A C-V2X information age optimization method

Abstract

The present invention discloses a C‑V2X information age optimization method based on NOMA and multi-priority queues, belonging to the field of vehicle network communication technology. The method includes: (1) multi-priority data packet queue design, (2) AoI evaluation model; (3) NOMA resource allocation strategy. The present invention reduces the impact of the AoI of messages in the C‑V2X system during the transmission process through NOMA, and reduces the impact of the AoI of high-priority messages during the queuing process through multi-priority queues, and effectively improves the optimization of the AoI in the C‑V2X system.

Description

Technical Field

[0001] The present invention belongs to the technical field of vehicle networking communications, and specifically relates to a C-V2X information age optimization method. Background Art

[0002] With the innovation and progress of Internet of Vehicles technology, the development of intelligent transportation systems has met a series of in-vehicle applications required by users, such as automatic navigation, collision warning and multimedia entertainment. Due to the high-speed movement of vehicles, the process in which vehicle users can obtain the required data and other content through requests needs to ensure timeliness. The traditional request method is that the user first communicates with the base station, and then accesses the data center where the data is stored by accessing the core backbone network, and the data center will then transmit the requested data back. However, this method has problems such as long end-to-end delay and limited return bandwidth. C-V2X proposes an interface called PC5 for communication between autonomous driving vehicles. PC5 provides two resource allocation methods. In mode 3, the vehicle requests transmission resources in the time domain and frequency domain from the base station, and then the base station allocates resources using semi-persistent and dynamic scheduling methods; in mode 4, the vehicle does not go through the cellular infrastructure and autonomously selects resources. In contrast, mode 4 not only does not have the disadvantage of limited coverage, but also has little interaction between the base station and the vehicle, which solves the problem of excessive delay and overhead.

[0003] In mode 4, the way vehicles automatically select communication resources is a distributed sensing-based semi-persistent resource scheduling protocol called SPS. Semi-persistence means that vehicles can choose one of several standardized message intervals (e.g., 10Hz, 20Hz, or 50Hz), but this increases the possibility that vehicles with the same message transmission interval occupy the same resources and thus collide, which will lead to an increase in block error rate. NOMA is a potential solution for C-V2X communication, which is a promising technology for improving spectrum efficiency and handling large-scale vehicle communications. It can be used to alleviate the performance degradation in terms of latency and packet reception probability caused by high vehicle density. Serial interference cancellation is currently a well-known multi-user detection technology for extracting overlapping signals. It can use the different power strengths of multiple user signals received occupying the same resource for decoding. And after decoding, the high-power signal no longer acts as interference to other low-power signals, thereby improving the SINR of the low-power signal, reducing transmission failures caused by collisions, and reducing the block error rate.

[0004] Reliability and transmission delay are often used in existing engineering and 3GPP standards to evaluate the performance of vehicle communications. However, these two indicators are mutually exclusive, that is, reliability performance can be improved at the expense of increased delay. Therefore, it is necessary to find a new metric to comprehensively reflect the performance of reliability and delay, such as AoI. A lower average AoI can reflect lower delay with the same reliability or higher reliability with the same delay. The data packet received by the receiver describes the information of the transmitter at the time of packet generation, so the packet queuing process needs to be taken into account. The AoI generated by the data packet in the queue is affected by the packet generation rate and the service rate. The service rate in CV2X depends on the RRI. However, with the same RRI, the AoI of the transmitter experiences a short queuing time and the collision probability is relatively large, resulting in a large AoI at the receiver.

