Relaying in the Internet of Things (IoT): A Survey

The deployment of relays between Internet of Things (IoT) end devices and gateways can improve link quality. In cellular-based IoT, relays have the potential to reduce base station overload. The energy expended in single-hop long-range communication can be reduced if relays listen to transmissions of end devices and forward these observations to gateways. However, incorporating relays into IoT networks faces some challenges. IoT end devices are designed primarily for uplink communication of small-sized observations toward the network; hence, opportunistically using end devices as relays needs a redesign of both the medium access control (MAC) layer protocol of such end devices and possible addition of new communication interfaces. Additionally, the wake-up time of IoT end devices needs to be synchronized with that of the relays. For cellular-based IoT, the possibility of using infrastructure relays exists, and noncellular IoT networks can leverage the presence of mobile devices for relaying, for example, in remote healthcare. However, the latter presents problems of incentivizing relay participation and managing the mobility of relays. Furthermore, although relays can increase the lifetime of IoT networks, deploying relays implies the need for additional batteries to power them. This can erode the energy efficiency gain that relays offer. Therefore, designing relay-assisted IoT networks that provide acceptable trade-offs is key, and this goes beyond adding an extra transmit RF chain to a relay-enabled IoT end device. There has been increasing research interest in IoT relaying, as demonstrated in the available literature. Works that consider these issues are surveyed in this paper to provide insight into the state of the art, provide design insights for network designers and motivate future research directions

Relay assisted device-to-device communication with channel uncertainty

The gains of direct communication between user equipment in a network may not be fully realised due to the separation between the user equipment and due to the fading that the channel between these user equipment experiences. In order to fully realise the gains that direct (device-to-device) communication promises, idle user equipment can be exploited to serve as relays to enforce device-to-device communication. The availability of potential relay user equipment creates a problem: a way to select the relay user equipment. Moreover, unlike infrastructure relays, user equipment are carried around by people and these users are self-interested. Thus the problem of relay selection goes beyond choosing which device to assist in relayed communication but catering for user self-interest. Another problem in wireless communication is the unavailability of perfect channel state information. This reality creates uncertainty in the channel and so in designing selection algorithms, channel uncertainty awareness needs to be a consideration. Therefore the work in this thesis considers the design of relay user equipment selection algorithms that are not only device centric but that are relay user equipment centric. Furthermore, the designed algorithms are channel uncertainty aware. Firstly, a stable matching based relay user equipment selection algorithm is put forward for underlay device-to-device communication. A channel uncertainty aware approach is proposed to cater to imperfect channel state information at the devices. The algorithm is combined with a rate based mode selection algorithm. Next, to cater to the queue state at the relay user equipment, a cross-layer selection algorithm is proposed for a twoway decode and forward relay set up. The algorithm proposed employs deterministic uncertainty constraint in the interference channel, solving the selection algorithm in a heuristic fashion. Then a cluster head selection algorithm is proposed for device-to-device group communication constrained by channel uncertainty in the interference channel. The formulated rate maximization problem is solved for deterministic and probabilistic constraint scenarios, and the problem extended to a multiple-input single-out scenario for which robust beamforming was designed. Finally, relay utility and social distance based selection algorithms are proposed for full duplex decode and forward device-to-device communication set up. A worst-case approach is proposed for a full channel uncertainty scenario. The results from computer simulations indicate that the proposed algorithms offer spectral efficiency, fairness and energy efficiency gains. The results also showed clearly the deterioration in the performance of networks when perfect channel state information is assumed

Physical layer security in 5G and beyond wireless networks enabling technologies

Information security has always been a critical concern for wireless communications due to the broadcast nature of the open wireless medium. Commonly, security relies on cryptographic encryption techniques at higher layers to ensure information security. However, traditional cryptographic methods may be inadequate or inappropriate due to novel improvements in the computational power of devices and optimization approaches. Therefore, supplementary techniques are required to secure the transmission data. Physical layer security (PLS) can improve the security of wireless communications by exploiting the characteristics of wireless channels. Therefore, we study the PLS performance in the fifth generation (5G) and beyond wireless networks enabling technologies in this thesis. The thesis consists of three main parts. In the first part, the PLS design and analysis for Device-to-Device (D2D) communication is carried out for several scenarios. More specifically, in this part, we study the underlay relay-aided D2D communications to improve the PLS of the cellular network. We propose a cooperative scheme, whereby the D2D pair, in return for being allowed to share the spectrum band of the cellular network, serves as a friendly jammer using full-duplex (FD) and half-duplex (HD) transmissions and relay selection to degrade the wiretapped signal at an eavesdropper. This part aims to show that spectrum sharing is advantageous for both D2D communications and cellular networks concerning reliability and robustness for the former and PLS enhancement for the latter. Closed-form expressions for the D2D outage probability, the secrecy outage probability (SOP), and the probability of non-zero secrecy capacity (PNSC) are derived to assess the proposed cooperative system model. The results show enhancing the robustness and reliability of D2D communication while simultaneously improving the cellular network’s PLS by generating jamming signals towards the eavesdropper. Furthermore, intensive Monte-Carlo simulations and numerical results are provided to verify the efficiency of the proposed schemes and validate the derived expressions’ accuracy. In the second part, we consider a secure underlay cognitive radio (CR) network in the presence of a primary passive eavesdropper. Herein, a secondary multi-antenna full-duplex destination node acts as a jammer to the primary eavesdropper to improve the PLS of the primary network. In return for this favor, the energy-constrained secondary source gets access to the primary network to transmit its information so long as the interference to the latter is below a certain level. As revealed in our analysis and simulation, the reliability and robustness of the CR network are improved, while the security level of the primary network is enhanced concurrently. Finally, we investigate the PLS design and analysis of reconfigurable intelligent surface (RIS)-aided wireless communication systems in an inband underlay D2D communication and the CR network. An RIS is used to adjust its reflecting elements to enhance the data transmission while improving the PLS concurrently. Furthermore, we investigate the design of active elements in RIS to overcome the double-fading problem introduced in the RISaided link in a wireless communications system. Towards this end, each active RIS element amplifies the reflected incident signal rather than only reflecting it as done in passive RIS modules. As revealed in our analysis and simulation, the use of active elements leads to a drastic reduction in the size of RIS to achieve a given performance level. Furthermore, a practical design for active RIS is proposed