# A Survey of Autonomous Vehicles: Enabling Communication Technologies and Challenges
## 1 Introduction  
Technology facilitates humans, improves productivity and leads to a better quality of life. Technological developments and automation in vehicular networks will lead to better road safety and lower congestion in present urban areas where the traditional transport system is becoming increasingly disorganised and inefficient [1]. Therefore, the development of the intelligent transport systems (ITS) concept has been proposed, with the aim and focus on improving traffic safety and providing different services to its users [2, 3]. There has been considerable research in ITS resulting in significant contributions (see [4] and references therein). Figure 1 shows the three main applications of the ITS system: (1) Applications for transport efficiency; (2) Safety-critical applications; and (3) Infotainment applications. The main task of the transport efficiency application includes the calculation of the optimal speed and the route to navigate the vehicle to the destination by taking into account the traffic information [5]. Safety-critical applications deal with the response to emerging vehicles and intersection collision avoidance [5]. Infotainment applications deal with cooperative local services such as the location of petrol stations, hotels, etc. [5]. One of the most significant contributions towards the ITS has been the development of autonomous vehicles (AVs). These vehicles perceive the environment through sensing using various sensors, and then this information is utilised to drive without the need for any human intervention [6]. In Figure 2, the technical evolution of AVs is illustrated in detail. The modern cruise control system was developed in 1948 and since then self-drivingcars have been gradually developing different features [7; 8]. Significant developments were made in the early 2000s when the lane departure warning system (LDWS), adaptive cruise control (ACC), self-parking assistance (SPA), auto-pilot and traffic sign recognition (TSR) were developed for AVs [9; 10; 11]. It is planned that by 2022 we will have intelligent speed adaptation systems and by 2030 fully automated AVs will be available with no driver backup required. This advancement will further lead to the next emerging and evolutionary stage of the Internet of vehicles (IoV). In IoV networks, besides human as users of the Internet, physical sensors, machine-type devices and vehicles will be the users of the IoV ecosystem. The Society of Automotive Engineers (SAE) classified the vehicles into six different levels of automation. The same classification is also adopted by other organisations such as the International Organisation of Motor Vehicle Manufacturer (OICA) and the German Federal Highway Research Institute (BASt). The US National Highway Traffic Safety Administration (NHTSA) initially had a different classification, as compared in Table 1[12]. Since then, NHTSA has also adopted SAE's six levels of automation in the Federal Automated Vehicle Policy. Levels 0-2 can be broadly categorised as driver assisted, while Levels 3 and 4 can be termed as semi-automatic and Level 5 is fully autonomous. The detailed features of each level are described below:  
* Level 0 is not automated, and all the tasks are performed by the driver. * Level 1 provides driver assistance with some functions such as ACC, TSR, etc., but the driver controls the accelerator and the brakes while monitoring the surroundings. * Level 2 provides partial driving assistance, and the vehicle can perform the steering and acceleration functions. However, the driver is responsible for many safety-critical actions. * Level 3 provides conditional driving automation where the vehicle performs the entire monitoring of the surroundings. The driver is no longer responsible for safety-critical issues. Figure 1: Different applications for intelligent transport system (ITS). Figure 2: Technical evolution in autonomous cars. * Level 4 provides high driving automation, and the driver holds control only if the automated situation turns unsafe.