# Social Internet of Vehicles for Smart Cities
## 4 Social Network Analysis in SIoVs
### Social Clustering of Vehicles  
Clustering is a method widely used in all kinds of networks, ranging from mobile _ad hoc_ networks [76] to sensor networks [77]. Clustering exists in different forms and can be performed using a wide range of network characteristics, bringing significant benefits to a network. For example, clustering can alleviate the problem of an overwhelming number of broadcasts in dense networks (broadcast storm) [78], increasing the system throughput and improving the bit error rate. It can significantly decrease packet delays and provide better spectrum utilization in time and space. Clusters that partition the network cleverly can also allow data aggregation and increase network longevity. By creating small groups of nodes that share common characteristics, the network appears to be smaller and more stable. To form clusters, vehicles can combine LTE and DSRC communication capabilities, as shown in Figure 3. Cluster heads can perform special operations inside a cluster, such as regulation of channel use, aggregation of data, scheduling and packet routing. A special characteristic of a VANET environment is diversity in the mobility patterns that vehicles follow, which is based on the road network's local condition, for example reacting to traffic congestion or accidents. This characteristic makes a VANET a very dynamic network where the vehicle density exhibits large variations. Another problem that the communication between moving vehicles has arises when the receiver and the source are moving towards different directions. The problem that is called Doppler shift has attracted much attention, and many solutions have been proposed [79]. The authors in [80] identified the key factors that affect the delivery of warning messages between vehicles, which are the radio propagation model, the density of vehicles and the roadmap. This work showcased how dynamic a VANET environment can be and how small variations in only one feature can heavily affect the performance of the network. Having this dynamic characteristic of a VANET in mind, a good clustering method that can reassure good stability and high cluster lifetime must incorporate many different factors during both the cluster formation and cluster maintenance processes [63; 81]. In addition, an effective clustering method has to take into account signal fading and channel interference. [PERSON] _et al._[82] presented APROVE, a protocol for distributed election of cluster heads using affinity propagation from a communications perspective. [PERSON] _et al._[81] proposed a distributed clustering algorithm, which applies virtual forces among the nodes to form stable clusters. The applied force is relative to the current and predicted future distance among each pair of vehicles and to their relative velocities. The method assigns positive virtual forces among vehicles that move towards each other and negative forces to vehicles that follow different directions. Nodes decide whether to become cluster heads, join a nearby cluster or leave their cluster depending on their current state and the relation of the total virtual forces applied to it from its neighbours. [PERSON] _et al._[83] introduced a new cluster-based routing  
Figure 3: Clustering of vehicles. protocol that reduces network overload, message duplication and packet collisions by allowing vehicles to selectively transmit messages. When moving on the social aspect of vehicular communications, new parameters, like the frequency of interactions between entities, historical data of driver behaviour and driver habits, must be taken into account. As drivers tend to follow similar routes when moving in a city or on a highway, past mobility information can be used to build distinct social profiles of the drivers. These social profiles can become a basic characteristic to create social groups of vehicles and drivers. In [84], the nodes are divided into different groups based on the regularity of contact between vehicles with fixed routes.