Hybrid satellite–terrestrial networks toward 6G : key technologies and open issues

Future wireless networks will be required to provide more wireless services at higher data rates and with global coverage. However, existing homogeneous wireless networks, such as cellular and satellite networks, may not be able to meet such requirements individually, especially in remote terrain, including seas and mountains. One possible solution is to use diversified wireless networks that can exploit the inter-connectivity between satellites, aerial base stations (BSs), and terrestrial BSs over inter-connected space, ground, and aerial networks. Hence, enabling wireless communication in one integrated network has attracted both the industry and the research fraternities. In this work, we provide a comprehensive survey of the most recent work on hybrid satellite–terrestrial networks (HSTNs), focusing on system architecture, performance analysis, design optimization, and secure communication schemes for different cooperative and cognitive HSTN network architectures. Different key technologies are compared. Based on this comparison, several open issues for future research are discussed

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

5G embraces satellites for 6G ubiquitous IoT : basic models for integrated satellite terrestrial networks

Terrestrial communication networks mainly focus on users in urban areas but have poor coverage performance in harsh environments, such as mountains, deserts, and oceans. Satellites can be exploited to extend the coverage of terrestrial fifth-generation (5G) networks. However, satellites are restricted by their high latency and relatively low data rate. Consequently, the integration of terrestrial and satellite components has been widely studied, to take advantage of both sides and enable the seamless broadband coverage. Due to the significant differences between satellite communications (SatComs) and terrestrial communications (TerComs) in terms of channel fading, transmission delay, mobility, and coverage performance, the establishment of an efficient hybrid satellite-terrestrial network (HSTN) still faces many challenges. In general, it is difficult to decompose a HSTN into a sum of separate satellite and terrestrial links due to the complicated coupling relationships therein. To uncover the complete picture of HSTNs, we regard the HSTN as a combination of basic cooperative models that contain the main traits of satellite-terrestrial integration but are much simpler and thus more tractable than the large-scale heterogeneous HSTNs. In particular, we present three basic cooperative models, i.e., model X, model L, and model V, and provide a survey of the state-of-the-art technologies for each of them. We discuss future research directions towards establishing a cell-free, hierarchical, decoupled HSTN. We also outline open issues to envision an agile, smart, and secure HSTN for the sixth-generation (6G) ubiquitous Internet of Things (IoT)

How Physicality Enables Trust: A New Era of Trust-Centered Cyberphysical Systems

Multi-agent cyberphysical systems enable new capabilities in efficiency, resilience, and security. The unique characteristics of these systems prompt a reevaluation of their security concepts, including their vulnerabilities, and mechanisms to mitigate these vulnerabilities. This survey paper examines how advancement in wireless networking, coupled with the sensing and computing in cyberphysical systems, can foster novel security capabilities. This study delves into three main themes related to securing multi-agent cyberphysical systems. First, we discuss the threats that are particularly relevant to multi-agent cyberphysical systems given the potential lack of trust between agents. Second, we present prospects for sensing, contextual awareness, and authentication, enabling the inference and measurement of ``inter-agent trust" for these systems. Third, we elaborate on the application of quantifiable trust notions to enable ``resilient coordination," where ``resilient" signifies sustained functionality amid attacks on multiagent cyberphysical systems. We refer to the capability of cyberphysical systems to self-organize, and coordinate to achieve a task as autonomy. This survey unveils the cyberphysical character of future interconnected systems as a pivotal catalyst for realizing robust, trust-centered autonomy in tomorrow's world

Leveraging 6G Technologies to Optimize Information Freshness for Time-Sensitive Applications

