Internet of vehicles data transmission fairness method based on virtual long frame

Abstract

The invention provides an Internet of Vehicles data transmission fairness method based on a virtual long frame, relates to the technical field of network and communication, and solves the technical problem of data volume fairness of task unloading under the condition of high vehicle speed difference. According to the technical scheme, the method comprises the following steps of S1, throughput modeling; s2, designing and realizing a virtual long frame; s3, designing a loss acknowledgement frame, constructing a'loss acknowledgement frame 'structure by the receiver, and sending the'loss acknowledgement frame' structure to the sender; a sender designs an FEC subframe retransmission mechanism, and the sender only needs to retransmit the lost FEC subframe according to the received'loss acknowledgement frame '. The method has the beneficial effects that the transmission data volume of the high-speed vehicle is increased, the transmission data volume of the low-speed vehicle is maintained, and the unloading data volume balance of the high-speed vehicle and the low-speed vehicle is realized.

Description

Technical Field

[0001] The present invention relates to the field of network and communication technologies, and particularly to a method for fairness of vehicle-to-everything (V2X) data transmission based on virtual long frames. Background Art

[0002] The Internet of Vehicles (IoV) is an integrated system that combines computer networks, sensors, and related software to enable data exchange between vehicles and other communication entities, thereby constructing a multi-dimensional collaborative intelligent interaction system such as vehicle-to-vehicle (V2V), vehicle-to-road (V2R), vehicle-to-pedestrian (V2P), and vehicle-to-cloud (V2C). The multi-modal information interaction mechanism constructed by the IoV not only provides real-time decision-making support for advanced driver assistance systems but also provides technical support for dynamic traffic collaborative management. Therefore, the IoV has become a core infrastructure in intelligent transportation systems. With the deep integration of new-generation information technologies and manufacturing industries, the innovative development of the IoV is of great strategic significance for achieving frontier industrial development goals such as improving traffic safety, optimizing energy resource allocation, and reducing carbon emissions.

[0003] In terms of hardware, the IoV includes two core components: on-board units (OBUs) and roadside units (RSUs). An OBU is a communication device installed on a vehicle and is responsible for sending vehicle dynamic data to an RSU. An RSU is a communication device installed near a road, which collects information from OBUs and can also provide information to OBUs. RSUs and OBUs together form the communication backbone of the IoV and are key communication devices for realizing IoV applications.

[0004] In terms of communication, the IEEE 802.11 protocol family is an important standard for realizing IoV communication. In response to the characteristics of the in-vehicle environment, the 802.11 protocol improves traditional wireless short-range network technologies to support mobile in-vehicle environments. Based on the 802.11 protocol, in-vehicle communication can be carried out between vehicles or between vehicles and roadside infrastructure.

[0005] With the development of traffic information systems, in-vehicle computing tasks have become increasingly heavy, and the demand for the processing capabilities of in-vehicle devices has been growing continuously. However, due to limitations in computing power and storage space, vehicles often struggle to efficiently process computing tasks, while traditional cloud computing faces problems such as long distances, large communication delays, and congestion at network bottleneck nodes. To overcome these limitations, researchers have proposed edge computing technology, deploying edge servers on the roadside to assist vehicles in completing computing tasks. For this purpose, the computing tasks of vehicles first need to be offloaded to the roadside unit. Task offloading refers to transferring the computing tasks, data processing, or storage requirements of in-vehicle terminals to the edge servers on the roadside, leveraging the rich computing and storage capabilities of the edge servers to improve the efficiency of task processing and reduce the computing burden on in-vehicle devices.

[0006] In the vehicle-to-everything (V2X) environment, the performance of task offloading is affected by many factors such as wireless communication bandwidth, obstacle occlusion, and communication duration. Among them, communication duration has a relatively significant impact on the total amount of wireless network data transmission. Since the signal coverage range of roadside units (RSUs) is usually limited, the communication duration between vehicles and RSUs mainly depends on vehicle speed, thus giving rise to the issue of data transmission fairness between high-speed and low-speed vehicles. Specifically, low-speed vehicles stay within the RSU signal coverage range for a longer time and have more time to communicate with RSUs, so they can transmit more data. In contrast, high-speed vehicles stay within the RSU signal coverage range for a shorter time and have less time to communicate with RSUs, so they can only transmit less data. It can be seen that there is an unfair phenomenon in data transmission when high-speed and low-speed vehicles offload tasks to RSUs, and the main reason is the different vehicle speeds of each vehicle.

[0007] Regarding the above-mentioned V2X transmission fairness problem, the existing research has mainly carried out the following work.

