1 Introduction 
In recent years, as car ownership continues to grow, road carrying capacity has reached saturation in many cities, and traffic safety, travel efficiency, and environmental protection have become increasingly prominent. In this context, the intelligentization and networking of automobiles has become an important way to solve these problems, and has been highly valued by the industry. The birth and rapid development of the Internet of Vehicles has revolutionized the automotive industry and changed people's lives. Internet of Vehicles has become the only means to achieve the goal of China Intelligent Networked Cars in 2025 (see Figure 1).
In the development of vehicle networking, wireless communication technology plays a key role in the development and evolution of vehicle networking communication. In the era of the Internet of Things, where vehicle-mounted terminals are rapidly increasing and vehicle communication needs are increasing, how to meet the low-latency, high-reliability, high-transmission rate, and high-capacity requirements of vehicle communication in high-density scenarios is the key to wireless communication networks. challenge.
2 The necessity of car network communication The Internet of Vehicles (V2X: Vehicle to Everything) is a vehicle (V2X: Vehicle to Everything) that utilizes in-vehicle electronic sensing devices to enable vehicles, roads, and vehicles through mobile communication technologies, car navigation systems, intelligent terminal devices, and information network platforms. Real-time networking between vehicles, vehicles and people, vehicles and the Internet to realize information interconnection and interoperability, thus effectively monitoring, scheduling, and managing network systems for vehicles, people, objects, roads, and locations. It is the foundation and key technology for smart cars, autonomous driving and intelligent transportation systems in the future.
As the intelligent bicycle technology of the vehicle, the Advanced Driver Assistant System (ADAS) is the initial stage of vehicle intelligence and is an active safety technology for improving safety. ADAS uses various sensors (cameras, radar, lasers, and ultrasonics) installed in the car to collect environmental data inside and outside the vehicle, and to identify and detect static and dynamic objects, so that the driver can get the fastest time. Identify the dangers that may occur and thus increase safety. The early ADAS technology was mainly based on passive alarms. When the vehicle detected a potential hazard, it would issue an alarm to alert the driver to abnormal vehicle or road conditions. The latest ADAS technology has achieved active intervention. However, there are certain limitations in the application of ADAS technology:
(1) ADAS's sensing system with millimeter wave radar plus camera cannot achieve accurate perception of all-weather road conditions. Because ADAS is a bicycle intelligent technology, the sensor cost is high and the working distance is limited. At the same time, it is greatly affected by weather conditions such as rain, snow and smog, which is easy to cause system error, which leads to safety accidents.
(2) ADAS technology cannot cover the basic application of intelligent transportation. The European Telecommunications Standardization Institute (ETSI: European Telecommunica TIons Standards InsTItute) defines a set of basic applications for intelligent transportation in its technical report TR 102 638, which contains 54 applications. TelemaTIcs in-vehicle information service provides 25 information service applications, ADAS can provide 18 early warning driving assistance applications, but the remaining 11 key driving assistance functions ADAS can not provide services, such as intersection collision warning.
The vehicle network V2X technology can extend the vehicle's sensing range to hundreds of meters, which is superior to the radar and optical camera detection range in the ADAS system. Combining the vehicle network V2X technology with various detection methods of the ADAS system, with the help of the integrated information processing technology, it can effectively improve the driving safety and traffic efficiency. The organic combination of bicycle intelligence and vehicle networking ultimately leads to driverless driving.
3 car networking communication scene The Internet of Vehicles mainly includes four application scenarios: V2N (Vehicle to Network), Vehicle to Vehicle (V2V: Vehicle to Vehicle), Vehicle to Infrastructure (V2I: Vehicle to Infrastructure), and Vehicle Connect (V2P: Vehicle to Pedestrian), the purpose of intelligent transportation through the effective coordination of people, cars and roads.
(1) V2N is currently the most widely used car networking scenario. Its main function is to enable vehicles to connect to cloud servers through mobile networks, using navigation, entertainment and anti-theft applications provided by cloud servers.
(2) V2V is used for bidirectional data transmission between vehicles. Through V2V, the vehicle can collect the speed, position, direction and alarm information of the surrounding vehicles in real time, mainly used in the anti-collision safety system between vehicles. In addition, the exchange function of pictures, text messages, audio and video, etc. can also be realized through inter-vehicle communication.