Next-generation wireless networks (Beyond 5G, 6G) aim to provide tremendous improvements over previous generations by promising a massive connectivity, ultra-reliable and low-latency communications, and soaring broadband speeds. Such transformation will give rise to a wide range of propitious Internet-of-Things (IoT) applications such as intelligent transportation systems (ITS), tactile internet, augmented/virtual reality, industry 4.0, etc. These applications possess stringent requirements of fresh and timely information updates to make critical decisions. Out-dated or stale information updates are highly undesirable for these applications as they may call forth unreliable or erroneous decisions. The conventional performance metrics such as delay and latency may not fully characterize the freshness of information for time-critical IoT applications. Recently, information freshness has been investigated through defining a new performance metric termed as Age of Information (AoI). AoI offers a rigorous way to quantify the information freshness as compared to other performance metrics and is deemed suitable for real-time IoT applications. In reality, the limited energy and computing resources of IoT devices (IoTDs) is a significant challenge towards realizing the timely delivery of information updates. To address this challenge, the first aim of this dissertation is to examine the capability of multi-access edge computing (MEC) towards minimizing the AoI. In fact, MEC offers an expedited computation of resource-intensive tasks, which, if processed locally at the IoTDs, may experience excessive computational latency. In this context, an optimization problem is setup to determine the optimal scheduling policy with the goal of minimizing the expected sum AoI of multiple IoTDs, while considering the combined impact of unreliable channel conditions and random packet arrivals. Another acute challenge is the high randomness and uncontrollable behaviour of wireless communication environments, which may severely impede the timely and reliable delivery of information updates. Towards addressing this challenge, reconfigurable intelligent surface (RIS) is leveraged to mitigate the propagation-induced impairments of the wireless environment and enhance the quality of wireless links to preserve the information freshness. First, a wireless network consisting of a base station (BS) that is forwarding information updates of multiple real-time traffic streams to their destinations is studied. The considered multiple access technique is frequency division multiple access (FDMA), which is an orthogonal multiple access (OMA) technique. A joint user scheduling and phase-shift matrix (passive beamforming) optimization problem is formulated with the objective of minimizing the expected sum AoI of the coexisting multiple traffic streams. The resulting problem is a mixed integer non-convex optimization problem. To evade the high coupling of the invoked optimization variables, the bi-level optimization technique is utilized, where the original problem is decomposed into an outer traffic stream scheduling problem and an inner RIS phase-shift matrix problem. Owing to the stochastic nature of packet arrivals, a deep reinforcement learning (DRL) solution is employed to solve the outer problem. To do so, the traffic stream scheduling is modeled as a Markov Decision Process (MDP) and Proximal Policy Optimization (PPO) is invoked to solve it. On the other hand, the inner problem that determines the RIS configuration is solved through semi-definite relaxation (SDR). Due to the limitations of OMA techniques in terms of the number of served IoTDs and the spectral efficiency, the focus of this dissertation shifts to explore non-orthogonal multiple access (NOMA) scheme towards achieving the goal of minimizing the AoI in an uplink setting. In this context, an optimization problem is formulated to optimize the RIS configuration, the transmit power of IoTDs and their clustering policy. To solve this mixed-integer non-convex problem, the RIS configuration is obtained first by resorting to difference-of-convex (DC) along with successive convex approximation (SCA). On the other hand, the bi-level optimization is used to solve the power allocation and the clustering problems. Optimal closed-form expressions are derived for the power control scheme and the one-to-one matching is employed to solve the clustering problem. Aiming to further improve the information freshness in time-critical IoT applications, an extended version of NOMA, termed as Cooperative-NOMA (C-NOMA), is adopted. In C-NOMA, the cooperation between IoTDs through device-to-device (D2D) communication and full-duplex (FD) relaying is invoked within the NOMA scheme. In this context, the integration of RIS and C-NOMA is investigated towards achieving the goal of minimizing the average sum AoI. Precisely, it is investigated how much performance gain in terms of AoI reduction can be brought by the RIS-enabled uplink C-NOMA system compared to the conventional C-NOMA and NOMA schemes, both with and without RIS. Results elucidate the superiority of our proposed approaches against other baseline schemes. The findings in this dissertation shed light on the choice of effective design of wireless communication networks leveraging the core future enabling technologies

Mobile Ad Hoc Networks

Guiding readers through the basics of these rapidly emerging networks to more advanced concepts and future expectations, Mobile Ad hoc Networks: Current Status and Future Trends identifies and examines the most pressing research issues in Mobile Ad hoc Networks (MANETs). Containing the contributions of leading researchers, industry professionals, and academics, this forward-looking reference provides an authoritative perspective of the state of the art in MANETs. The book includes surveys of recent publications that investigate key areas of interest such as limited resources and the mobility of mobile nodes. It considers routing, multicast, energy, security, channel assignment, and ensuring quality of service. Also suitable as a text for graduate students, the book is organized into three sections: Fundamentals of MANET Modeling and Simulation—Describes how MANETs operate and perform through simulations and models Communication Protocols of MANETs—Presents cutting-edge research on key issues, including MAC layer issues and routing in high mobility Future Networks Inspired By MANETs—Tackles open research issues and emerging trends Illustrating the role MANETs are likely to play in future networks, this book supplies the foundation and insight you will need to make your own contributions to the field. It includes coverage of routing protocols, modeling and simulations tools, intelligent optimization techniques to multicriteria routing, security issues in FHAMIPv6, connecting moving smart objects to the Internet, underwater sensor networks, wireless mesh network architecture and protocols, adaptive routing provision using Bayesian inference, and adaptive flow control in transport layer using genetic algorithms

Mobile Ad-Hoc Networks

Being infrastructure-less and without central administration control, wireless ad-hoc networking is playing a more and more important role in extending the coverage of traditional wireless infrastructure (cellular networks, wireless LAN, etc). This book includes state-of the-art techniques and solutions for wireless ad-hoc networks. It focuses on the following topics in ad-hoc networks: vehicular ad-hoc networks, security and caching, TCP in ad-hoc networks and emerging applications. It is targeted to provide network engineers and researchers with design guidelines for large scale wireless ad hoc networks