# On the Needs and Requirements Arising from Connected and Automated Driving
## 6 Related Challenges and Existing Research Works  
V2X services and technologies have been maturing for almost two decades. The IEEE 802.11p standard has emerged and 3 GPP also addressed these needs to respond to the requirements of the day one applications. They support delivering safety messages, such as cooperative awareness messages (CAM) and Decentralized Environmental Notification Message (DENM) [24]. Subsequently, after recognizing the increasing demand for vehicular communications, 3 GPP developed cellular vehicle-to-everything (C-V2X) communication, known as LTE-V2X based on the LTE standard. The 3 GPP-based V2X communications can be utilized for day one safety applications as well as non-safety (e.g., infotainment) purposes [25]. Although, both 802.11p and LTE-V2X provide satisfying results in low channel load and good propagation conditions. The contention-based MAC of 802.11p and the high collision probability of broadcasted packets in high load networks prevent the support of efficient and reliable safety critical services, such as cooperative maneuvers. Additionally, although LTE-V2X [26] introduces the sensing mechanism to reduce the collision probability, Fit does not completely eliminate the collision risk, especially in the case of high traffic loads where all of the resources might be sensed occupied and still the transmitter has to choose one of them. Several technical challenges have been identified as being associated to the new V2X applications:  
* **Meeting the requirements of low latency and high reliability simultaneously**: this is a major research challenge, especially in highly mobile vehicular scenarios, and requires overcoming the challenges that are created by the dynamics of the V2X wireless environment, including high Doppler and delay spread due to moving transmitters, moving receivers, and moving scatterers. This highly dynamic environment has large impacts on the direct device-to-device (sidelink-based) V2X communication, in terms of resource efficiency, latency reduction, as well as out-of-coverage support [27]. However, for vehicle-to-vehicle (V2V) services that require high reliability and/or high data rate, the sidelink (SL) alone might not be sufficient to meet the requirements, especially in very complex environments. Therefore, it is possible to use only cellular uplink/downlink (Uu) or alternatively multi-connectivity combining SL with cellular Uu. These settings can be explored for enhancing reliability as well as data rate for advanced V2X communications. Dual- or, more generally, multi-connectivity in LTE and 5G allows for UE to be configured with two or more Uu connections with different BSs. This is different from the dual or multi-connectivity with SL and Uu connections for direct end-to-end communication in V2X. Optimal data path routed via eNB using Uu links and possible switch between SL and optimal data path for V2V is discussed in 3 GPP and literature [28], but not multi-connectivity with SL and Uu link. * **Prioritization of data packets and Quality of Service (QoS) management**: the diversity of the requirements of the new V2X applications raises new challenges for efficient multiplexing of different services and interference handling. Although the dynamic sharing of the same resources for different services is certainly beneficial in terms of spectral efficiency, it brings challenges on the system design that has to optimally multiplex different service flows with different QoS requirements in a high load system without compromising the V2X critical safety nature. Similarly, different types of interference arise, especially when considering all different V2X communication forms and also the potential operation over unlicensed band. Interference due to co-channel operation or from adjacent channels should be handled to achieve high reliability. * **Adaptive and Robust Beam Management in mmWave Spectrum Bands**: several V2X applications impose a requirement to deliver a common message to a set of vehicles over the V2I communication link.