A communication system and protocol method for a networked intelligent intersection without traffic lights

Abstract

The present invention discloses a communication system and protocol method for a networked intelligent intersection without traffic lights. The present invention combines the CSMA / CA channel listening and conflict avoidance ideas with the time division multiple access method in VeMAC to construct a CVMAC protocol that can dynamically adjust the working mode according to the channel busyness, solving the problem of node information access conflict; CVMAC determines the protocol type according to the estimated channel node access density, uses CSMA / CA protocol scheduling when the density is low, and adopts the VeMAC protocol mode for transmission when the density is high; the present invention also builds a MAC protocol prototype verification system based on the Raspberry Pi platform to simulate and verify the scenario, and through the statistics of a large number of data packets, it can calculate the bad packet rate, communication delay, and vehicle throughput. The present invention can effectively solve the data conflict problem when high-density vehicle nodes in networked intelligent intersections without traffic lights access the dispatching center, and can greatly reduce the cost of physical scenario verification through the simulation of the scenario.

Description

Technical Field

[0001] The present invention relates to the field of vehicle networking technology, and primarily to a communication system and protocol method for a networked intelligent intersection without traffic lights. Background Art

[0002] With the rapid development of technologies like autonomous driving and the Internet of Vehicles (IoV), traffic light-free intersection scheduling scenarios have attracted widespread attention and research within the field of intelligent transportation systems. Solving the problem of collision-free scheduling at intersections without traffic lights relies not only on the intelligent perception and decision-making capabilities of individual vehicles but also heavily on vehicle-infrastructure collaboration (V2X) technology based on vehicle-to-vehicle communication. By leveraging onboard communication systems to achieve V2X collaboration, the intersection scheduling control center collects driving information from surrounding vehicles, calculates the optimal driving parameters for each vehicle through a specific algorithm, and transmits this information back to the dispatched vehicles, thus achieving collision-free scheduling at the intersection.

[0003] Vehicular communication systems consist of vehicles and roadside units. Communication nodes provide each other with information such as safety warnings and traffic data, forming a computer network that enables intelligent transportation. In the early 21st century, vehicular ad hoc networks (VANETs) emerged as a specialized type of mobile ad hoc network (MANET) within communication system applications. These systems utilize vehicle-to-vehicle (V2V) and vehicle-to-infrastructure (V2I) data exchange through vehicle relays. Their primary goals are to improve road safety, enhance road traffic monitoring and management, and extend high-quality data services to the connected vehicle network.

[0004] Communications within VANETs include dedicated short-range communications (DSRC). The USDOT's Intelligent Transportation Systems (ITS) document describes a DSRC-based vehicle communication system. In this system, vehicles are equipped with onboard units (OBUs) whose transceivers are integrated with GPS and other sensors. Roadside units (RSUs) are placed along the road as infrastructure. The 5.9 GHz Industrial, Scientific, and Medical (ISM) band provides a dedicated 75 MHz bandwidth for DSRC, which has six service channels (SCHs) and one control channel (CCH). System construction requires a high level of infrastructure and high node penetration.

[0005] MAC protocols are responsible for allocating wireless resources between vehicles. Currently, many protocols are being developed to standardize MAC for VANETs. These protocols share a common goal: to address the real-world challenges of high vehicle node mobility, rapidly changing network topologies, stringent latency requirements, and high QoS demands. For example, IEEE 802.11P, introduced in the "IEEE 802.11 Standard for Information Technology," uses Carrier Sense Multiple Access (CSMA / CA), a contention-based access mechanism. In the event of a busy channel or collision, a random backoff procedure is used, with data transmitted only when the medium is idle. However, it does not utilize the Request to Send / Clear to Send (RTS / CTS) mechanism to reduce hidden nodes. Furthermore, the paper "VeMAC: A TDMA-based MAC Protocol for Reliable Broadcast in VANETs" proposes a distributed TDMA-based VeMAC protocol. VeMAC is a multi-channel protocol using two transceivers: one for the CCH and one for the SCH. Different TDMA time slots are assigned to different directions on the road to reduce the probability of time slot collisions. The time synchronization of the frame is completed by the GPS 1PPS signal. The broadcast information of the idle channel is adaptively determined by the vehicle single-hop set and two-hop set. The vehicle node selects the appropriate idle time slot to send data information. If it finds that all the time slots of the nodes moving in its direction are occupied, it will try to access the available time slots in the opposite direction.

[0006] In summary, the problems existing in the existing technology include: 1) Communication protocols based solely on the random access method competition mechanism cannot adapt to scenarios with high-density node access, resulting in large communication delays and high probability of data packet collisions; 2) The VeMAC protocol based on distributed TDMA will cause waste of idle time slots when dealing with low-complexity node access scenarios, thereby reducing the system's transmission efficiency; 3) The cost of the on-board communication system is high, and the verification of the communication protocol physical platform is difficult; 4) It is difficult to implement a networked intelligent intersection without traffic lights, and there is an urgent need for a design method for an on-board communication system that is highly scalable and easy to transplant. Summary of the Invention

[0007] Purpose of the invention: In response to the problems existing in the above-mentioned background technology, the present invention provides a communication system and protocol method for a networked intelligent intersection without traffic lights, designs an efficient CVMAC transmission protocol, and further builds a system platform suitable for semi-physical verification. It conducts inter-protocol verification and analysis on the scenario of the intersection without traffic lights, ensuring the stable and efficient operation of the scheduling of the networked intelligent intersection without traffic lights.