[0008] The literature "An Adaptive Packet Size Adjustment Scheme for Improving Fairness in Vehicular Networks" (Acta Electronica Sinica, 2007) proposed an adaptive packet size adjustment scheme. The algorithm first designs a new type of control frame to announce the current speed of the on-board unit (OBU). Secondly, it models and analyzes the functional relationships between packet size, the relative speed of oncoming vehicles, the data stream access probability, and throughput. Thirdly, it calculates the optimal packet size in different situations to improve the channel competition fairness and average throughput of each vehicle. Finally, simulation experiments show that the optimized packet size helps to improve throughput and fairness.

[0009] The literature "Adaptive Backoff Algorithm for 802.11p Vehicular Networks Based on Relative Speed" (《Computer Applications Research》, 2011) proposed an adaptive Media Access Control (MAC) contention window backoff algorithm. According to the relative speed difference between vehicles and the number of vehicle neighbor nodes, this scheme dynamically adjusts the size of the contention window and optimizes the MAC backoff mechanism. Experiments show that this method can effectively improve channel access fairness and reduce the packet loss rate.

[0010] The literature "Velocity-Adaptive Access Scheme for MEC-Assisted Platooning Networks: Access Fairness Via Data Freshness" (《IEEE Internet of Things Journal》, 2022) proposed a Velocity-Adaptive Access Scheme (VAAS) algorithm based on vehicle speed. According to the platoon spacing and vehicle speed, it dynamically adjusts the MAC initial contention window to ensure that the data volume transmitted by high-speed and low-speed vehicles is relatively balanced. Simulations show that this scheme significantly improves the data transmission fairness of vehicles with different speeds.

[0011] In the above research, the literature "Packet Size Adaptive Adjustment Scheme for Improving Fairness in Vehicular Networks" only designed an adaptive algorithm for the data frame size that adapts to environmental conditions such as vehicle speed, but ignored the problem that long frames are more likely to encounter packet collisions. The VAAS scheme designed in the literature "Velocity-Adaptive Access Scheme for MEC-Assisted Platooning Networks: Access Fairness Via Data Freshness" is relatively simple to implement. By simply dynamically adjusting the contention window, it can better balance the data volume unloaded by vehicle nodes with different vehicle speeds to the roadside base station. However, when the speed difference between fast and slow vehicles becomes larger, there is still a large difference in the data throughput between fast and slow vehicles. In addition, in order to achieve fairness in data transmission between fast and slow vehicles, the VAAS algorithm increases the MAC layer initial contention window of slow vehicle nodes, which essentially reduces the transmission efficiency of slow vehicles. Summary of the Invention

[0012] Aiming at the technical problem of data transmission fairness when vehicles with different speeds unload tasks in the 802.11 vehicle-to-everything (V2X) network, the present invention proposes a V2X data transmission fairness method based on virtual long frames. According to throughput modeling, the appropriate payload size is first calculated to improve the throughput of high-speed vehicles and enhance the transmission fairness between fast and slow vehicles. Secondly, the present invention appends a sub-frame structure based on Forward Error Correction (FEC) to the original payload position of the MAC frame in 802.11 to construct a payload with an appropriate length. When the collision probability of the sub-frame is small, the FEC mechanism can assist the receiving node in automatically recovering the lost sub-frame without the need for the sending node to retransmit the sub-frame. Thirdly, a "Loss-ACK" frame is designed. When the collision probability of the sub-frame is large and the receiving node cannot use the FEC mechanism to recover the lost sub-frame, the source node can retransmit several lost sub-frames according to the information in the "Loss-ACK" frame.

[0013] To achieve the above object of the invention, the technical solution adopted by the present invention is specifically as follows: A V2X data transmission fairness method based on virtual long frames, comprising the following steps:

[0014] S1: Throughput modeling. According to the frame transmission process and related parameters of the 802.11 protocol, a mathematical model of throughput is derived.

[0015] S2: Design and implementation of virtual long frames. First, based on the throughput mathematical model, the length of the virtual long frame is determined. Secondly, the FEC sub-frame structure of the virtual long frame is designed. Thirdly, the number k of normal sub-frames and the number h of redundant sub-frames of the FEC mechanism are calculated. For the low bit error rate condition of the wireless channel, through the above three steps, the reliable transmission of the virtual long frame can be realized.

[0016] S3: Design of Loss-ACK frame. When the number of lost FEC sub-frames in a certain virtual long frame exceeds h and the receiving party cannot use the FEC mechanism to recover the lost FEC sub-frames. First, the receiving party designs the structure of the "Loss-ACK" frame and sends it to the sending party. Secondly, the sending party designs an FEC sub-frame retransmission mechanism. According to the received "Loss-ACK" frame, only the lost FEC sub-frames need to be retransmitted. For the high bit error rate condition of the wireless channel, through the above two steps, the reliable transmission of the virtual long frame can be realized.

