# NB-IoT for D2D-Enhanced Content Uploading with Social Trustworthiness in 5G Systems
## 1 Introduction  
The expected drastic increase in Internet of things (IoT) connected devices will definitely produce huge demands for data transmission over wireless systems. At the same time, a plethora of new IoT use cases are emerging across the domains of intelligent transportation systems, smart grid automation, remote health care, smart metering, industrial automation and control, remote manufacturing, and public safety surveillance, among others [1]. Most IoT devices operate through their virtual representations within a digital overlay information system, built over the physical world. Therefore, the majority of current IoT solutions rely on cloud services, leveraging on their virtually unlimited capabilities to effectively exploit the potential of massive tiny sensors and actuators towards the so-called cloud of things. Given the complexity and the challenging requirements of future IoT ecosystems, experts in the field share the opinion that the upcoming fifth generation (5G) cellular systems will represent a strong boost for actual IoT deployment [2]. This vision is sustained by the fervent activities, aimed at designing IoT-oriented 5G wireless systems, conducted by academic, industrial, and standardization bodies [3; 4], worldwide. Several types of interactionsmay coexist within an IoT ecosystem, including machine-to-machine (M2M), machine-to-human, human-to-machine, and machine-to-cloud interactions. All require ubiquitous connectivity. For this purpose, device-to-device (D2D) communications appears as a promising paradigm to support the interconnection of heterogeneous objects [5]. Short-range D2D cooperation among devices may introduce benefits in terms of improved spectrum utilization, higher throughput, and lower energy consumption, which is important for constrained IoT devices. However, there are still several open issues that need to be solved in order to achieve a seamless, effective, and reliable deployment of proximity-based communications for IoT systems [6; 7]. For an effective implementation of proximity communications, one of the most important challenges is to understand how the node-originated information shall be processed so as to build a reliable system on the basis of the objects' behavior, namely the need for _trustworthiness_[8]. Indeed, in realistic scenarios, where human interactions and human behavior are also to be considered, the presence of _malicious_ nodes in the network is a constant threat for successful cooperation. Accordingly, without effective trust management foundations, attacks and malfunctions are likely to outweigh any possible cooperation benefits [8]. For the reference scenario in this paper, we consider that groups of devices in close proximity are willing to upload contents to the Cloud or to a central server and end users may not be aware of whom they are going to forward the data to. Typical sample scenarios are small-scale crowded environments (e.g., stadiums, university campuses, music theaters, or fairs) where devices can exploit opportunistic data forwarding over other devices in proximity. In these contexts, malicious nodes may decide to drop the data packets they are expected to forward or even modify the data packets before forwarding the corrupted content. To cope with these threats, _reliability_ and _reputation_ notions will be considered to model the level of _trust_ among the involved entities. By taking inspiration from recent social Internet of things (SIoT) models, in this paper we consider the sociality level of the devices to model the _reliability_ of the communication. The historical _reputation_ of the cooperative users will be considered to offer rational users the possibility to filter out untrusted users and avoid unsuccessful opportunistic hop-by-hop D2D interactions. An initial investigation in this direction was made in our previous paper [9] in long-term evolution-advanced (LTE-A) scenarios where multihop cooperative uploading is implemented over cellular D2D resources [10].