[0008] Technical solution: To achieve the above-mentioned purpose, the present invention provides a communication system for a networked intelligent intersection without traffic lights, including a vehicle node and a dispatching center node. The vehicle node and the dispatching center node complete data transmission in accordance with the CVMAC protocol; the CVMAC protocol combines the CSMA / CA and VeMAC protocols, and determines the protocol type according to the estimated channel node access density. When the density is low, the CSMA / CA protocol scheduling is used, and when the density is high, the VeMAC protocol mode is adopted for transmission; the high and low access density distinction standard is determined according to the number of vehicles calculated when the throughput of the two protocols is equivalent. When the actual number of access vehicles exceeds the vehicle number standard, the access density is considered to be high, otherwise the access density is considered to be low.

[0009] Furthermore, the throughput of the CSMA / CA protocol Throughput of the core TDMA protocol of VeMAC design Where n represents the number of vehicles, p represents the transmission probability, s represents the packet transmission delay, d represents the DIFS transmission delay, and R represents the number of lanes.

[0010] Specifically, the number of vehicles that switch the protocol is

[0011] Furthermore, the vehicle node is provided with the following processing unit:

[0012] The listening unit is used to listen to whether a channel idle signal is received when the dispatch demand is triggered at the intersection, parse the access density information in the packet when a channel idle data packet is received, and determine the protocol type based on the density;

[0013] The CSMA / CA protocol scheduling unit is used to send a data packet containing its own vehicle information and demand information to the scheduling center node when the access density is low, and receive feedback from the scheduling center node. If the request fails, it uses the collision avoidance algorithm to resend the request and add the number of failures to the data packet. The failure number is used by the scheduling center node to determine whether the data of the conflicting vehicle nodes has intensified, so as to determine whether to calculate the number of access vehicle nodes and the vehicle number standard for switching protocols, thereby determining whether to change the transmission protocol type.

[0014] In addition, the VeMAC protocol transmission unit is used to receive the idle time slot identifier sent by the dispatching center node when the access density is high, select the time slot adapted to its own frame according to the road direction of its own vehicle, and send information to the dispatching center node, and add the number of times the vehicle node failed to avoid selecting the time slot request into the data packet and feed it back to the dispatching center node, so that the dispatching center node can determine whether to dynamically adjust the width of the time slot in each direction under the time slot frame.

[0015] Furthermore, in the CSMA / CA protocol scheduling unit, a binary exponential backoff algorithm is used to handle access conflicts. When the number of vehicle nodes whose request failure times reach a set threshold increases, the scheduling center is triggered to calculate the vehicle number standard for switching from the CSMA / CA protocol to the VeMAC protocol. When the scheduling center node uses the VeMAC protocol for transmission, it statistically averages the time slot occupancy rates within multiple groups of frame periods. If the time slot occupancy rate is less than the set threshold, the VeMAC protocol is switched to the CSMA / CA protocol to transmit data.

[0016] Furthermore, the dispatch center node is provided with the following processing units:

[0017] An access density analysis unit is used to analyze the conflict avoidance information or channel utilization information in the data packets of existing vehicle nodes, obtain the vehicle node access density, and send the access density information to the vehicle nodes to guide the vehicle nodes to adopt the CSMA / CA protocol or VeMAC protocol transmission mode;

[0018] And, a vehicle demand processing unit is used to parse the demand information in the data packet of the vehicle node, calculate the time when the vehicle enters the intersection, and send it to the vehicle node waiting to receive the time information packet.

[0019] The present invention also provides a prototype verification system for a communication system of a networked intelligent intersection without traffic lights. The prototype verification system is built on a Raspberry Pi platform and uses road vehicle nodes, dispatching center nodes, and time slot synchronization nodes to simulate real intersection scenarios. The road vehicle nodes are used to randomly generate vehicles in various directions to simulate vehicle nodes arriving at the intersection, the dispatching center nodes are used to simulate roadside computing units, and the time slot synchronization nodes are used to simulate the time slot synchronization function of outdoor GPS modules. The road vehicle nodes and the dispatching center nodes complete data transmission in accordance with the CVMAC protocol. The CVMAC protocol combines the CSMA / CA and VeMAC protocols, and determines the protocol type based on the estimated channel node access density. When the density is low, the CSMA / CA protocol is used for scheduling, and when the density is high, the VeMAC protocol mode is used for transmission. The standard for distinguishing between high and low access density is determined based on the number of vehicles calculated when the throughput of the two protocols is equivalent. When the actual number of access vehicles exceeds the vehicle number standard, the access density is considered to be high, otherwise the access density is considered to be low.