[0005] In the C-V2X scenario, there are generally four types of messages, namely High-Priority Data (HPD), Decentralized Environmental Notification Message (DENM), Cooperative Awareness Message (CAM), and Miscellaneous High-Density Data (MHD). If multiple data packets are in a queue, the AoI of the key data packet will be affected by other data packets. In addition, in C-V2X, there are fewer available resources in the smaller selection window (SW), which may lead to duplicate occupation of resources as the number of vehicles increases. RRI is usually equal to SW, so it not only affects the AoI during the queuing process, but also affects the change of AoI during the transmission process. Summary of the invention

[0006] In order to reduce the impact of AoI of key data packets in the queuing process and the impact of the transmission process, the present invention proposes a C-V2X information age optimization method based on NOMA (Non-orthogonal multiple access) and multi-priority queues, thereby optimizing AoI from the queuing process and the transmission process respectively, and effectively improving the optimization of AoI in the C-V2X system.

[0007] To achieve the above object, the present invention provides the following technical solutions:

[0008] A C-V2X information age optimization method, comprising:

[0009] (1) Multi - priority packet queue design: Establish a multi - priority queue for the four types of packets in the C - V2X system, and allocate them to FIFO queues with different priorities according to the urgency of the packets; thus, when the vehicle has a transmission opportunity, it can directly transmit the messages with a higher degree of urgency to reduce their delay;

[0010] (2) AoI evaluation model: Construct an evaluation model to comprehensively consider the impact of the multi - message queue queuing process and the transmission process on AoI, including factors such as packet generation rate, transmission delay, and success rate;

[0011] (3) NOMA resource allocation strategy: Implement the resource allocation mechanism of NOMA to reduce resource collisions through shared spectrum and power allocation, and optimize the success rate of packet transmission.

[0012] Furthermore, the specific steps of the queue design in step (1) are as follows: Mode 4 of C - V2X includes four types of messages, namely HPD, DENM, CAM, and MHD, and their priorities are HPD > DENM > CAM > MHD; among them, the new packet generation probability of CAM is 1 / Tc, where Tc is the fixed packet generation period of CAM; the new packet generation probabilities of HPD, DENM, and MHD are represented by the Poisson distribution:

[0013]

[0014] where n ∈ [HPD, DENM, MHD], represents the generation probability of new packets of n types of packet types for vehicle i at time t, and λ n represents the number of generations of each type of packet within a certain period of time, is a normalization constant used to ensure that the sum of the probabilities of all possible events is 1;

[0015] Their queue capacities are all L, and the queue length is q; when the queue satisfies q < L, a corresponding new packet can be added, and they share the same transmission opportunity to leave the queue; the transmission opportunity refers to the time when the vehicle can use the predetermined resources, defined as t, at which time the vehicle will transmit the packets in each queue according to the priority; when the queue q of the high - priority queue is not 0, the corresponding transmission action s takes 1, and the s corresponding to other lower - priority queues takes 0; only when the q of the high - priority queue is 0 can the s of the next - priority queue take 1; s = 1 means that the packets in this queue can be transmitted, otherwise they cannot be transmitted. Therefore, the expression of the multi - priority queue transmission action s is:

[0016]

[0017] where represents the transmission opportunity of the HDP queue at time t, represents the transmission opportunity of the CAM queue, Indicates whether the HDP queue has a data packet at time t. Indicates whether there are data packets in the CAM queue; when the q of all queues is 0, it means that no data packets can be transmitted, and the s of all queues is 0.

[0018] Furthermore, the specific implementation steps of the AoI evaluation model in step (2) are as follows: when a data packet is in a queue, it will increase every subframe, and when it leaves the queue, it will no longer increase, so the AoI expression of each message in each queue is:

[0019]

[0020] Where n contains four queues, b∈[1,q-1] represents the position of the data packet in the queue. is the AoI of the message at position b in queue n of vehicle i at time t, Whether the data packet in queue n of vehicle i at time t can be transmitted;

[0021] The accumulated AoI during the communication process is regarded as the time taken to complete the transmission between the receiver and the transmitter, so the AoI expression of each receiver j to the transmitter i is:

[0022]

[0023] in is the AoI of vehicle j to vehicle i at time t, indicates that vehicle i successfully transmits the data packet to j, and This indicates reception failure.