[0017] The throughput modeling in step S1 includes the following steps:

[0018] Calculate the backoff time. In an ideal situation, when there is no collision in the data transmission between the two communication parties, the backoff time T BO is equal to the average contention window The product of the time slot. For the 802.11 communication protocol, according to the initial contention window size CW min and the time slot T slot , the backoff time T of the sender BO can be written as

[0019]

[0020] S12. Calculate the transmission time of the data frame header. According to the data frame header length L H_DATA and the data frame transmission rate R trans , the transmission time T of the data frame header H-DATA is

[0021]

[0022] Calculate the transmission time of the data frame header at the physical layer. Let T P be the time to transmit the physical layer preamble, and T PHY be the time to transmit the physical layer header. The transmission time T of the data frame header at the physical layer D_H-DATA is written as

[0023] T D_H-DATA = T P + T PHY + T H-DATA (3)

[0024] Calculate the transmission time of the data frame payload. According to the data frame payload length L DATA and the data frame transmission rate, the data frame payload transmission time T DATA is

[0025]

[0026] Calculate the transmission time of the entire data frame. According to the frame structures of the physical layer and MAC layer of 802.11, the transmission time T of the entire data frame D_DATA is

[0027] T D_DATA = T D_H-DATA + T DATA (5)

[0028] S13. Calculate the transmission time of the ACK acknowledgment frame. After receiving the above data frame, the receiver sends an ACK acknowledgment frame back to the sender. According to the ACK frame length L ACK and the ACK frame transmission rate R basic , the transmission time T of the ACK frame ACK is

[0029]

[0030] Calculate the transmission time of the ACK confirmation frame at the physical layer. From the physical layer of 802.11 and the frame structure of ACK, the transmission time T of the ACK frame D_ACK is

[0031] T D_ACK = T P + T PHY + T ACK (7)

[0032] From formulas (1) to (7), let τ be the delay time of the transmitted frame, and the throughput of the one-way communication is

[0033]

[0034] If T DIFS + T BO + T D_H-DATA + τ + T SIFS + T D_ACK + τ is briefly recorded as T other , and at the same time, according to the definition of formula (4), the throughput formula (8) of the one-way communication can be written as

[0035]

[0036] The design and implementation of the virtual long frame in step S2 include the following steps:

[0037] S21: Determine the length of the virtual long frame. Observing formula (9), it can be found that since R trans and T other are fixed values, as the payload L DATA increases, the throughput will gradually increase, but finally approach a certain value, and this value is slightly lower than the transmission rate R trans of the data frame. Therefore, the data frame length of the express node can be adjusted to increase the amount of data transmitted from the express to the RSU, and finally achieve the fairness of data transmission between the fast and slow vehicles.

[0038] During wireless transmission, the larger the data frame length, the greater the transmission time and the greater the probability of collision. Once the data frame collides, the receiving node will not be able to correctly parse the content of the collided data frame, resulting in a waste of channel resources. Therefore, for the long frame transmission mechanism, it is necessary to design an error correction and retransmission scheme to ensure the reliable transmission of the long frame. Of course, the value of the payload length L DATA should not be too large, usually not exceeding 11000 bytes.

[0039] S22: Design the FEC sub-frame structure of the virtual long frame. Sending long frames by the source node in the vehicle-to-everything (V2X) network can improve throughput, but it is also more likely to collide with frames sent by other nodes, resulting in waste of channel resources and retransmission delay. To achieve reliable transmission of the virtual long frame, in this solution, an FEC sub-frame structure is designed in the application payload of the virtual long frame, as Figure 2 shown.

[0040] As Figure 2 shown, in the application payload area of the virtual long frame, the following six fields are sequentially added. The "long frame number" field (1 byte) fills in the number of this virtual long frame to distinguish different long frames sent by the same source node. The "retransmission flag bit" field (1 bit) fills in 0 to represent that this frame is not a retransmitted data frame, and fills in 1 to represent that this frame is a retransmitted data frame. The "k value" (1 byte) and "h value" (1 byte) fields fill in the number of normal sub-frames and redundant sub-frames of the FEC mechanism respectively, and the specific filled values can be seen in the following calculation formula (10). The "sub-frame number" field (1 byte) fills in the number of a specific sub-frame in this long frame to distinguish different sub-frames in the same long frame, and its value range is a positive integer. The "sub-frame payload" field fills in the specific user payload. A short interframe space (SIFS) is used to separate the "sub-frame payload" area of the previous sub-frame and the "sub-frame number" area of the next sub-frame.

