# A Survey of Autonomous Vehicles: Enabling Communication Technologies and Challenges
## 6 Conclusions and Future Research Directions  
Due to the increase in road traffic, the possibility of accidents and collisions is also increasing. The main goal of the development of self-driving/autonomous vehicles is to make driving safer and more reliable, as well as reduce congestion. For this purpose, these vehicles need to precisely communicate with other vehicles and their surroundings. This process starts at the perception stage when the self-driving vehicle gather information from the environment using various sensors. Even though the data obtained from the sensors might be inaccurate, the fusion process allows the AVs to synthesise this raw data and obtain accurate results. After analysing the surrounding and making a decision, the ad-hoc network topology enables the AVs to communicate with each other and with the network infrastructure to ensure road safety and traffic flow. Different types of vehicular communication (V2V, V2I and V2X) performed by AVs use different technologies. This survey differentiates these technologies on the basis of transmission range. As a result, Bluetooth, BLE, ZigBee and UWB are grouped as short-range technologies, while DSRC and Wi-Fi are considered medium-range technologies. Similarly, C-V2X and 5G-NR are identified as long-range technologies. This survey has compared all these technologies with the aim of identifying the applications of V2X communication where each technology is best utilised. For short-range communication technology, this survey discusses its key features and proposes its use for some V2X applications which do not have strict latency requirements such as forward collision warning, toll check and vehicle identification. However, they cannot support applications such as remote driving, remote maintenance, etc. due to their lack of data rates and latency. Out of the four short-range technologies, UWB stands out because it can produce the highest data rate and low latency, and, hence, it can be used for above mentioned vehicular applications. Medium-range technologies such as DSRC and Wi-Fi can provide higher mobility to the AVs. This survey discussed why DSRC technology provides higher flexibility and is better suited for V2X communication compared to Wi-Fi. Furthermore, the DSRC technology facilitates AVs with different applications such as collision avoidance, lane changing assistance, etc. However, DSRC technology has certain limitations such as congestion issue, Doppler spread and hand-over issue. Based on these limitations and the ultra-low latency requirements, the paper concludes that DSRC technology does not completely fulfil the diverse needs for V2X mobility. Finally, the survey presents long-range technologies and identifies that C-V2X technology and the 5G-NR are enabling technologies for future intelligent transport systems. This survey discusses the two modes for C-V2X operations: Mode 3 and Mode 4. Mode 3 works within cellular coverage and Mode 4 is suitable for direct V2V communication using the PC5 interface. Moreover, different vehicular applications require high dates, low latency and reliable communication and the 5G-NR technology is able to provide massive machine type communication (mMTC), enhanced mobile broadband (eMBB) and ultra-reliable low