[0024] Furthermore, the specific implementation steps of step (3) NOMA resource allocation are as follows: if the receiving end receives at most one signal in one resource, the signal-to-noise ratio between vehicles at time t is:

[0025]

[0026] is the transmission power of vehicle i at time t, is the channel gain of communication between car i and j on resource n, σ 2 is the noise energy; however, due to the half-duplex resource selection scheme of C-V2X, vehicles may select the same resource block when selecting resources. When they use the same resource for transmission, collisions will occur. At this time, the wireless information transmission rate is defined as:

[0027]

[0028] Where m is the other vehicle in the C-V2X scenario except i, representing the interfering vehicle. is their transmission energy; it can be seen that when multiple vehicles use the same resource, the transmission rate is reduced, and when the transmission rate is too low, it will cause transmission failure; therefore, NOMA based on serial interference cancellation is introduced to deal with this situation; the receiving end sorts the multiple signals received according to the size of the received power, first takes the signal with the maximum received power as the target signal and the other signals as interference, and removes them after decoding; then the operation is repeated until the SINR of all signals is calculated, let is a group of vehicles with weaker receiving power than vehicle i, k is the vehicle number except i among all vehicles N, then the SINR at vehicle i is:

[0029]

[0030] It can be seen that using NOMA when a collision occurs can reduce the interference to the target signal and thus increase the SINR, reducing the possibility of transmission failure due to the SINR being lower than the threshold.

[0031] Compared with the prior art, the present invention has the following beneficial effects:

[0032] NOMA reduces the impact of the AoI of messages in the C-V2X system during the transmission process, and multi-priority queues reduce the impact of the AoI of high-priority messages during the queuing process, effectively improving the optimization of the AoI in the C-V2X system. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] The drawings described herein are used to provide a further understanding of the embodiments of the present invention, constitute a part of this application, and do not constitute a limitation of the embodiments of the present invention. In the drawings:

[0034] Figure 1 A C-V2X communication system based on NOMA and multi-priority message queues is provided in an embodiment of the present application.

[0035] Figure 2 The embodiment of the present application provides the AoI change trends of various data packets when the number of vehicles is set to 50 and Γ is set to 100 ms.

[0036] Figure 3 The average AoI in queues with different Γ values ​​when the number of vehicles provided in the embodiment of the present application is 30 and 50.

[0037] Figure 4 The average AoI of the receiving end under different Γ when the number of vehicles provided in the embodiment of the present application is 30. DETAILED DESCRIPTION

[0038] In order to further explain the technical means and effects adopted by the present invention to achieve the predetermined invention purpose, the specific implementation mode, structure, characteristics and effects of the present invention are described in detail below in combination with the accompanying drawings and preferred embodiments.

[0039] A C-V2X information age optimization method based on NOMA and multi-priority queues, specifically comprising:

[0040] Step (1): Composition structure of C-V2X vehicle networking system

[0041] Consider a cellular-based V2X communication system with N half-duplex vehicles. All communication resources in C-V2X mode 4 are in the resource pool. The minimum unit of the resource pool in the frequency dimension is RB, which is 180kHz, and the subchannel generally contains 10 RBs. The time is set as a discrete value, and each time is divided into a time slot lasting 1ms, which is the size of a subframe in the resource pool. The four priority message types generate data packets in different ways. When the multi-priority queue is not empty, the vehicle will reserve communication resources from the resource pool. The time interval between the reserved resources is RRI, which is generally 20ms, 50ms, and 100ms. The number of reserved resources is the initial value of RC, defined as 500 / RRI+rand(1000 / RRI). And because of half-duplex, the vehicle cannot receive signals when using communication resources.

