Interpretation of China C-V2X communication application layer standards

The cooperative intelligent transportation system vehicle communication system aims to realize various applications such as road safety, traffic efficiency, and information services through information interaction between various subsystems of the intelligent transportation system. To this end, there must be interconnection between vehicles from different manufacturers and between these vehicles and the road infrastructure in the areas they can reach.

Taking the on-board unit (OBU On-Board Unit) in the automotive communication system as an example, the on-board equipment usually includes subsystems as shown in the following figure:

The vehicle-mounted equipment is connected to the application electronic control unit through an interface, and runs the program in the application electronic control unit to realize the application of the vehicle communication system, and realizes reminders to the driver through the Human Machine Interface (HMI, Human Machine Interface), including images. , sound, vibration, etc.

Based on the seven-layer communication system reference model specified by the International Organization for Standardization (ISO) and the vehicle communication system architecture being formulated in Europe and the United States, vehicle communication systems can usually be divided into system applications, application layers, transmission layers, network layers, and data links. layer and physical layer. The DAY1 standard focuses on the application layer and the data interaction interface between the application layer and the upper and lower adjacent layers, as shown in Figure 2. The application layer protocol mainly includes the message set and the data frames and data elements in the message set, as well as the data structure and encoding method of the message.

Through the analysis of basic applications such as road safety, traffic efficiency and information services, the DAY1 standard defines the information interaction content, interaction protocols and information exchange between the vehicle and other vehicles, road traffic facilities and other traffic participants when implementing various applications. Interface etc.

The DAY1 standard selects three aspects: safety, efficiency, and information services to cover 17 application scenarios, including forward collision warning, intersection collision warning, abnormal vehicle reminder, green wave speed guidance, forward congestion reminder, near-field vehicle payment, etc. As shown in Table 1:

The DAY1 standard defines 17 scenarios in six aspects: application definition, main scenarios, basic system principles, communication methods, basic performance requirements and data interaction requirements. It also defines 17 scenarios based on the different requirements of applications for communication frequency and delay. Phase I applications are divided into two categories :

First, applications with high latency (>100 ms) and low frequency (<10 Hz) can be realized through 4G cellular communication technology;

The second is low-latency (≤100 ms), high-frequency (≥10 Hz) applications, which require the support of LTE-V2X, DSRC or 5G communication technology.

By comparing the requirements of the two versions of CSAE53-2017 and CSAE53-2020 in the DAY1 stage, it is not difficult to find that compared with the CSAE53-2017 protocol, CSAE53-2020 puts forward higher requirements for positioning accuracy, as shown in Table 2:

Table 2 Classification table of Phase I applications according to communication requirements

Except for the positioning accuracy of the ahead congestion reminder which remains at 5m and the positioning accuracy of smart car near-field payment which is canceled, other applications have increased the positioning accuracy from 5m to 1.5m, which puts forward higher requirements for GNSS positioning accuracy.

The DAY2 standard is based on the DAY1 standard and is targeted at safety, efficiency, information services, traffic management, advanced intelligent driving and other fields. It defines 12 typical applications, including sensing data sharing, collaborative lane changing, collaborative vehicle merging and other applications. As shown in Table 3:

Table 3 DAY2 application list

The DAY2 standard also describes 12 second-stage application scenarios in seven aspects: application definition, expected effects, main scenarios, basic system principles, communication methods, basic performance requirements and data interaction requirements.

Comparing the DAY1 and DAY2 standards, it is not difficult to find that the application scenarios of the DAY1 standard focus more on the realization of the early warning function of bicycles in a vehicle-road collaborative environment, while in the DAY2 stage, more scenarios focus on vehicle-road collaboration and transportation. Control management, etc. The author distinguished the scenes in DAY1 and DAY2 from the scene classification dimension, as shown in Table 4:

From Table 4, we can see that in the DAY2 standard, the three scenarios of blind spot warning/lane change warning, collision warning for vulnerable traffic participants, and emergency vehicle reminder are discussed in more depth.

Perception data sharing, collaborative vehicle merging, and collaborative intersection application scenarios have been added to the corresponding safety classification, and road toll services, dynamic lane management, collaborative vehicle merging, and collaborative intersection have been added to the efficiency classification. In the peer-to-port scenario, differential data services, station route guidance services, and road toll service application scenarios have been added to the information service classification.

At the same time, two new categories, traffic management and advanced intelligent driving, have been added. Dynamic lane management and floating vehicle data collection application scenarios have been added to the traffic management category, and collaborative vehicle formation management application scenarios have been added to the advanced intelligent driving category.

It is worth noting that the DAY2 standard focuses more on "collaboration". The naming of vehicles in the DAY2 standard has changed from HV (Host Vehicle) and RV (Remote Vehicle) in the DAY1 standard to EV-1 (Equipped Vehicle 1). EV-2 (Equipped Vehicle 2), this also means that the standard has shifted from focusing on the "rights and interests" of own vehicles in the DAY1 stage to focusing on the "rights and interests" of traffic participants in the DAY2 stage. It also proves that the author's DAY2 standard is more focused on "collaboration" the opinion of.

This article takes the blind spot warning/lane change warning scenario in DAY1 and the collaborative lane change scenario in DAY2 as examples.

The common focus and expected effect of the two scenarios of blind spot warning/lane change warning and cooperative lane change is how to safely change lanes between two vehicles to eliminate the driving risks caused by driving in blind spots.

Figures 3, 4, and 5 are respectively the lane change warning scene of DAY1 and the cooperative lane change scene of DAY2, showing a vehicle (HV in the blind spot warning/lane change warning scene, EV-1 in the cooperative lane change scene). ) intends to change lanes while driving, and another vehicle (RV in the blind spot warning/lane change warning scenario, EV-2 in the collaborative lane change scenario) has a safety risk in the process, in order to avoid the risk. .

The difference is that the intended purpose of the blind spot warning/lane change warning scenario is that after receiving the warning, the HV driver will have enough time to take measures to avoid a collision with the RV in the adjacent lane. The intended purpose of the collaborative lane change scenario is that the relevant vehicle (EV-2) receives the EV-1 lane change intention information, makes a judgment based on its own driving status and surrounding environment information, and adopts a driving behavior of slowing down to give way or accelerating to pass.

In the blind spot warning/lane change warning scenario, the focus is on the driving safety and status of the HV, while in the collaborative lane change scenario, more attention is paid to the collaborative behavior between collaborative vehicles.

The CSAE application layer standard follows the three-layer nested structure rules of "message set-data frame-data element" to formulate the application layer data set; it uses ASN.1 abstract syntax for definition, achieving cross-platform and compatibility with different programming languages; Considering radio frequency performance, the encoding and decoding method of data set interaction follows the unaligned compression encoding rules UPER (Unaligned Packet Encoding Rules), effectively compressing the size of air interface communication data packets.

The DAY1 protocol defines five basic message types: BSM, SPAT, RSI, MAP, and RSM, as shown in Figure 6. DAY2 expands the MAP and RSI messages based on DAY1, and also proposes new message bodies such as PAM, PSM, and RSC, as shown in Figure 7.

The launch of the DAY1 standard and the DAY2 standard is of great significance to the establishment and deepening of China's V2X communication standards, and has greatly promoted the development, verification and commercialization of vehicle communication system application scenarios.

This time I briefly introduce the Chinese V2X communication standard application layer standard, and we will bring you more relevant content based on C-V2X testing in the future. At the same time, inquiries and communication are also welcome to discuss together.