[0041] S23: Calculate the number k of normal sub-frames and the number h of redundant sub-frames of the FEC mechanism. The length of the virtual long frame is L DATA_VLF , the length of each sub-frame payload is L_sub, and the symbol represents rounding up, then the number of sub-frames can be written as

[0042]

[0043] In the FEC mechanism, the number k of normal sub-frames takes a positive integer, and its value range is 1 ≤ k < n, then the number of redundant sub-frames h = n - k. According to the FEC rule, when the destination node receives a virtual long frame, if the number of lost sub-frames is less than or equal to h, the destination node can directly recover the lost sub-frames based on the existing sub-frames, thereby reducing the retransmission times of the sending node and saving the bandwidth overhead of repetitive information.

[0044] The step S3 includes the steps:

[0045] S31: The receiver designs the "Loss-ACK" structure. As Figure 3As shown in the figure, in the ACK frame data payload area, two fields, namely "long frame number" and "missing sub-frame number", are added respectively. The "type" field of the "lost acknowledgment frame" is filled with the binary "01", indicating that this frame is a control frame. The "sub-type" field is filled with the binary "0001" as a reserved field, and this value represents the "lost acknowledgment frame". The functions of the "protocol version" field, "period" field, "receiving address" field, and "frame tail FCS check" field of the ACK frame remain unchanged. The receiving node fills the unrecoverable sub-frame number and the long frame number where the sub-frame is located into the "missing sub-frame number" and "long frame number" fields of the "lost acknowledgment frame" respectively. The receiving node transmits this "lost acknowledgment frame" to the sending node.

[0046] S32: The sender designs a retransmission mechanism for the FEC sub-frame. After receiving a certain frame, the sender first reads its "sub-type" field. If its value is "0001", it can be determined that this frame is a "lost acknowledgment frame". Secondly, the sending node reads the "long frame number" and "missing sub-frame number" contained in the "lost acknowledgment frame" to extract the relevant information of the lost sub-frame. Thirdly, the sending node constructs a long frame, sets the "retransmission flag bit" to 1, and fills the values of the "long frame number" and "missing sub-frame number" contained in the "lost acknowledgment frame" into the corresponding field positions of the retransmission frame in sequence. The "sub-frame payload" field is filled with the payload corresponding to the "missing sub-frame number". Finally, the sending node retransmits the constructed long frame.

[0047] In summary, the present invention proposes a fairness scheme for vehicle-to-everything (V2X) data transmission based on virtual long frame (VLF).

[0048] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0049] 1. In a wireless network under the traditional 802.11 communication standard, when a vehicle passes through the signal coverage area of a roadside unit, due to different speeds (fast and slow), there is a significant difference in the communication duration between each vehicle and the roadside unit, resulting in a significant difference in the amount of data for offloading computing tasks between fast and slow vehicles. Existing research (the literature "Velocity-Adaptive Access Scheme for MEC-Assisted Platooning Networks: Access Fairness Via Data Freshness") adjusts the MAC layer contention window (CW) to reduce the waiting time for fast vehicles to compete for MAC channel resources (equivalent to reducing the pre-transmission waiting time for fast vehicles) and appropriately increase the waiting time for slow vehicles to compete for MAC channel resources (equivalent to increasing the pre-transmission waiting time for slow vehicles), so as to improve the offloading data volume of fast vehicles and achieve relative fairness in data transmission volume. However, the waiting time that the contention window can adjust is limited after all. As the speed difference between fast and slow vehicles continues to increase, the fairness improvement effect of this literature is not good. To address the above fairness issue, the present invention proposes a transmission scheme for virtual long frames to increase the amount of data for fast vehicle offloading tasks while maintaining the amount of data for slow vehicle offloading tasks, thereby achieving fairness in the transmission of offloading tasks.

[0050] 2. Due to the vulnerability of the wireless channel, the transmitted information is easily subject to wireless shielding, attenuation, and interference, which may cause data frame loss. In addition, when the number of sending nodes competing for the wireless channel increases, data frames are prone to collision. Since the signals transmitted by multiple nodes are superimposed on each other, it may also cause the receiving node to be unable to correctly parse the data frame, and at this time, the protocol determines that the collided data frame is lost. As the length of the MAC layer data frame increases, the transmission time also increases, and the data frame is more likely to collide. Therefore, in the payload area of the 802.11 protocol MAC frame, a sub-frame structure based on FEC is designed to load the payload of an appropriate length. When the loss probability of the sub-frame is small, the FEC mechanism can assist the receiving node to automatically recover the lost sub-frame, and the sending node does not need to retransmit the above lost sub-frame, saving the communication overhead of retransmission.

