# A Survey of Autonomous Vehicles: Enabling Communication Technologies and Challenges
## 2 Literature Review  
The focus of the paper is to cover the number of sensors employed for AVs and the virtual server for data transmission. Most applications for ITS require a multitude of sensors, which assist automation and communication. These are mounted within a vehicle or on the network infrastructure. Depending on the transmission range of the wireless technology, the sensors are divided into three main categories, namely short-range, medium-range and long-range. Each category has a separate communication latency and, thus, is deployed in different scenarios. A summary of the wireless technologies, their categorisation and use cases in existing literature are referenced and discussed in Table 3.  
In [14], the architecture design and implementation of an AV is discussed, and, in [15], various strategies of multi-sensor fusion are discussed. The recent developments and advancements in the perception and sensor technologies for AVs are presented in [16]. In [17], multiple-target and multiple-source are combined to form a framework for on-board sensors. In [1], a five-layer architecture for AVs is discussed. However, none of these survey papers discuss the key wireless technologies for data communication. In [18], resource allocation schemes for DSRC and C-V2X are discussed. However, both these technologies are suitable only for medium- and long-range communication and lack low-latency application. This paper contributes to the current state-of-the-art by providing a holistic review of the key enabling wireless technologies for ITS. It not only identifies the main aspects of network architecture behind AVs, but it also analyses the evolution of the technological trends, the challenges and the constraints that AVs must address before being fully implemented. The paper links abstract technological concepts with the expected real implications that AVs can have in human mobility, transportation systems and infrastructure. Most of the recent papers address these issues separately and they focus on technological innovations, adoption or regulatory issues. Therefore, this paper tries to gather the conceptual frameworks with the practical aspects by illustrating the main architecture concepts and compiling the outcomes of some of the latest research about VANET technologies, including their applications, advantages and limitations. For short-range communications where transmission range is less than 25 m, four technologies, namely, Bluetooth [19], Bluetooth low energy (BLE) [20], ZigBee [21] and UWB [22], have been proposed. Several papers have been published which compare these wireless technologies, evaluating their main features, such as power consumption, data rates and transmission time (see, e.g., [23; 24] and references therein). \begin{table}
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**Advantages** & **Disadvantages** \\ \hline Casualty: AVs can significantly reduce the number of accidents. & Law: The definition of legal responsibilities can hinder the implementation of AVs. \\ \hline Fewer Expenses: Precise autonomous driving can reduce fuel consumption and increase the conservation of other parts. & Threat: AVs can be more vulnerable to network hacks because of the present computer-controlled functions. \\ \hline Productivity: The journey can be productive by performing other activities than driving. & Employment: There will be many job losses due to AVs in the transportation sector. \\ \hline Comfort: Interiors of AVs can be comfortable and spacious. & Price: The price of AVs is initially high, but, after greater adoption, the price is going to decrease. \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Advantages and disadvantages of AVs. For medium-range communication, where transmission range is between 25 and 100 m, WiFi and DSRC are useful wireless technologies [25; 26]. However, IEEE 802.11p/DSRC is the prevailing technology for vehicular communications as it supports high-mobility.