[0042] Step (2): AoI composition in C-V2X

[0043] Mode 4 of C-V2X includes four types of data packets: High-Priority Data (HPD), Decentralized Environmental Notification Message (DENM), Cooperative Awareness Message (CAM), and Miscellaneous High-Density Data (MHD). Their priorities are HPD>DENM>CAM>MHD. The generation of CAM type data packets is periodic, while the other three types of data packets are triggered. The probability of generating a new CAM packet is 1 / Tc, where Tc is the fixed packet generation period of CAM; the probability of generating a new HPD, DENM, and MHD packet is represented by Poisson distribution:

[0044]

[0045] where n∈[HPD,DENM,MHD], represents the probability of vehicle i generating new packets of n packet types at time t, λ nIndicates the number of packets generated for each type within a certain period of time. is a normalization constant used to ensure that the sum of probabilities of all possible events is 1. The new packets generated by DENM and HDM need to be retransmitted multiple times to ensure the successful transmission of information, and this retransmission process is periodic. Therefore, four corresponding FIFO queues are established according to different signal types, and their queue capacities are all L, and the queue length is q. Only when the queue satisfies q < L can a corresponding new packet be added, and they share the same transmission opportunity to leave the queue.

[0046] The transmission opportunity refers to the time when the vehicle can use the predetermined resources, defined as t. At this time, the vehicle will transmit the data packets in each queue according to the priority. When the queue q of the high-priority queue is not 0, the corresponding transmission action s takes 1, and s takes 0 for other lower-priority queues; only when the q of the high-priority queue is 0 can the s of the next-priority queue take 1. When s is 1, it means that the data packet in this queue can be transmitted, otherwise it cannot be transmitted. Therefore, the expression for the transmission action s of the multi-priority queue is:

[0047]

[0048] Because it is judged in order of priority, the relationship of s of different queues can be reflected by using the expressions of two priority queues. Among them represents the transmission opportunity of the HDP queue at time t, represents the transmission opportunity of the CAM queue, represents whether there is a data packet in the HDP queue at time t, represents whether there is a data packet in the CAM queue. When the q of all queues is 0, it means that there is no data packet to be transmitted, and at this time, the s of all queues is 0.

[0049] The AoI accumulated by the data packet in the queue is generated by queuing, defined as the time experienced by the data packet from generation to transmission. Therefore, the processing rate that affects the queue length will also affect AoI. The data packet grows every sub-frame when queuing and stops growing when leaving the queue. Therefore, the AoI expression for each data packet in each queue is:

[0050]

[0051] where n includes four queues, and b ∈ [1, q - 1] represents the position of the data packet in the queue, is the AoI of the data packet at position b in queue n of vehicle i at time t, is whether the data packet in queue n of vehicle i at time t can be transmitted. When, the positions of all data packets except the head packet in this queue will move, The position of all packets in the queue remains unchanged. The probability of a packet being transmitted is the processing rate. Since the processing rate in C-V2X is expressed as 1 / RRI, RRI will affect the size of AoI.

[0052] The accumulated AoI during the communication process can be regarded as the time it takes to complete the transmission between the receiver and the transmitter, that is, the transmission delay and whether the transmission is successful will affect it. And because the received data packet reflects the situation at the time when the transmitter generates the packet, the receiver needs to inherit its size to indicate the degree of understanding of the situation at the transmitter. The AoI expression of each receiver j for the transmitter i is:

[0053]

[0054] in is the AoI of vehicle j to vehicle i at time t, It means that vehicle i successfully transmits the data packet to vehicle j. At this time, the AoI at vehicle j is the AoI of the first position data packet in the highest priority queue broadcast by vehicle i plus the transmission delay; If the reception fails, for Add the subframe size. From equations 3 and 4, we can see and When increasing at the same time and the transmission fails Increase unchanged, so That is, transmission failure will result in And due to the unique way of reserving resources in C-V2X Mode 4, each transmission failure must wait for a time interval of the size of RRI, that is, The main reason for transmission failure is that the SINR is too low due to collision, and NOMA can solve this problem.

[0055] Step (3): Consider the NOMA-based communication model in C-V2X.