[0020] The present invention also provides a vehicle node communication protocol method for a networked intelligent intersection without traffic lights, wherein the vehicle node and the dispatch center node complete data transmission according to the CVMAC protocol; the CVMAC protocol determines the protocol type based on the estimated channel node access density, uses the CSMA / CA protocol scheduling when the density is low, and adopts the VeMAC protocol mode for transmission when the density is high; wherein the high and low access density distinction standard is determined based on the number of vehicles calculated when the throughput of the two protocols is equivalent; when the actual number of access vehicles exceeds the vehicle number standard, the access density is considered high; otherwise, the access density is considered low; the vehicle node communication protocol process includes the following steps:

[0021] The vehicle node that reaches the intersection and triggers the dispatching demand listens to see if it receives the channel idle signal;

[0022] After receiving the channel idle data packet, the vehicle node parses the data packet. If it determines that the access density is low, the vehicle node sends a data packet containing its own vehicle information and demand information to the dispatch center node. If the information request is successful and an ACK feedback is received from the dispatch center node, the transmission is completed. If the request fails, the collision avoidance algorithm is used to send the request again, and the number of failures is added to the data packet. The number of failures is used by the dispatch center node to determine whether the data of the conflicting vehicle nodes has intensified, so as to determine whether to calculate the number of access vehicle nodes and the vehicle number standard for switching protocols, thereby determining whether to change the transmission protocol type.

[0023] After receiving a channel idle packet, the vehicle node parses the packet. If it detects high access density, it receives the idle time slot identifier from the dispatch center node based on the VeMAC protocol. It then selects a time slot that matches its own frame based on the vehicle's road direction and sends this information to the dispatch center node. The vehicle node also adds the number of times it has avoided failed time slot requests to the packet and feeds it back to the dispatch center node, allowing the dispatch center node to determine whether to dynamically adjust the width of the time slots in each direction within the time slot frame.

[0024] Furthermore, when using CSMA / CA protocol scheduling, a binary exponential backoff algorithm is adopted to handle access conflicts. When the number of vehicle nodes whose request failure times reach the set threshold increases, the scheduling center is triggered to calculate the vehicle number standard for switching from CSMA / CA protocol to VeMAC protocol. When using VeMAC protocol transmission, the time slot occupancy rate within multiple groups of frame periods is statistically averaged. If the time slot occupancy rate is less than the set threshold, the VeMAC protocol is switched to CSMA / CA protocol for data transmission.

[0025] The present invention also provides a dispatch center node communication protocol method for a networked intelligent intersection without traffic lights, wherein the dispatch center node and the vehicle node complete data transmission according to the CVMAC protocol; the CVMAC protocol determines the protocol type based on the estimated channel node access density, using CSMA / CA protocol scheduling when the density is low, and adopting the VeMAC protocol mode for transmission when the density is high; wherein the high and low access density distinction standard is determined based on the number of vehicles calculated when the throughput of the two protocols is equivalent; the dispatch center node communication protocol process includes the following steps:

[0026] The dispatch center node analyzes the conflict avoidance information or channel utilization information in the data packets of existing vehicle nodes to obtain the vehicle node access density. When the access density is low, the dispatch center node adds the low access density information to the channel idle signal to guide the vehicle nodes to adopt the CSMA / CA protocol transmission mode. When the access density is high, the dispatch center node adopts the VeMAC protocol transmission mode, broadcasts the idle time slots in all directions to the vehicle nodes, and adds the high access density information to the channel idle signal to guide the nodes to adopt the VeMAC protocol transmission mode.

[0027] The dispatcher parses the demand data packet of the vehicle node, calculates the time when the vehicle enters the intersection, and sends it to the vehicle node waiting to receive the time information packet.

[0028] Beneficial effects: The present invention designs a polymorphic CVMAC protocol, combining the CSMA / CA channel listening and backoff ideas with the time division multiple access method in VeMAC, which maximizes the efficiency of node communication when the node access density is low, and effectively reduces the communication delay. When the node access density is high, the VeMAC protocol is used for transmission, and the dynamically updated time slots, the allocation of idle time slots in other directions, and the competitive time slot acquisition method effectively improve the node transmission success rate; in addition, the present invention designs a personalized communication prototype system for the de-traffic light networked intelligent intersection scenario, which has an open link layer verification platform, and can verify the performance of the protocol through simulation of the scenario, greatly reducing the physical scenario verification cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 It is a schematic diagram of the node structure and protocol design of the scheduling prototype system in an embodiment of the present invention.

[0030] Figure 2 This is a schematic diagram of the design time slots of the CVMAC protocol in different scenarios in an embodiment of the present invention.

[0031] Figure 3 This is a flow chart of the vehicle node execution protocol in an embodiment of the present invention.

[0032] Figure 4This is a flowchart of the execution of the scheduling center node in an embodiment of the present invention.

[0033] Figure 5 2 is a schematic diagram of analyzing the protocol bad packet rate in an embodiment of the present invention.

[0034] Figure 6 4 is a schematic diagram of protocol delay analysis in an embodiment of the present invention.

[0035] Figure 7 4 is a schematic diagram of protocol throughput analysis in an embodiment of the present invention. DETAILED DESCRIPTION

[0036] In order to make the objectives, technical solutions, and advantages of the present invention more clearly understood, the following detailed description of an embodiment of the present invention is given in conjunction with the accompanying drawings. This embodiment is implemented based on the technical solutions of the present invention, and provides a detailed implementation method and specific operation process. It should be understood that the specific examples described herein are only used to illustrate the present invention, and the scope of protection of the present invention is not limited to the following embodiments.