[0051] 3. As the shielding, attenuation, and interference intensify or the source node's competition for the channel becomes more intense, when the loss probability of the sub-frame is large, the receiving node cannot use the FEC mechanism to recover the lost sub-frame. Therefore, a "loss confirmation frame" is designed. The receiving node will send this "loss confirmation frame" to the source node. The source node can retransmit several lost sub-frames according to the information contained in the "loss confirmation frame" without retransmitting the entire long frame, thereby reducing the reuse of the channel and saving wireless bandwidth resources. Description of the Drawings

[0052] The accompanying drawings are used to provide a further understanding of the present invention, and constitute a part of the specification. Together with the embodiments of the present invention, they are used to explain the present invention, and do not constitute a limitation to the present invention.

[0053] Figure 1 This is a schematic diagram of the scenario where the express vehicle and the slow vehicle pass through the RSU signal coverage range in the present invention.

[0054] Figure 2 This is the frame structure design diagram of the virtual long frame in the present invention.

[0055] Figure 3 This is the frame structure design diagram of the lost confirmation frame in the present invention.

[0056] Figure 4 This is a schematic diagram of the comparison results between the present invention, the 802.11 algorithm, and the VAAS (Velocity-Adaptive Access Scheme) algorithm in the prior art document "Velocity-Adaptive Access Scheme for MEC-Assisted Platooning Networks: Access Fairness Via Data Freshness". Detailed implementation manners

[0057] In order to make the objectives, technical solutions, and advantages of the present invention clearer and more understandable, the present invention will be further described in detail below in conjunction with the accompanying drawings and embodiments. Of course, the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0058] Embodiment 1

[0059] Refer to Figures 1 to 4 The technical solution provided in this embodiment is as follows: A method for the fairness of vehicle-to-infrastructure data transmission based on virtual long frames, including the following steps:

[0060] S1: Throughput modeling. According to the frame transmission process and related parameters of the 802.11 protocol, a mathematical model of throughput is derived.

[0061] S2: Design and implementation of virtual long frames. First, based on the throughput mathematical model, the length of the virtual long frame is determined. Second, the FEC sub-frame structure of the virtual long frame is designed. Third, the number k of normal sub-frames and the number h of redundant sub-frames of the FEC mechanism are calculated. For the low bit error rate condition of the wireless channel, through the above three steps, the reliable transmission of the virtual long frame can be achieved.

[0062] S3: Design of Loss-ACK. When the number of lost FEC sub-frames in a certain virtual long frame exceeds h, the receiver cannot recover the lost FEC sub-frames using the FEC mechanism. First, the receiver designs the "Loss-ACK" structure and sends it to the sender. Second, the sender designs an FEC sub-frame retransmission mechanism. According to the received "Loss-ACK", only the lost FEC sub-frames need to be retransmitted. For the high bit error rate condition of the wireless channel, through the above two steps, the scheme can achieve reliable transmission of virtual long frames.

[0063] The throughput modeling of step S1 includes the following steps:

[0064] Calculate the backoff time. In an ideal situation, when there is no collision in data transmission between the two communication parties, the backoff time T BO is equal to the product of the average contention window and the time slot. For the 802.11 communication protocol, according to the initial contention window size CW min and the time slot T slot , the backoff time T of the sender BO can be written as

[0065]

[0066] S12. Calculate the transmission time of the data frame header. According to the data frame header length L H_DATA and the data frame transmission rate R trans , the transmission time T of the data frame header H-DATA is

[0067]

[0068] Calculate the transmission time of the data frame header at the physical layer. Let T P be the time to transmit the physical layer preamble, and T PHY be the time to transmit the physical layer header. The transmission time T of the data frame header at the physical layer D_H-DATA is written as

[0069] T D_H-DATA = T P + T PHY + T H-DATA (3)

[0070] Calculate the transmission time of the data frame payload. According to the data frame payload length L DATA and the data frame transmission rate, the data frame payload transmission time T DATA is

[0071]

[0072] Calculate the transmission time of the entire data frame. According to the frame structures of the physical layer and MAC layer of 802.11, the transmission time T of the entire data frame is obtained. D_DATA For

[0073] T D_DATA = T D_H-DATA + T DATA (5)

[0074] S13. Calculate the transmission time of the ACK confirmation frame. After receiving the above data frame, the receiver sends an ACK confirmation frame back to the sender. According to the ACK frame length L ACK and the ACK frame transmission rate R basic , the transmission time T of the ACK frame ACK is

[0075]

[0076] Calculate the transmission time of the ACK confirmation frame at the physical layer. From the physical layer of 802.11 and the frame structure of ACK, the transmission time T of the ACK frame D_ACK is

[0077] T D_ACK = T P + T PHY + T ACK (7)

[0078] From formulas (1) to (7), let τ be the delay time of the transmission frame, and the throughput of the one-way communication is obtained as

[0079]

