# Software-Defined Heterogeneous Vehicular Networking: The Architectural Design and Open Challenges
## 2 Background and Motivation
Automotive cameras are typically responsible for generating a considerable proportion of sensor data on the vehicles, and the required data rates are typically around 100 Mbps and 700 Mbps for low- and high-resolution raw images, respectively, after significant compression has been applied [14]. Practically, the maximum data rate for DSRC is only around 6-27 Mbps, while 4G cellular systems are still limited to approximately 100 Mbps in high mobility scenarios, though much lower data rates are typical. In this context, millimetre wave communication (mmWave) remains a pivotal approach for realizing the aim of higher bandwidth next-generation connected vehicles. The mmWave band has already been rolled out in the market in the form of the IEEE802.11 ad and supports a data rate of 7 Gbps [15]. There are substantial challenges, i.e., lack of accurate mmWave vehicular channel models, insufficient penetration rates and beam alignment overhead, that still prove critical in realizing the full potential of mmWave V2X communication systems. However, it can still prove attractive for a number of powerful vehicular safety applications such as the _bird's eye view_ and _see-through_ highlighted in the _5G Automotive Vision_ of the 5G Public Private Partnerships Group (5G-PPP Group) [16]. Hence, a heterogeneous combination of diverse wireless technologies appears to be one of the most viable options for next-generation ITS communication platforms so that the advantages of one technology reasonably offset the disadvantages of the other. Table 1 depicts the salient characteristics of candidate networking technologies that can match the challenging requirements of the diverse vehicular networking applications. Heterogeneity is also supported in the _5G Vision_[17] promulgated by the 5G-PPP Group, which regards the future 5G networks to be a heterogeneous set of air interfaces comprise of both existing and future wireless networking technologies (especially as terahertz communication is currently being explored for vehicular networking [18]). Seamless handovers among heterogeneous technologies (vertical handovers) are also a native feature of the 5G-PPP's 5G Vision. Hence, heterogeneity can help achieve better network performance guarantees. Nevertheless, heterogeneity itself is an intricate task to handle and leads to network fragmentation and inefficacy in network resource utilization. Furthermore, transitioning from one radio access technology to another and the multi-hop process involved in the routing of the network traffic could add to the overall end-to-end delay and needs to be carefully tackled. Especially in the case of dense vehicular environments where resource demand is particularly high and several network routing paths are available, there is a need to look for optimal paths within the shortest possible time. This could be addressed with the help of intelligent routing algorithms and via efficient network resource management. The emerging paradigm of SDN proposes a possible solution to these networking challenges by providing an intelligent orchestration of the network through its salient characteristics of reprogrammability, agility, scalability, elasticity and flexibility. An illustration of a heterogeneous vehicular networking architecture is depicted in Figure 2.