[0056] Collisions in C-V2X: In C-V2X Mode 4, vehicles autonomously allocate resources and transmit in a broadcast manner; when multiple vehicles reserve resources in a close time, they may select the same resource, and because of the broadcast method, they can communicate with the same receiver at the same time. When multiple vehicles use the same resources to communicate with the same receiver, a collision will occur. The collision probability in C-V2X Mode 4 can be expressed as:

[0057]

[0058] Here, π represents the probability that the vehicle is ready to select resources, that is, the probability that the vehicle queue is not empty, the reselection counter is 0, and a new resource is rebooked. Here, i represents a time slot in the selection window, P rk represents the probability of the vehicle reselecting a new resource when the reselection counter returns to zero, CSR represents the total number of resources in the selection window, and N v is the total number of vehicles, Γ is the size of the selection window, so CSR increases as Γ increases. However, the range of Γ is very small, usually 20, 50 or 100, so when Γ selects a value, For a random N v The value of the change, so N v The larger the P col The larger the value is. Moreover, Γ, as the time interval for vehicles to transmit data packets, may increase the waiting time of data packets in the queue while reducing the probability of collision, which will make the AoI larger. Therefore, it is considered to use NOMA to reduce the impact of collisions based on changing Γ.

[0059] SINR calculation: In C-V2X mode 4, the size of the transmission resource is not fixed. Vehicle i calculates the bandwidth B and transmission rate threshold required for successful transmission within 1 subframe based on the size Q of the data packet to be transmitted. The expression is:

[0060]

[0061] in Because the time for a data packet to complete data transmission needs to be less than the length of a subframe, that is, Q / R ≤ 1. Then the signal-to-noise ratio threshold SINR is calculated based on the bit width of the transmission channel and the transmission rate threshold th , whose expression is as follows:

[0062]

[0063] Since the distances between vehicles are different at different times, the communication will suffer different channel impairments, resulting in the receiver receiving at different communication rates in different channels; and since multiple vehicles may occupy different communication resources in the same subframe, the receiver also receives at different rates in different communication resources. If the receiver receives at most one signal in one resource, the signal-to-noise ratio between vehicles at time t is:

[0064]

[0065] is the transmission power of vehicle i at time t, is the channel gain of communication between car i and j on resource n, σ 2is the noise energy. However, due to the half-duplex resource selection scheme of C-V2X, vehicles may select the same resource block when selecting resources. When they use the same resource for transmission, collisions will occur. At this time, the wireless information transmission rate is defined as:

[0066]

[0067] Where m is the other vehicle in the C-V2X scenario except i, representing the interfering vehicle. is their transmission energy. It can be seen that when multiple vehicles use the same resource, the transmission rate is reduced, and when the transmission rate is too low, it will cause transmission failure. Therefore, NOMA based on serial interference cancellation is introduced to deal with this situation. The receiving end sorts the multiple signals received according to the size of the received power. First, the signal with the maximum received power is used as the target signal and the other signals are used as interference. After decoding, they are removed. Then the operation is repeated until the SINR of all signals is calculated. Let is a group of vehicles with weaker receiving power than vehicle i, k is the vehicle number except i among all vehicles N, then the SINR at vehicle i is:

[0068]

[0069] It can be seen that using NOMA when a collision occurs can reduce the interference to the target signal, thereby increasing the SINR and reducing the possibility of transmission failure due to the SINR being below the threshold. c ij is the coefficient between vehicle i and vehicle j, d ij is the distance between vehicle i and vehicle j, η is the path loss exponent, where d ij Depends on the positions of vehicles i and j at time t.