[0037] An embodiment of the present invention discloses a communication system for a networked intelligent intersection without traffic lights, mainly including vehicle nodes and a dispatch center node. The vehicle nodes and the dispatch center node complete data transmission according to the CVMAC protocol. The CVMAC protocol combines the CSMA / CA and VeMAC protocols, and determines the protocol type according to the estimated channel node access density. When the density is low, the CSMA / CA protocol scheduling is used, and when the density is high, the VeMAC protocol mode is adopted for transmission. Here, the standard for distinguishing between high and low access density is determined based on the number of vehicles calculated when the throughput of the two protocols is equivalent. When the actual number of access vehicles exceeds the vehicle number standard, the access density is considered to be high; otherwise, the access density is considered to be low.

[0038] Specifically, use G CSMA represents the throughput of the CSMA / CA protocol, n represents the number of vehicles, p represents the transmission probability, s represents the packet transmission delay, d represents the DIFS transmission delay, S represents the total amount of data transmitted, T all Indicates the transmission delay of the protocol:

[0039]

[0040] G TDMA represents the throughput of the core TDMA protocol of the VeMAC design, and R represents the number of lanes:

[0041]

[0042] When the two agreements are equal, the number of vehicles to be switched can be calculated:

[0043]

[0044] The detailed implementation process of the embodiment of the present invention is described below with reference to specific experimental scenarios.

[0045] This embodiment adopts a four-way four-lane intersection scenario, such as Figure 1 At each intersection, vehicles can choose to turn left or go straight (right turns have dedicated lanes and will not cause collisions, so they are not considered). Moreover, the left turn of the vehicle is an arc curve, which is more in line with reality. In order to build a similar communication verification system, the road vehicle node, the dispatch center node, and the time slot synchronization node are used to simulate the real intersection scene. The road vehicle node is used to randomly generate vehicles in various directions to simulate the vehicle nodes arriving at the intersection. The dispatch center node is used to simulate the roadside computing unit. The time slot synchronization node is used to simulate the time slot synchronization function of the outdoor GPS module. The specific steps are as follows:

[0046] Step 1: Build a communication node prototype verification system based on the Raspberry Pi platform, debug the RF communication module, GPS, and DS3231 module functions, and exchange information through the SPI protocol, UART protocol, and operating system.

[0047] Step 2: Modify the lower computer's underlying code, initialize the communication module parameters, select the corresponding communication channel (Pipe), set the channel (Channel) and transmission data rate (DataRate) and other information. In an ideal physical scenario, the data transmission time is 20-25 milliseconds. Initialize the GPS module and DS3231 module, and verify the synchronization of the indoor simulated GPS signal and the outdoor 1PPS time. The GPS synchronization accuracy is maintained within 50ns, and the indoor simulated GPS synchronization is maintained within 1 millisecond. The error increase can be reduced according to real-time synchronization.

[0048] In order to ensure the quality of communication system design, steps one and two are to first debug the modules of each node.

[0049] Communication module: The NRF module connects to the Raspberry Pi via the SPI protocol. The initialization data settings are as follows:

[0050] Table 1 Communication module data settings

[0051]

[0052] Time Slot Synchronization Module: The outdoor GPS module extracts time = sdata[1][0:2] + sdata[1][2:4] + sdata[1][4:6] through the serial port connection, collecting the 1PPS time synchronization signal. This module and method are used to extract the time signal when the system is outdoors. Indoors, a time synchronization node automatically calculates the clock signal and sends the time content to each node by determining the synchronization time slot. Each node uses the DS3231 high-precision clock synchronization module to collect the time slot accumulation information time_data = ds3231.read_datetime() to achieve time slot sub-synchronization of each node.

[0053] Step 3: Design the data packet format for vehicle nodes, dispatch center nodes, and time slot synchronization nodes. The vehicle node data frame structure includes a header address (Header_ADDR), which is mainly used to confirm the channel of the data packet and provide transmission permissions; the vehicle data packet (Vehicle_Data), which mainly carries the information of the vehicle itself, which is the generated vehicle information in this scenario; the request data packet (Request_Data), which mainly contains the target direction of the vehicle and the channel busyness information generated by the MAC protocol that needs to be carried to the dispatch center. The last section of information is used to add security verification information (Security_Message) to prevent external interference. The structure is as follows:

[0054] Table 2 Data frame structure

[0055] Header_ADDR Vehicle_Data Request_Data Security_Message

[0056] In addition to the normal data packet transmission timeframe, the scheduling node also needs to broadcast the idle timeframe, broadcasting the current time slot allocation status and the status of reserved time slots. The time synchronization node mainly verifies the header address data, matches the time information packet with the security packet, and ensures high-quality real-time synchronization of the time slots.

[0057] Step 4: Design a personalized link layer CVMAC protocol, and coordinate the vehicle nodes, dispatch center nodes, and time synchronization (in actual smart intersection scenarios, where GPS signals are good, time synchronization does not require special consideration) to complete data transmission.