[0080] If T DIFS + T BO + T D_H-DATA + τ + T SIFS + T D_ACK + τ is abbreviated as T other , and at the same time according to the definition of formula (4), the throughput formula (8) of the one-way communication can be written as

[0081]

[0082] Step S2 The design and implementation of the virtual long frame include the following steps:

[0083] S21: Determine the length of the virtual long frame. Observing formula (9), it can be found that since R trans and T other are fixed values, as the payload L DATAWith the increase of [[ID=]], the throughput will gradually increase, but eventually approach a certain value, which is slightly lower than the transmission rate R of the data frame. trans Therefore, the data frame length of the express vehicle node can be adjusted to increase the amount of data transmitted from the express vehicle to the RSU, and finally achieve the fairness of data transmission between the fast and slow vehicles.

[0084] In the wireless transmission process, the larger the data frame length, the greater the transmission time and the greater the probability of collision. Once the data frame collides, the receiving node will not be able to correctly parse the content of the collided data frame, resulting in a waste of channel resources. Therefore, for the long frame transmission mechanism, it is necessary to design an error correction and retransmission scheme to ensure the reliable transmission of long frames. Of course, the value of the payload length L DATA should not be too large, usually not exceeding 11000 bytes.

[0085] S22: Design the FEC sub-frame structure of the virtual long frame. Sending long frames by the source node in the vehicle network can increase the throughput, but it is also more likely to collide with the frames sent by other nodes, resulting in waste of channel resources and retransmission delay. To achieve the reliable transmission of the virtual long frame, this scheme designs the FEC sub-frame structure in the application payload of the virtual long frame, as Figure 2 shown.

[0086] As Figure 2 shown, in the application payload area of the virtual long frame, the following six fields are sequentially added. The "long frame number" field (1 byte) fills in the number of this virtual long frame to distinguish different long frames sent by the same source node. The "retransmission flag bit" field (1 bit) fills in 0 to represent that this frame is not a retransmitted data frame, and fills in 1 to represent that this frame is a retransmitted data frame. The "k value" (1 byte) and "h value" (1 byte) fields respectively fill in the number of normal sub-frames and redundant sub-frames of the FEC mechanism. The specific filled values are shown in the following calculation formula (10). The "sub-frame number" field (1 byte) fills in the number of a specific sub-frame in this long frame to distinguish different sub-frames in the same long frame, and its value range is a positive integer. The "sub-frame payload" field fills in the specific user payload. A short interframe space (SIFS) is used to separate the "sub-frame payload" area of the previous sub-frame and the "sub-frame number" area of the next sub-frame.

[0087] S23: Calculate the number k of normal sub-frames and the number h of redundant sub-frames of the FEC mechanism. The length of the virtual long frame is L DATA_VLF , the length of each sub-frame payload is L_sub, and the symbol represents rounding up, then the number of sub-frames can be written as

[0088]

[0089] In the FEC mechanism, the number k of normal sub - frames takes positive integers, and its value range is 1 ≤ k < n. Then the number h of redundant sub - frames is h = n - k. According to the FEC rule, when the destination node receives a virtual long frame, if the number of lost sub - frames is less than or equal to h, the destination node can directly recover the lost sub - frames based on the existing sub - frames, thereby reducing the re - transmission times of the sending node and saving the bandwidth overhead of repetitive information.

[0090] Step S3 includes the steps:

[0091] S31: The receiver designs the structure of the "Loss - ACK" frame. As Figure 3 shown, in the ACK frame data payload area, two fields, namely "long frame number" and "missing sub - frame number", are added respectively. The "type" field of the "Loss - ACK" frame is filled with binary "01", indicating that this frame is a control frame. The "sub - type" field is filled with binary "0001" as a reserved field, and this value represents the "Loss - ACK" frame. The functions of the "protocol version" field, "period" field, "reception address" field, and "frame - tail FCS check" field of the ACK frame remain unchanged. The receiver fills the sub - frame number that cannot be recovered and the long frame number where this sub - frame is located into the "missing sub - frame number" and "long frame number" fields of the "Loss - ACK" frame respectively. The receiver transmits this "Loss - ACK" frame to the sender.

[0092] S32: The sender designs the re - transmission mechanism for FEC sub - frames. After receiving a certain frame, the sender first reads the "sub - type" field. If its value is "0001", it determines that this frame is a "Loss - ACK" frame. Secondly, the sending node continues to read the "long frame number" and "missing sub - frame number" contained in the "Loss - ACK" frame, and extracts the relevant information of the lost sub - frames. Thirdly, the sending node constructs a long frame, but sets the "re - transmission flag bit" to 1, and fills the values of the "long frame number" and "missing sub - frame number" contained in the "Loss - ACK" frame into the corresponding field positions of the re - transmission frame in sequence. The "sub - frame payload" field is filled with the payload corresponding to the "missing sub - frame number". Finally, the sending node re - transmits the constructed long frame.

