# Advancing C-V2X for Level 5 Autonomous Driving from the Perspective of 3 GPP Standards
## 3 Taxonomy of C-V2X Related 3 GPP Standard Documents  
With the view to align the standard documents with current trends and guide the readers to the right documents, we suggest a simple but descriptive taxonomy of the standard documents in this section. In this regard, we present Table 1, Figures 2 and 3. The idea is to:  
* Categorize the standard documents following the natural evolution path of communication technologies,
* Evolution of technical specifications showcasing the dependency among specification documents,
* Decompose the services on the system and network segments including core and access network,
* Map the standard documents to the aforementioned system and network segments. We believe that the contributed categorization and taxonomy will significantly aid the stakeholders in their pursuits. The standard documents encamp rich V2X relevant guidelines for future scenarios, however, distributed across a wide range of documents. As can be seen in Table 1, we ensure the arrangement of relevant documents in terms of their technological evolution e.g., 4G, 4G & 5G, and 5G based V2X services. Furthermore, we have also arranged the documents according to the release number under each technological category, which clearly visualizes the continuation and focus of various topics in different releases. The use-cases and services (from _TR 22.885_) support V2X communication types and address the safety and non-safety issues through 4G technology [17]. Therefore, it goes without saying that the technical report (_TR 22.885_) is the foundual building-block for identifying 4G V2X use-cases and their potential requirements. Since each 4G V2X use-case provides description, pre-conditions, service flows, post-conditions, and potential requirements, the readers are encouraged to go through _TR 22.885_ as we simply list few use-cases here owing to the page limitation i.e., emergency vehicle warning, road safety services, wrong-way driving warning, curve speed warning, etc. Furthermore, the report also presented coverage, spectrum, security, mobility, future-proofness, and deployment considerations and listed various important V2X deployment example scenarios. Hence, the identified use cases and other important details complement the 3 GPP standardization documents for 4G (before) and 5G (now) V2X communication services. Figure 1: Evolution of 3 GPP C-V2X Standardization. To equip the readers with a simplified version of the technical specification (TS) documents evolution for C-V2X services starting from 2016 until 2021, Figure 2 presents technical specifications' categorization fed from Table 1 and yearly evolution of the produced specifications. The annual division of the TSs highlights the updated production of documents. For example, the majority of the updated documents are produced in the year 2020, which are complemented by documents from the years 2018 and 2019. It can be noted that _Rel-16_ supported the realization of V2X services by producing almost half of the TS documents, which are complemented by TS documents from _Rel-14_ and _Rel-15_. However, the _Rel-16_'s TS documents outreached the support for 5G V2X services. Therefore, _Rel-17_'s TS documents (complemented by _Rel-16_) started addressing the support for V2X service through 5G technology. The 3 GPP includes three important technical specification groups (TSG) i.e., radio access network (RAN) [18], service & system aspects (SA), and core network & terminals (CT) [19]. These TSGs focus on requirements, functions, interfaces, architectures, execution environments, etc., for supporting V2X services. Inspired by the TSGs and missing support of literature, we decompose the C-V2X-based 3 GPP technical documents into system and network (including core and access network) support. Figure 3 shows a more precise yet easy-to-understand system and network support categories for cellular technologies over three axes, where each axis presents different services based on different cellular technologies i.e., 4G, 4G & 5G (Mix), and 5G.