[0058] like Figure 2 As shown in (a) and (b), facing a four-way four-lane intersection, the CVMAC protocol access method is adopted, which combines CSMA / CA channel listening and binary exponential backoff ideas with VeMAC time division multiple access method. Figure 2 (a) shows that vehicles from four directions access in sequence. If there is an access conflict between vehicles A and C, they will access in a later time slot according to the backoff idea. Figure 2(b) follows the principle of time division multiple access (TDMA). Time frames are pre-set, with 25mm time slots set for each of the four directions, resulting in a frame size of 100mm. Time synchronization signals are checked every 10 frames. Vehicles arriving from each direction receive channel status information every three frames and select the closest frame and time slot for transmission.

[0059] Supplement: If the CVMAC protocol is used for a four-way eight-lane scenario, then the time division multiple access is as follows Figure 2 As shown in (c) and (d), in low-density access, simplifying the time slot distribution of four-way eight-lane to four-way four-lane time slot can greatly save time redundancy in a larger time frame. When lanes in the same direction compete for time slots in one direction at the same time, they can temporarily occupy idle time slots in other directions.

[0060] To identify the performance dividing line between CSMA / CA and VeMAC, we derived mathematical models for the two protocols and found that the performance dividing line is when the throughput of the CSMA / CA protocol and the VeMAC protocol are similar. When the number of vehicles is less than a certain number n, the CSMA / CA protocol is suitable for the two protocols. When the number of vehicles is greater than n, the VeMAC protocol performs better.

[0061] The transmission probability p can be expressed as the minimum value of the contention window CW min To calculate, it is

[0062]

[0063] Then the transmission success rate and collision rate of the p-persistent CSMA model can be expressed as

[0064]

[0065] P c =1-P s

[0066] Since the transmission speed is uniform, it can be omitted. The idle time T before successful transmission idle

[0067]

[0068] The time loss caused by the collision can be expressed as

[0069]

[0070] The transmission throughput of the CSMA / CA model is expressed as the maximum available data load divided by the average successful transmission time. The successful transmission time also includes the transmission delay of the successfully sent data packet, the idle time between each successful transmission, and the DIFS (Distributed Inter-frame Spacing) delay of the listening. The throughput of the CSMA / CA protocol can be expressed as G CSMA express:

[0071]

[0072] This protocol also uses the core TDMA protocol concept of the VeMAC protocol. The number of lanes is 4 in 4 directions, so 4 small slots are set in 1 large time slot as the transmission interval for vehicles arriving from different directions:

[0073]

[0074] Through the performance analysis of the two protocols, it is found that when the throughput of the CSMA / CA protocol and the VeMAC protocol is close, it is the dividing point between the two performances. It can be deduced that when the number of vehicles reaches a certain level, using different protocols can maximize the benefits.

[0075] G CSMA =G TDMA

[0076] Get the number of vehicles switching the protocol n

[0077]

[0078] Specifically, the vehicle node is provided with a listening unit, a CSMA / CA protocol scheduling unit and a VeMAC protocol transmission unit, which are used to realize data transmission with the scheduling center node. Among them, the listening unit is used to listen for the reception of a channel idle signal when arriving at an intersection to trigger a dispatch demand. When receiving a channel idle data packet, it parses the access density information in the packet and determines the protocol type based on the density. The CSMA / CA protocol scheduling unit is used to send a data packet containing its own vehicle information and demand information to the dispatch center node when the access density is low, and receive feedback from the dispatch center node. If the request fails, it uses the collision avoidance algorithm to resend the request and add the number of failures to the data packet. The failure number is used by the dispatch center node to determine whether the data of the conflicting vehicle nodes has intensified, so as to determine whether to calculate the number of access vehicle nodes and the vehicle number standard for switching protocols, thereby determining whether to change the transmission protocol type. The VeMAC protocol transmission unit is used to receive the idle time slot identifier sent by the dispatch center node when the access density is high, select a time slot adapted to its own frame based on the road direction of its own vehicle, and send information to the dispatch center node. The number of times the vehicle node failed to select a time slot request is avoided is added to the data packet and fed back to the dispatch center node so that the dispatch center node can determine whether to dynamically adjust the width of the time slot in each direction of the time slot frame.

[0079] The dispatch center node is equipped with an access density analysis unit and a vehicle demand processing unit to facilitate data transmission with the vehicle nodes. The access density analysis unit analyzes the collision avoidance information or channel utilization information in the data packets of existing vehicle nodes to obtain the vehicle node access density. This access density information is then sent to the vehicle nodes, guiding them to adopt the CSMA / CA protocol or VeMAC protocol transmission method. The vehicle demand processing unit parses the demand information in the vehicle node data packets, calculates the time when the vehicle enters the intersection, and sends it to the vehicle nodes waiting to receive the time information packet.

[0080] The following combination Figure 3 and Figure 4 The detailed processing steps of the vehicle node and the dispatch center node during the execution of the CVMAC protocol are as follows: Figure 3 As shown:

[0081] Step (a.1): The vehicle node that reaches the intersection and triggers the scheduling demand listens to see if it receives the channel idle signal;

[0082] Step (a.2): After receiving the channel idle data packet and parsing the data in the packet to find out that the node access density is low within a fixed time period, the vehicle node sends a data packet containing its own vehicle information, demand information, etc. to the Master dispatch center node;

[0083] Step (a.3): If the information request is sent successfully and the ACK feedback from the scheduling center is received, the transmission is completed once. If the request fails k times, wait (2 k +T r )mm,T r Indicates a randomly generated period of time. In the scenario where k <= 9, it is set to a random value within 50 milliseconds.

