Contents I. Introduction
Recently, research into wireless networks has focused on the development of fifth-generation (5G) enabling technologies, building on the achievements of the previous generations, aiming to enhance the quality of service (QoS) offered to the users. However, the economic and environmental sustainability of 5G networks has to be promoted through the Green Communications paradigm, targeting the reduction of the operational cost of the network, as well as its carbon footprint. Hence, technologies that offer energy-efficient QoS will play a major role [1]. To this end, cooperative relaying improves the wireless channel's characteristics through multihop transmissions and multipath fading mitigation via increased diversity [2]. In cases where buffer-aided relays are available, interesting tradeoffs arise between increased diversity and controlled delay [3]. These technologies are exposed to a wide number of attacks, as analyzed in [4], some of which can be mitigated when relay selection is implemented considering physical-layer security techniques [5]. For example, relay selection can result in avoiding the eavesdropper if channel state information (CSI) is available. Moreover, cooperative jamming can be employed to produce artificial noise and confuse the eavesdropper, as long as the trusted nodes' QoS is guaranteed.