[0093] Embodiment 2

[0094] Based on Embodiment 1, a simulation experiment is carried out. The performance comparison of each algorithm is shown in Figure 4 .

[0095] The task offloading scenario of the vehicle - to - everything network is as Figure 1As shown in the figure. On a one-way two-lane road, low-speed and high-speed vehicles travel from west to east and transmit data to the roadside base station RSU respectively. The speed of the low-speed vehicle is set to v_low (m / s), the speed of the high-speed vehicle is set to v_high (m / s), and the communication radius of the roadside base station is r (m), and its signal coverage range is represented by a dotted semi-circle. Since the RSU communication distance (usually several hundred meters) is much larger than the lane width (usually 2 - 3 meters), the lane width is ignored, and the maximum communication durations of the low-speed and high-speed vehicles are respectively and

[0096] Task offloading uses the 802.11 default algorithm, the VAAS algorithm, and the VLF algorithm respectively to simulate the offloading data volumes of fast and slow vehicles for the three algorithms. The parameters for simulation calculation are shown in Table 1.

[0097] Table 1 Experimental parameters, their values, and meanings

[0098]

[0099]

[0100] The offloading data volume situations of high-speed and low-speed vehicles under the three algorithms are as Figure 4 shown. The difference in the offloading data volumes of the two vehicles using the 802.11 default algorithm is the largest because the 802.11 default algorithm allocates equal channel usage rights to fast and slow vehicles, and the vehicle speed determines the communication duration between the vehicle and the RSU, resulting in the problem of data transmission fairness between high-speed and low-speed vehicles. Under the 802.11 default algorithm, the ratio of the data volume transmitted by the slow vehicle to that of the fast vehicle is approximately 2.00, which is consistent with the speed ratio of the fast and slow vehicles.

[0101] The VAAS algorithm sets a smaller MAC layer contention window value for fast vehicles and a larger MAC layer contention window value for slow vehicles. Therefore, fast vehicles obtain a shorter pre-transmission waiting time, improving their transmission efficiency, while slow vehicles obtain a longer pre-transmission waiting time, reducing their transmission efficiency, so as to achieve relative fairness in the task offloading data volume between fast and slow vehicles. However, the VAAS algorithm achieves fairness at the cost of reducing the throughput of slow vehicles, and the data offloading volume of slow vehicles is only about 80% of that of the 802.11 default algorithm. When the vehicle speed difference is 16 m / s in the simulation, the ratio of the total amount of data transmitted by slow vehicles to that of fast vehicles under the VAAS algorithm reaches 1.40, and the fairness improvement effect is not ideal. This is because the waiting time that can be adjusted by the MAC layer contention window is limited. When the vehicle speed difference between the two vehicles reaches 16 m / s, the adjustment of the pre-transmission waiting time is not sufficient to improve the transmission fairness between the two vehicles, and it also limits the data transmission volume of slow vehicles. In the VLF scheme of this embodiment, through the setting of virtual long frames and the design of FEC sub-frames and acknowledgment frames, the ratio of the total amount of data transmitted by slow vehicles to that of fast vehicles is 0.77. The VLF scheme significantly improves the throughput of fast vehicles by increasing the length of the effective payload of fast vehicle data frames, while maintaining the throughput of slow vehicles unchanged. At the same time, it reduces the retransmission caused by packet collisions and saves channel resources. Therefore, the data transmission fairness is better than that of the VAAS algorithm.

[0102] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A method for fairness of vehicle-to-everything (V2X) data transmission based on virtual long frames, characterized in that, It includes the following steps: S1: Throughput modeling According to the frame transmission process and related parameters of the 802.11 protocol, derive the mathematical model of throughput; S2: Design and implementation of virtual long frames Firstly, based on the throughput mathematical model, determine the length of the virtual long frame; Secondly, design the FEC sub-frame structure of the virtual long frame; Thirdly, calculate the number k of normal sub-frames and the number h of redundant sub-frames of the FEC mechanism, and achieve reliable transmission of the virtual long frame for the low bit error rate condition of the wireless channel; S3: Design of Loss-ACK for lost acknowledgment frames When the number of lost FEC sub-frames in a certain virtual long frame exceeds h and the receiving party cannot recover the lost FEC sub-frames using the FEC mechanism, firstly, the receiving party designs the structure of "Loss-ACK for lost acknowledgment frames" and sends it to the sending party; Secondly, the sending party designs a retransmission mechanism for FEC sub-frames. According to the received "Loss-ACK for lost acknowledgment frames", the sending party only needs to retransmit the lost FEC sub-frames, and achieve reliable transmission of the virtual long frame for the high bit error rate condition of the wireless channel.