[0084] Step (a.4): Set the number of failed requests k <= 16. When k>10, k=10 remains unchanged. If k is equal to 16 three times in a row, k is cleared and retransmitted until the transmission is completed. If the number of conflicting nodes increases and the number of nodes with k values ​​equal to 16 increases, the dispatch center will start calculating the number of vehicles and the n value when switching from the CSMA / CA protocol to the VeMAC transmission protocol, so that the vehicle nodes can change the transmission mode. Since the transmission probability is affected by the size of the contention window, and the contention window is affected by the number of transmission backoffs, and the backoff number has a range of 0-16, the impact on the n value at different times will also have a fixed range. In order to more accurately determine the number of vehicles, the n value will be recalculated each time the switching threshold is triggered;

[0085] Step (a.5): When receiving a channel idle data packet and parsing it to determine that the node access density is high, VeMAC is used to transmit data. The second control channel receiving module of the vehicle node is turned on to receive the idle time slot data packet of the special channel sent by the dispatch center. The vehicle node selects the time slot that is suitable for its own frame according to the road direction of the vehicle to send information.

[0086] Step (a.6): In a four-way, four-lane intersection scenario, time slots in all four directions are not subject to competition, and vehicle nodes in each lane successfully complete data transmission tasks. To improve the performance of the CVMAC protocol, the vehicle density in the VeMAC transmission mode is identified. The dispatch center node statistically averages the time slot occupancy rate P_slot over multiple three-frame periods. If P_slot is less than 50%, it indicates that the current number of vehicle nodes is low. Information about low node access density is added to the channel idle signal, and steps (a.3) and (a.4) are executed.

[0087] Step (a.7): When there are multiple lanes in the same direction that can enter the intersection or vehicles enter in a tight manner, the size of the frame and time slot needs to be dynamically expanded. In addition, the congested direction can occupy the idle time slots in other directions. If the occupancy rate of each time slot is high, the direction time slot of the next time frame is selected based on the idea of ​​collision avoidance. As in step (a.4), the size and frequency of the k value are recorded to determine the degree of congestion and feedback is given to the dispatch center. Here, the k value is used to represent the degree of vehicle congestion in the same direction when there are many vehicle nodes. If vehicles in the same direction collide in the same direction time slot due to problems such as transmission delay or lane sharing, the VeMAC protocol should expand the number of directional time slots under the frame to improve the scheduling throughput under high vehicle density conditions.

[0088] The schematic diagram of the steps for the dispatch center node to execute this CVMAC protocol is as follows: Figure 4 As shown:

[0089] Step (b.1): The central node analyzes the vehicle information data packets of existing vehicle nodes through service data packets, and obtains the node access density by understanding the number of backoffs experienced when the vehicle accesses or calculating the time slot occupancy rate. If the density is low, the central node sends a packet with low node access density in the signal to guide the node to adopt the transmission methods (4.3) and (4.4);

[0090] Step (b.2): When the node access density is high, the second control channel transmission module is turned on and the time division multiple access transmission method in CVMAC is used to broadcast the idle time slots in all directions to the vehicle nodes. At the same time, a packet with information about the high node access density is sent in the Signal.

[0091] Step (b.3): Analyze the vehicle node's demand data packet, calculate the time it takes for the vehicle to enter the intersection using the traffic light-free intersection scheduling algorithm, and send the time information packet to the vehicle node waiting to receive it. For specific intersection scheduling algorithms, refer to existing literature (e.g., "A scalable unsignalized intersection system for automated vehicles and semi-physical implementation. Peer-to-Peer Netw. Appl. 14(1):202-214(2021)").

[0092] To further verify the protocol verification capability of this system at the de-traffic light connected intelligent intersection, four nodes were set as road nodes, and vehicles were generated at random times to make the traffic flow in the four lanes roughly the same. The exponential distribution function for generating the interval between two vehicles was changed (the parameter is λ, 1 / λ represents the expectation of the vehicle interval time) to simulate traffic complexity scenarios. The number of nodes generated on each road per minute represents the busyness of the traffic flow. The 802.11P protocol, VeMAC protocol, and CVMAC protocol were transplanted to perform performance analysis of the prototype verification system.

[0093] In the simulation, each test was repeated 5 times and the average value was taken. Figure 5 The Number of Collision Events (NCEs) is a measure of channel contention. When several adjacent nodes decide to access and transmit their data packets simultaneously, collisions are generated and accumulated. This only considers the number of collisions, regardless of whether all packets are fully transmitted. We can see that 802.11p still sees an increase in collisions as the number of vehicles increases. VeMAC, on the other hand, experiences very few collisions due to strict transmission within allocated time slots. However, occasional collisions may occur due to accumulated errors in time slot synchronization. CVMAC conflicts are primarily due to the number of collisions caused by using CSMA transmission during idle time. Time slots change during transmission, and the binary exponential backoff algorithm, which selects idle frame slots, also increases the number of collisions. However, due to the allocated frame size and fixed time slots used for transmission, collisions are significantly reduced compared to CSMA.

