# NB-IoT for D2D-Enhanced Content Uploading with Social Trustworthiness in 5G Systems
## 2 Related Work  
Security is one of the key issues for an effective and widespread adoption of D2D communications [12] in IoT scenarios [13]. This is particularly relevant in a cooperative context such as the one studied in this paper, where the multihop D2D data forwarding paradigm is based on the assumption that the involved devices behave in a trusted and secure way [14]. Unfortunately, this is not always the case as _malicious_ nodes may be active in the network by either dropping or manipulating the data to be forwarded.  
Generally, trust is defined as the quantified belief of a truster with respect to the competence, honesty, security and dependability of a trustee within a specified context [15]. When two users want to cooperate, one of them (the truster) assumes the role of a service requester and the other (the trustee) acts as the service provider. Specifically, in our cooperative D2D multihop scenario, the node acting as relay/gateway towards another node will be the trustee and the source node of the relayed data is the truster. The cooperative topology formation exploits a game theoretic coalition formation model as proposed in [10]. Game theoretic approaches have found several applications also in the field of D2D communications given the potential to model the user behavior (see e.g., [16; 17]). The trustworthiness of the truster with respect to the trustee can be determined by considering reliability and/or reputation. The former is a direct measure derived by subjective observations of the truster during its interactions with the trustee; the latter is an indirect measure based on the opinions that other actors in the community have about the trustee.  
In the literature, several trust models have been proposed to represent both reliability and reputation [15]. A way to reach trustworthiness in communication is to exploit sociality among devices [9; 18]. The mechanism we propose enhances classic trust models through the exploitation of _social relationships_ among the involved devices (to improve device _reliability_) and of recommendation exchange (to the purpose of reputation definition). Socially-aware D2D communications have attracted high interest in recent research activity, such as for instance in [19; 20; 21; 22]. With respect to the works in the literature, we consider the potential of the SIoT model defined in [23], to embrace the social networking concepts and build trustworthy relationships among devices [24; 25]. In particular, mobility patterns and relevant context can be considered to configure the appropriate forms of socialization among the UE. Specifically, the so-called _co-location object relationships_ (C-LOR) and _co-work object relationships_ (C-WOR) are established between devices in a similar manner as among humans, when they share personal (e.g., cohabitation) or public (e.g., work) experiences. Another type of relationship may be defined for the objects owned by a single user, which is named _ownership object relationship_ (OOR). The parental object relationship (POR) is defined among similar devices built in the same period by the same manufacturer, where the production batch is considered a family. Finally, the social object relationship (SOR) is established when objects come into contact, sporadically or continuously, for reasons related to relations among their owners.