# Connected Vehicles: Technology Review, State of the Art, Challenges and Opportunities
## 2 Enabling Technologies
### DSRC  
For over a decade, DSRC was one of the key enabling wireless communication technologies standards designed specifically for vehicular communication [10; 19]. The IEEE standard for wireless access in vehicular environments (WAVE) presents the fundamental specifications of DSRC [20; 21]. WAVE incorporates IEEE 802.11p, which deals with the medium-access control (MAC) layer and which tackles design challenges at the physical (PHY) layer of DSRC as illustrated in Figure 1.  
The need for reliable and efficient wireless communication technology to support many of the rising innovative vehicular applications has resulted in further enhancements and developments in the current DSRC technology.  
In spite of these efforts, in November 2020, the FCC has ruled on new regulations to reallocate the 5.850 to 5.925 GHz (i.e., 5.9 GHz spectrum band) dedicated for DSRC standard operations since 1999. The spectrum reallocation announced by the FCC comes as a drastic measure towards the slow deployment of the DSRC technology. The existing 75 MHz safety spectrum will be subdivided into two bandwidth allocations. The upper band of 30 MHz ranging from 5.850 to 5.925 GHz will no longer support DSRC and will be instead reassigned to advance and speed up the deployment of cellular-vehicle-to-everything (C-V2X) technology. The remaining 45 MHz of the bandwidth will be taken away and reserved for unlicensed wireless usage, presumably Wi-Fi [23].  
In the automotive industrial field, there has been minimal adoption of the DSRC-based technology in any of the past original equipment manufacturers' (OEMs) lineup production models. GM was the only automotive brand to adopt the DSRC technology in their Cadillac lineup production with only 50,000 DSRC-equipped vehicles rolled out. Although some auto manufacturers consider DSRC obsolete, LTE wireless communication technologies today cannot provide support for crash-imminent vehicle safety applications due to their high latency. 5G promises to handle this limitation, but it may take a while to be widely deployed and also need better training and understanding. Further concerns and skepticism are raised in regards to spectrum overloading and cross-interference that might occur from other radio-based devices utilizing adjacent frequencies to the 30 MHz spectrum where C-V2X technology is expected to operate. Even with the lack of research studies  
Figure 1: WAVE/DSRC communication’s layered architecture [22].  
towards the aforementioned issues, the momentum shift towards the C-V2X technology is accelerating. Even though Toyota was one of the first automotive companies to support DSRC and integrate it in their 2021 production lineup, this plan has been recently put on hold to reevaluate their deployment environment. GM is considering cellular technology but has already integrated DSRC capabilities in the Cadillac CT6 and super cruise level 3 autonomous vehicles. Nissan is on the neutral side and is studying the potential and drawbacks of each technology; they are open to the deployment of any technology depending on the demand. Auto manufacturers have transitioned from the phase of the wait-and-see approach to the adoption of C-V2X technology in their lineups. Ford has already announced their C-V2X technology deployment in their production line for 2022 [23; 24].  
In the same year, SAE International announced a new publication of the SAE J3216 standard, which primarily focuses on cooperative driving automation through the integration of both connectivity and automation [25].