(3) V2I is an application in which vehicles communicate with roads and even other infrastructure such as traffic lights, roadblocks, etc. Through the V2I system, the vehicle can obtain road management information such as traffic light signal timing and location-based vehicle service information, and is mainly applied to real-time information services, vehicle operation monitoring, and electronic toll management.
(4) V2P refers to the communication between pedestrians using mobile electronic devices, such as laptops, smart phones or other handheld devices and in-vehicle electronic devices. The important application scenario is that vehicles send security warnings to road pedestrians or non-motor vehicles.
4 Comparison of existing vehicle networking communication technologies At present, the vehicle networking communication technology is divided into two camps: IEEE 802.11p (the underlying wireless communication technology used for DSRC dedicated short-range communication) and 3GPP C-V2X (V2X wireless communication technology based on cellular network).
4.1 DSRC/IEEE 802.11p
Dedicated Short Range CommunicaTIons (DSRC) is a short-range wireless communication technology specifically designed for the V2VV2I communication standard. DSRC enables real-time, accurate and reliable bi-directional transmission of images, voice and data over a small area. DSRC development is relatively mature. The international DSRC standards mainly include the three camps of Europe, America and Japan, ASTM/IEEE in the US, ARIBSTD-T75 and ARIBSTD-T88 in Japan, and CEN/TC278 in Europe.
In order to promote the standardization and industrialization of DSRC, the IEEE established the Vehicle Wireless Access (WAVE) Working Group in 2004 to study and upgrade the DSRC standard of the 5.9 GHz band developed by ASTM in the United States, and to design and develop a unified and globally applicable Vehicle networking communication standards. In July 2010, the WAVE Working Group officially released the IEEE 802.11p car networking communication standard, and realized the evolution of the DSRC standard to the 802.11p protocol and the IEEE 1069/WAVE (Wireless Access in the Vehicles Environment) series of standards.
IEEE 802.11p is a communication protocol extended by the IEEE 802.11 standard. It can support the communication and data exchange of driving safety data between adjacent vehicles, and is in line with the relevant applications of intelligent transportation systems. From a technical point of view, the IEEE 802.11p standard has implemented a number of improvements to IEEE 802.11 for vehicle specific environments, such as enhanced hotspot switching, better support for mobile environments, enhanced security, and enhanced identity authentication.
The IEEE 1609 series of standards is proposed to improve the application layer functions of the DSRC standard. It is a high-level series of standards based on the IEEE 802.11p standard that defines data exchange formats between vehicles and between vehicles and roadside infrastructure. Security, etc.
4.2 3GPP C-V2X
C-V2X (Cellular Based V2X) is a car-network communication technology based on mobile cellular networks. The C-V2X based on the LTE cellular network is called LTE-V2X, and the 3GPP has completed the standardization of LTE-V2X in March 2017. In the future, the C-V2X based on the 5G New Radio cellular network is called NR-V2X.
The air interface of the LTE-V2X system is divided into two types, one is the Uu interface, and the base station is required as the control center. The vehicle and the infrastructure and other vehicles need to communicate by transferring the data at the base station to realize communication; the other is the PC5. The interface enables direct transmission of data between vehicles (see Figure 2). There are two working scenarios of LTE-V2X. One is based on the coverage of the cellular network. In this case, the Uu interface of the cellular network can provide services to achieve large bandwidth and large coverage communication, and can also provide services through the PC5 interface. The vehicle has low latency and high reliability direct communication with the surrounding environment nodes; the other is the work environment independent of the cellular network, and provides the vehicle network road service through the PC5 interface in the area without network deployment to meet the traffic safety requirements. In the scenario of cellular network coverage, data transmission can be flexibly and seamlessly switched between the Uu interface and the PC5 interface.
The Uu interface of the LTE-V2X is specifically enhanced based on the Uu interface of the LTE. For example, the LTE broadcast multicast technology is optimized to effectively support the car network, such as a small broadcast area and a flexible area, and the control channel is performed. Crop to further reduce the delay.
The PC5 interface of LTE-V2X is enhanced in many aspects based on Release 12 LTE-D2D (Device to Device) to support fast exchange of vehicle dynamic information (such as position, speed, driving direction, etc.) between vehicles. And an efficient radio resource allocation mechanism, in addition, the physical layer structure has been enhanced to support higher moving speeds (500km / h).
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