2. The method for fairness of vehicle networking data transmission based on virtual long frames according to claim 1, wherein The step S1 includes the following steps: S11. Calculate the backoff time If there is no collision in the data transmission between the two communication parties, the backoff time T BO is equal to the product of the average contention window and the time slot. For the 802.11 communication protocol, according to the initial contention window size CW min and the time slot T slot , the backoff time T of the sender BO is written as S12. Calculate the transmission time of the data frame header according to the length L of the data frame header H_DATA and the data frame transmission rate R trans , and the transmission time T of the data frame header H-DATA is Calculate the transmission time of the data frame header at the physical layer, and set T P as the time to transmit the physical layer preamble, T PHY as the time to transmit the physical layer header. The transmission time T of the data frame header at the physical layer D_H-DATA is written as T D_H-DATA = T P + T PHY + T H-DATA (3) Calculate the transmission time of the data frame payload according to the data frame payload length L DATA and the data frame transmission rate, the data frame payload transmission time T DATA is Calculate the transmission time of the entire data frame. According to the frame structure of the physical layer and MAC layer of 802.11, obtain the transmission time T of the entire data frame D_DATA be T D_DATA = T D_H-DATA + T DATA (5) S13. Calculate the transmission time of the ACK acknowledgment frame. After receiving the above data frame, the receiver sends an ACK acknowledgment frame back to the sender. According to the ACK frame length L ACK and the ACK frame transmission rate R basic , the transmission time T of the ACK frame ACK is Calculate the transmission time of the ACK confirmation frame at the physical layer. From the physical layer of 802.11 and the frame structure of the ACK, the transmission time T of the ACK frame D_ACK is T D_ACK = T P + T PHY + T ACK (7) From formulas (1) to (7), let τ be the delay time of the transmitted frame, and the throughput of one-way communication is If we take T DIFS +T BO +T D_H-DATA +τ+T SIFS +T D_ACK +τ and simply denote it as T other , and at the same time, according to the definition in formula (4), the throughput formula (8) for one-way communication is written as 3. The fairness method for vehicle networking data transmission based on virtual long frames according to claim 1, characterized in that The step S2 includes the following steps: S21: Determine the length of the virtual long frame From Equation 9, since R trans and T other are fixed values, as the payload L DATA increases, the throughput gradually rises and finally approaches a certain value, which is lower than the transmission rate R trans of the data frame, adjust the data frame length of the express node; S22: Design the FEC sub-frame structure of the virtual long frame In the vehicle-to-everything network, when the source node sends a long frame to improve throughput, it also collides with the frames sent by other nodes, resulting in waste of channel resources and retransmission delay. Design the FEC sub-frame structure; S23: Calculate the number k of normal subframes and the number h of redundant subframes of the FEC mechanism. The length of the virtual long frame is L DATA_VLF , and the length of each subframe payload is L_sub. The symbol represents rounding up. Then the number of subframes is In the FEC mechanism, the number k of normal sub-frames takes a positive integer, and its value range is 1 ≤ k < n, then the number h of redundant sub-frames is h = n - k. According to the FEC rule, when the destination node receives a virtual long frame, if the number of lost sub-frames is less than or equal to h, the destination node directly recovers the lost sub-frames based on the existing sub-frames.

4. The method for fairness of vehicle networking data transmission based on virtual long frames according to claim 1, wherein The step S3 includes the following steps: S31: The receiving party designs the structure of the "lost acknowledgment frame". In the data payload area of the ACK frame, add two fields, namely "long frame number" and "missing sub-frame number". The "subtype" field of the "lost acknowledgment frame" is filled with the binary "0001", and this value represents the "lost acknowledgment frame". The receiving node fills the sub-frame number that cannot be recovered and the long frame number where this sub-frame is located into the "missing sub-frame number" and "long frame number" fields of the "lost acknowledgment frame" respectively, and the receiving node transmits this "lost acknowledgment frame" to the sending node; S32: The sending party designs a retransmission mechanism for FEC sub-frames. After the sending party receives a certain frame, firstly, it reads its "subtype" field. If its value is "0001", then this frame is a "lost acknowledgment frame"; Secondly, the sending node reads the "long frame number" and "missing sub-frame number" contained in the "lost acknowledgment frame"; Thirdly, the sending node constructs a long frame, sets the "retransmission flag bit" to 1, fills the values of the "long frame number" and "missing sub-frame number" contained in the "lost acknowledgment frame" into the corresponding field positions of the retransmitted frame in sequence, fills the payload corresponding to the "missing sub-frame number" into the "sub-frame payload" field, and finally, the sending node retransmits the constructed long frame.

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