradually reduced. Comparing the specific data of the RAIB method and the periodic broadcast method under different vehicle densities, when the distance between vehicles is 10m, the RAIB method can notify up to 402 vehicles in one broadcast, while the periodic broadcast method can only notify 119 vehicles. When the distance between vehicles is 35m, the RAIB method can notify up to 116 vehicles at a time, while the periodic broadcast method can only notify 33 vehicles. It is proved that the RAIB method has a larger message warning range and more vehicles are notified.

Experiment 2: Broadcast Delivery Rate Performance Comparison

Experimental environment: Construct a traffic scene with 76 vehicles evenly placed in four lanes, that is, 19 vehicles in each lane, and the distance between vehicles is 15m. Different levels of communication pressure are set by changing the number of packets sent by vehicle nodes per second. 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 performance comparison of delivery rates between the periodic broadcast method and the RAIB method is shown in Figure 6.

When using traditional periodic broadcast warnings, when vehicle nodes send 15 packets per second, the delivery rate in the network is 88.68%. As the number of vehicle nodes sent increases, the network load increases. When the vehicle node sends 20 packets per second, the network delivery rate drops to 66.40%. A large number of early warning messages are lost during transmission, making it impossible to accurately warn of emergencies and threatening traffic safety.

When using the RAIB method to broadcast an early warning, the RSU monitors the PBC sending rate R _V0 of the vehicle node and sets its own PBC sending rate to 2*R _V0 , that is, when the vehicle node sends 10 packets per second, the RSU sends 20 packets per second. The RAIB method uses RSU to judge repeated broadcast warning events, filters redundant packets in network links, and improves broadcast efficiency while ensuring message transmission. When the vehicle node sends 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 only drops to 92.06%. Compared with the traditional periodic broadcast method, when the vehicle node sends 15 and 20 packets per second, the delivery performance is optimized by 8.84% and 25.66% respectively. It shows that the emergency warning message broadcast by the RAIB method has a higher message reception rate. More vehicles within the communication range can avoid risks based on the message warning, making traffic safer.

Experiment 3: Comparison of average broadcast delay performance

Experimental environment: Construct a traffic scene with 76 vehicles evenly placed in four lanes, that is, 19 vehicles in each lane, and the distance between vehicles is 15m.

The notification speed of traffic warning messages is particularly important in safe driving. The sooner the driver receives the message warning, the more time he has to adjust his driving strategy. In the RAIB method, the RSU adaptively adjusts the sending rate and competition window value of itself and vehicle nodes according to the emergency level of the event, thereby reducing the transmission delay of the early warning message. The delay comparison between the periodic broadcast method and the RAIB method is shown in Figure 7.

When using traditional periodic broadcast warnings, when vehicle nodes send 15 packets per second, the average delay for vehicle nodes to send messages is 7.905ms. As the number of vehicle nodes sent increases, the network load increases. When the vehicle node sends 20 packets per second, the average delay of the vehicle node sending messages is 10.905ms.

When using the RAIB method to broadcast warnings, when the vehicle node sends 15 packets per second, the average delay of the vehicle node is 6.671ms. As the number of vehicle nodes sent increases, the network load increases. When the vehicle node sends 20 packets per second, the average delay of the vehicle node is 7.255ms. Compared with vehicle nodes in the traditional periodic broadcast method, the average delay performance of the RAIB method is optimized by 15.6% and 33.47% respectively when the vehicle node sends 15 and 20 packets per second. When using the RAIB method to broadcast warnings, when the vehicle node sends 15 packets per second, the average delay of the RSU device is 5.767ms. As the number of vehicle nodes sent increases, the network load increases. When the vehicle node sends 20 packets per second, the average delay of the RSU device is 6.021ms. Compared with the vehicle nodes in the traditional periodic broadcast method, the RAIB method optimizes the average delay performance by 27.05% and 44.79% respectively when the vehicle nodes send 15 and 20 packets per second. It can be seen that the RAIB method optimizes the transmission delay broadcast by both RSU equipment and vehicle nodes. Among them, the delay optimization of RSU equipment is more obvious, ensuring that early warning messages are successfully transmitted in a shorter time.

The above are only preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention shall be included in the protection of the present invention. within the range.

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.