# NB-IoT for D2D-Enhanced Content Uploading with Social Trustworthiness in 5G Systems
## 5 The Social-Aware Trust Model  
In our scenario the eNodeB acts as a trusted third party that implements the coalition formation model based on social-aware trustworthiness. To this aim, we evaluate the potential of the SIOT model to embrace the social networking concepts and build trustworthy relationships among the devices [25]. The eNodeB will store information about the reliability, reputation and trust of the users in the network. We define a _player trust matrix_ (_PTM_) as the data structure stored in the eNodeB containing information for every pair of devices. This information will be used whenever a new coalition formation is triggered. Every element (\(i\)-th row and \(j\)-th column) of the PTM refers to a D2D link connecting the corresponding \(i\to j\) nodes in the coalition being considered at time \(t\), where node \(j\) is the relay/gateway for the data he receives from node \(i\) (its own and the preceding nodes in the topology); we consider \(i,j\) as the truster and the trustee respectively. The eNodeB will also act as a controller of the data uploading success as it will send an acknowledgment to the respective source nodes after each cooperative data transmission. Whenever data loss or data corruption is detected by the eNodeB, malicious behavior  
Figure 1: Cooperative multihop content uploading based on trustworthy device to device (D2D) links. will be detected and the information about the reliability of the interested devices will be respectively updated. We assume that control messages (sent over control channels) are very small compared to the main content to be sent and therefore, the relative transmission time and energy consumption are assumed to be negligible. The parameters used to define the level of trust are the following:  
_Social player reliability_\((spr_{i,j})\): this parameter has a value in \([0,1]\) and measures the reliability that node \(i\) assigns to player \(j\) based on the social relationship between the two devices;  
_Player reliability_\((pr_{i,j}^{t})\): this parameter has a value in \([0,1]\) and is representative of the reliability at time instant \(t\) that player \(i\) assigns to player \(j\). To determine this value, each player will consider both the _social player reliability_ and the outcome of past interactions (only those cases are considered where player \(j\) was expected to act as relay/gateway for player \(i\)). _Recommendation reliability_\((rr_{i,j})\): this parameter has a value in \([0,1]\) and is a measure of the reliability assigned by node \(i\) to the recommendations it receives from another device \(j\) about third devices in the network. In our model we consider this parameter to be influenced by the social relationship between the interested UE. _Player reputation_\((pp_{i,j}^{t})\): this parameter has a value in \([0,1]\) and measures the reputation of player \(j\) for player \(i\) according to the information he received through the recommendation values from third players in the network at a given time instant \(t\). _Player trust_\((pt_{i,j}^{t})\): this parameter has a value in \([0,1]\) and measures the level of trust for player \(j\) at time \(t\) as evaluated by player \(i\). This is the most important parameter as it will determine whether player \(i\) is willing to consider player \(j\) as relay/gateway node in a D2D-based cooperative coalition. For the computation of its value player \(i\) will use a weighted combination of the reliability \(pr_{i,j}^{t}\) and the reputation \(pp_{i,j}^{t}\) parameters. As commented above, the _player reliability_ parameter is a function of the time instant \(t\). In particular, its value is updated at every time instant based on the experienced behavior of the devices in the cooperative data uploading. To make this work, for each cooperative interaction the eNodeB sends an acknowledgment to the source nodes with information about the data being successfully received. Of note, this does not allow to determine which node in the cooperative topology has actually dropped or corrupted the data.