[0094] Delay Figure 6 As shown, with the increase in the number of vehicles in 802.11, collisions increase, backoff time increases, and latency increases accordingly. VeMAC, on the other hand, maintains a consistent level of latency. CVMAC, like CSMA, initially has a relatively small number of nodes, resulting in a low collision probability and transmission latency. However, as the number of vehicles increases, dynamic time slot allocation reduces CVMAC latency compared to TDMA.

[0095] Throughput Figure 7 As shown, CSMA continues its growth process, gradually reaching saturation or even decreasing. TDMA sacrifices latency to increase throughput, resulting in a slightly longer completion time, but it will always complete the transmission for all vehicles. CVMAC, based on TDMA, controls the frame size and slot size. When there are more vehicles, the transmission time is relatively shorter, resulting in better throughput.

[0096] The foregoing is merely a preferred embodiment of the present invention. It should be noted that those skilled in the art may make various improvements and modifications without departing from the principles of the present invention, and such improvements and modifications should be considered within the scope of protection of the present invention. Any components not specified in this embodiment may be implemented using existing technologies.

Claims

1. A communication system for a networked intelligent intersection without traffic lights, characterized by: The system includes vehicle nodes and dispatch center nodes, and the vehicle nodes and the dispatch center node complete data transmission according to the CVMAC protocol. The CVMAC protocol combines the CSMA / CA and VeMAC protocols, and determines the protocol type based on the estimated channel node access density. When the density is low, the CSMA / CA protocol is used for scheduling, and when the density is high, the VeMAC protocol mode is used for transmission. The standard for distinguishing high and low access density is determined by the number of vehicles calculated when the throughput of the two protocols is equivalent. When the actual number of access vehicles exceeds the vehicle number standard, the access density is considered high; otherwise, the access density is considered low. Throughput of CSMA / CA protocol Throughput of the core TDMA protocol of VeMAC design Where n represents the number of vehicles, p represents the transmission probability, s represents the packet transmission delay, d represents the DIFS transmission delay, and R represents the number of lanes.

2. The communication system for a networked intelligent intersection without traffic lights according to claim 1, characterized in that: The number of vehicles that switch the protocol is 3. The communication system for a networked intelligent intersection without traffic lights according to claim 1, characterized in that: The vehicle node is provided with the following processing unit: The listening unit is used to listen to whether a channel idle signal is received when the dispatch demand is triggered at the intersection, parse the access density information in the packet when a channel idle data packet is received, and determine the protocol type based on the density; The CSMA / CA protocol scheduling unit is used to send a data packet containing its own vehicle information and demand information to the scheduling center node when the access density is low, and receive feedback from the scheduling center node. If the request fails, it uses the collision avoidance algorithm to resend the request and add the number of failures to the data packet. The failure number is used by the scheduling center node to determine whether the data of the conflicting vehicle nodes has intensified, so as to determine whether to calculate the number of access vehicle nodes and the vehicle number standard for switching protocols, thereby determining whether to change the transmission protocol type. In addition, the VeMAC protocol transmission unit is used to receive the idle time slot identifier sent by the dispatching center node when the access density is high, select the time slot adapted to its own frame according to the road direction of its own vehicle, and send information to the dispatching center node, and add the number of times the vehicle node failed to avoid selecting the time slot request into the data packet and feed it back to the dispatching center node, so that the dispatching center node can determine whether to dynamically adjust the width of the time slot in each direction under the time slot frame.

4. The communication system for a networked intelligent intersection without traffic lights according to claim 3, characterized in that: In the CSMA / CA protocol scheduling unit, a binary exponential backoff algorithm is used to handle access conflicts. When the number of vehicle nodes whose request failures reach a set threshold increases, the scheduling center is triggered to calculate the vehicle number standard for switching from the CSMA / CA protocol to the VeMAC protocol. When the scheduling center node uses the VeMAC protocol for transmission, it statistically averages the time slot occupancy rate within multiple frame periods. If the time slot occupancy rate is less than the set threshold, the VeMAC protocol is switched to the CSMA / CA protocol for data transmission.

5. The communication system for a networked intelligent intersection without traffic lights according to claim 1, characterized in that: The dispatch center node is provided with the following processing units: An access density analysis unit is used to analyze the conflict avoidance information or channel utilization information in the data packets of existing vehicle nodes, obtain the vehicle node access density, and send the access density information to the vehicle nodes to guide the vehicle nodes to adopt the CSMA / CA protocol or VeMAC protocol transmission mode; And, a vehicle demand processing unit is used to parse the demand information in the data packet of the vehicle node, calculate the time when the vehicle enters the intersection, and send it to the vehicle node waiting to receive the time information packet.

