# Connected Vehicles: Technology Review, State of the Art, Challenges and Opportunities
## 2 Enabling Technologies
### 5G-Cellular Technology
In June 2019, the first use case \"Lane-merge coordination\" was exhibited in France by the consortium to demonstrate the optimization of merging vehicles entering the highway. The vehicles are able to share their current state data with a centralized maneuver planning system and hence upon these data sharing, recommendations regarding acceleration, deceleration, and lane changing is provided to the CVs. Another use case demonstrated by 5 GCAR is \"cooperative perception\" to show communications between vehicles and vulnerable road users (VRUs) [39; 40]. The rollout of 5G NR networks is considered indispensable to endorse the aforementioned use cases and many others. In this context, the deployment of 5G NR would be predominately classified into two main operation modes, namely, non-standalone (NSA) and standalone (SA). The NSA mode will include the deployment of the preliminary stages of 5G NR through the assistance of the current 4G infrastructure in order to speed up the 5G network's implementation. The NSA operation mode will hold a number of advantages due to the interconnection between 4G and 5G networks. This includes a reduction in the 5G network deployment time and support for dual 4G/5G operated devices. The core architectural components of the NSA mode are LTE BS (eNB), LTE evolved packet core (EPC), 5G BS (gNBs), and 5G core (5 GC) networks. This is in addition to incorporating multi-radio access technologies (RATs) and dual-supported connectivity for customers. On the other hand, the SA operation mode incorporates the implementation of the entire version of 5G NR. 5G NR operating in the SA mode will leverage complete end-to-end 5 GC-based networks and architecture that can provide use cases with ultra-low latency and massive capacity. Thereby, the interconnection between gNBs and 5 GC networks will exploit 5G cells for information transfer in user/control planes. 5G NR networks in the SA operating mode connected with cloud native 5G core will improve ultra-reliable low-latency communication (URLLC) and support a wide-range of use cases and advanced NS-based functionalities and services. Ease of implementation, improved dadio access technologies (RAT), and better performance are additional advantages offered in the SA operating mode. However, one of the challenges that pertain to the SA mode is the wide coverage adoption of 5G [41; 42]. New radio (NR) V2X is the latest advancement that 3 GPP is in the process of developing and standardizing for release 16. It is built on top of 5G NR, which has been standardized in release 15. NR V2X has great potential with respect to V2X applicationsthat demand higher QoS requirements in comparison to applications supported by cellular-V2X. Some of these applications require ultra-low latency that can reach up to 3 ms with a reliability of 99.999% [43]. These advancements in 5G-cellular technology are expected to further leverage the potential benefits of the CVs by enabling various add-on services that can meet the QoX and QoS requirements. QoX is the degree of delight or annoyance of the end-user resulting from the fulfillment of his or her expectations with respect to the utility and/or enjoyment of the technology with respect to the user's personality and current state. The QoX perception process is known to be dependent on a triad of influential factors, namely, the vehicle being used and its specific features and conditions, the context in which it is being used, and the subjective (or human) personality and mental state of the user. Context is any information that can be used to characterize the situation [44]. This contextual information answers situation-related questions such as who, what, when, and where in order to characterize the environment, the vehicle, and the users in terms of location, identity, activity, time, and status. Context can also include communication medium aspects such as bandwidth, latency, and topology.