[0070] Vehicle motion: Assume a two-way road with length D and U / 2 lanes in each direction. Establish a coordinate system to define the position of vehicle i at time t as The origin is the leftmost position of the road, the x-axis is the direction of the vehicle's movement, and the y-axis represents different lanes. Assuming that the vehicle updates its position every short period of time τ, the speed V of vehicle i can be considered constant:

[0071]

[0072] Where δ∈[-1,1] represents the direction of the vehicle, let is the initial position of the vehicle. Depends on the lane index of vehicle i (1≤u≤U), calculated as:

[0073]

[0074] where d y is the width of a lane, y0 is the distance from the vehicle to the edge of the lane, generally 1 / 2d y .

[0075] Table I Communication success rate

[0076]

[0077] The above description is only a preferred embodiment of the present invention and does not limit the present invention in any form. Although the present invention has been disclosed as a preferred embodiment as above, it is not used to limit the present invention. Any technical personnel in this field can make some changes or modify the technical contents disclosed above into equivalent embodiments without departing from the scope of the technical solution of the present invention. However, any brief modifications, equivalent changes and modifications made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solution of the present invention are still within the scope of the technical solution of the present invention.

Claims

1. A C-V2X information age optimization method, characterized by: Including: (1) Multi-priority data packet queue design: Establish multi-priority queues for four types of data packets in the Cellular Vehicle-to-Everything (C-V2X) system. The four types of data packets are High-Priority Data (HPD), Decentralized Environmental Notification Message (DENM), Cooperative Awareness Message (CAM), and Other High-Density Data (MHD). Their priorities are HPD > DENM > CAM > MHD. Allocate data packets to different priority First-In-First-Out (FIFO) queues according to their urgency. The new packet generation probability of CAM is 1 / Tc, where Tc is the fixed packet generation period of CAM. The new packet generation probabilities of HPD, DENM, and MHD are represented by Poisson distribution: where n∈[HPD,DENM,MHD], represents the probability of vehicle i generating new packets of n packet types at time t, λ n Indicates the number of times each type of data packet is generated within a certain period of time. is a normalizing constant used to ensure that the sum of the probabilities of all possible event numbers is 1; Their queue capacities are all L, and the queue length is q. Only when the queue satisfies q < L will the corresponding new packet be added, and they share the same transmission opportunity to leave the queue. The transmission opportunity is defined as t. At this time, the vehicle will transmit the data packets in each queue according to the priority. The expression of the multi-priority queue transmission action s is: in represents the transmission opportunity of the HDP queue at time t, represents the transmission opportunity of the CAM queue, Indicates whether the HDP queue has a data packet at time t. Indicates whether there are data packets in the CAM queue; when the q of all queues is 0, it means that there is no data packet transmission, and the s of all queues is 0; (2) Automatic Optical Inspection (AoI) evaluation model: The AoI expression of each message in each queue is: Where n contains four queues, b∈[1,q-1] represents the position of the data packet in the queue. is the AoI of the message at position b in queue n of vehicle i at time t, Whether the data packet in queue n of vehicle i at time t can be transmitted; The AoI expression of each receiving end j for the transmitting end i is: in is the AoI of vehicle j to vehicle i at time t, indicates that vehicle i successfully transmits the data packet to j, and It means reception failed; (3) Non-Orthogonal Multiple Access (NOMA) resource allocation strategy: If the receiving end receives at most one signal on one resource, then the signal-to-noise ratio between vehicles at time t is: is the transmission power of vehicle i at time t, is the channel gain of communication between car i and j on resource n, σ 2 is the noise energy; the wireless information transmission rate is defined as: Where m is the other vehicle in the C-V2X scenario except i, representing the interfering vehicle. is their transmission energy; when multiple vehicles use the same resource, the transmission rate is reduced, and when the transmission rate is too low, it will cause transmission failure; the receiving end sorts the multiple signals received according to the size of the received power, first taking the signal with the largest received power as the target signal and other signals as interference, and removes them after decoding; Then, the receiving end will perform the above operation of sorting the received multiple signals according to the received power until the SINR of all signals is calculated. is a group of vehicles with weaker receiving power than vehicle i, k is the vehicle number except i among all vehicles N, then the SINR at vehicle i is:

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