# Connected Vehicles: Technology Review, State of the Art, Challenges and Opportunities
## 2 Enabling Technologies
### 5G-Cellular Technology
The 5G network technology is deemed ideal for connecting IoT devices, and hence forming a separate subscription for every device is considered insignificant. This derives the investments to improve and enhance the infrastructure network to be capable of delivering QoS services at the edge. Moreover, the impact of 5G bandwidth on the backhaul networks and the frequent dynamic of the mobile devices impose a great challenge for delivering services in regards to virtualization and edge computing [35]. * **Centralized Architecture of Cellular Networks. ** The centralized-based architecture of the cellular networks will pose a great challenge in correspondence to CV environments. By design, transmitted information by vehicles have to be initially sent to the base station (BS), and hence it extends the latency for message delivery. This imposes drawbacks in safety time-critical applications, where these applications require ultra-low latency. For example, in a unicast mode situation, a vehicle transmits safety-critical information to the cellular BS, and this message is then either broadcast to all the vehicles within the BS's cell range or to the pertinent vehicles only. In these two situations, the downlink channel becomes traffic jammed even when there are a considerable number of vehicles [28; 36]. Proposed solutions for broadcasting safety messages include multimedia broadcast and multicast services (MBMS) along with the evolved multimedia broadcast and multicast services (eMBMS), which are incorporated in the 3 GPP standard. In broadcasting situations, the safety data is broadcast by BS to all vehicles in its cellular range. In this manner, vehicles decide on the relevancy of the received safety data whether to use this information or discard it. Thus, vehicles may perform unnecessary computation. The utilization of multicast services, where data is delivered to a multicast group of vehicles, is considered to be one of the proposed solutions for this issue [28]. * **Interference at Low-level Altitude. ** In CV environments, vehicles should be able to discover and communicate with nearby vehicles occasionally. A significant specification of 5G-enabled telecommunication is the availability of the proximity service (ProSe) [37; 38]. The main purpose of ProSe is to enable vehicles to have a better perception by discovering devices and services based on the position and geographical location information [21; 38]. ProSe is considered critical for many applications and communication opportunities within a specified range. Moreover, it provides communication and discovery in an ad-hoc manner, which is considered most convenient for safety-critical applications. However, the paradigm shift in ProSe gives rise to radio propagation among vehicles that is caused by high buildings, towers, and many other obstructions in urban canyons and metropolitan areas, for example, which lead to a high level of interference [38]. (2) Current Status of 5G-Cellular Technology: Current cellular technologies are able to provide some of the requirements needed by CVs. For example, LTE-CAT-M and narrowband Internet of Things (NB-IoT) are considerably reliable and adopt minimum power sensor telecommunication technologies. Yet, broadly speaking, cellular technologies fall short to meet many of the demanding requirements needed by V2X applications such as high speed, minimal latency, complicated vehicle maneuvering, and many others. Many parties including some of the leading automotive, telecommunication industries, and governments are driven towards 5G to leverage its benefits into ITS. In an effort to support 5G technology, a board of leading companies are aiding to advance a comprehensive 5G system architecture to leverage an optimized end-to-end V2X connectivity. A project named 5 GCAR financed by the European Commission, which incorporates 14 European companies such as Bosch, Orange, Volvo, PSA group, and many others, has been launched in support of these efforts.