6. A prototype verification system for a communication system for a networked intelligent intersection without traffic lights, characterized by: The prototype verification system is built on the Raspberry Pi platform, and uses road vehicle nodes, dispatch center nodes, and time slot synchronization nodes to simulate real intersection scenarios. Road vehicle nodes are used to randomly generate vehicles in various directions to simulate vehicle nodes arriving at intersections, dispatch center nodes are used to simulate roadside computing units, and time slot synchronization nodes are used to simulate the time slot synchronization function of outdoor GPS modules. The road vehicle nodes and dispatch center nodes complete data transmission according to the CVMAC protocol. The CVMAC protocol combines CSMA / CA and VeMAC protocols, and determines the protocol type according to the estimated channel node access density. When the density is low, CSMA / CA protocol scheduling is used, and when the density is high, VeMAC protocol mode is used for transmission. The standard for distinguishing high and low access density is determined according to the number of vehicles calculated when the throughput of the two protocols is equivalent. When the actual number of access vehicles exceeds the vehicle number standard, the access density is considered to be high, otherwise the access density is considered to be low. The throughput of the CSMA / CA protocol Throughput of the core TDMA protocol of VeMAC design Where n represents the number of vehicles, p represents the transmission probability, s represents the packet transmission delay, d represents the DIFS transmission delay, and R represents the number of lanes.

7. A vehicle node communication protocol method for a networked intelligent intersection without traffic lights, characterized in that: The vehicle node and the dispatch center node complete data transmission according to the CVMAC protocol; the CVMAC protocol determines the protocol type based on the estimated channel node access density, uses the CSMA / CA protocol scheduling when the density is low, and adopts the VeMAC protocol mode for transmission when the density is high; the high and low access density distinction standard is determined by the number of vehicles calculated when the throughput of the two protocols is equivalent, when the actual number of access vehicles exceeds the vehicle number standard, the access density is considered high, otherwise it is considered low; the throughput of the CSMA / CA protocol Throughput of the core TDMA protocol of VeMAC design Where n represents the number of vehicles, p represents the transmission probability, s represents the packet transmission delay, d represents the DIFS transmission delay, and R represents the number of lanes. The vehicle node communication protocol process includes the following steps: The vehicle node that reaches the intersection and triggers the dispatching demand listens to see if it receives the channel idle signal; After receiving the channel idle data packet, the vehicle node parses the data packet. If it determines that the access density is low, the vehicle node sends a data packet containing its own vehicle information and demand information to the dispatch center node. If the information request is successful and an ACK feedback is received from the dispatch center node, the transmission is completed. If the request fails, the collision avoidance algorithm is used to send the request again, and the number of failures is added to the data packet. The number of failures is used by the dispatch center node to determine whether the data of the conflicting vehicle nodes has intensified, so as to determine whether to calculate the number of access vehicle nodes and the vehicle number standard for switching protocols, thereby determining whether to change the transmission protocol type. After receiving the channel idle data packet, the vehicle node parses the data packet. If it is known that the access density is high, it receives the idle time slot identifier sent by the dispatch center node based on the VeMAC protocol, selects the time slot adapted to its own frame according to the road direction of its own vehicle, and sends information to the dispatch center node. It also adds the number of times the vehicle node's time slot selection request failed to be avoided into the data packet and feeds it back to the dispatch center node so that the dispatch center node can determine whether to dynamically adjust the width of the time slot in each direction under the time slot frame.

8. The vehicle node communication protocol method for a networked intelligent intersection without traffic lights according to claim 7, characterized in that: When using the CSMA / CA protocol for scheduling, a binary exponential backoff algorithm is used to handle access conflicts. When the number of vehicle nodes whose request failures reach a set threshold increases, the scheduling center is triggered to calculate the number of vehicles required to switch from the CSMA / CA protocol to the VeMAC protocol. When using the VeMAC protocol for transmission, the time slot occupancy rate within multiple frame periods is statistically averaged. If the time slot occupancy rate is less than the set threshold, the VeMAC protocol is switched to the CSMA / CA protocol for data transmission.

9. A communication protocol method for dispatching center nodes of a networked intelligent intersection without traffic lights, characterized in that: The dispatch center node and the vehicle node complete data transmission according to the CVMAC protocol; the CVMAC protocol determines the protocol type based on the estimated channel node access density, using the CSMA / CA protocol scheduling when the density is low and the VeMAC protocol mode for transmission when the density is high; the high and low access density distinction standard is determined by the number of vehicles calculated when the throughput of the two protocols is equivalent; the throughput of the CSMA / CA protocol is Throughput of the core TDMA protocol of VeMAC design Where n represents the number of vehicles, p represents the transmission probability, s represents the data packet transmission delay, d represents the DIFS transmission delay, and R represents the number of lanes. The dispatch center node communication protocol process includes the following steps: The dispatch center node analyzes the conflict avoidance information or channel utilization information in the data packets of existing vehicle nodes to obtain the vehicle node access density. When the access density is low, the dispatch center node adds the low access density information to the channel idle signal to guide the vehicle nodes to adopt the CSMA / CA protocol transmission mode. When the access density is high, the dispatch center node adopts the VeMAC protocol transmission mode, broadcasts the idle time slots in all directions to the vehicle nodes, and adds the high access density information to the channel idle signal to guide the nodes to adopt the VeMAC protocol transmission mode. The dispatcher parses the demand data packet of the vehicle node, calculates the time when the vehicle enters the intersection, and sends it to the vehicle node waiting to receive the time information packet.

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