Contents In 6th generation (6G), a plethora of novel services is unfolding to cater to a myriad of Internet of Things (IoT) devices, surpassing the conventional offerings of mobile phones and internet connectivity. Instances of these services encompass extended reality gadgets, sensors, as well as ground and aerial robots. Nonetheless, the implementation of these sophisticated services presents hurdles for the wireless network, specifically in its capacity to uphold pervasive connections while adhering to varied quality-of-service (QoS) specifications. To sustain capacity growth at an economical cost and low energy consumption while meeting diverse QoS requirements, the advent of innovative technologies is imperative. Recent considerations include millimeter-wave, TeraHertz, and optical frequencies, holographic multiple-input multiple-output (MIMO), ultramassive MIMO, reconfigurable intelligent surfaces (RIS), and ultra-dense networks for various applications. Additionally, the overall system performance is further enhanced through various techniques like beamforming-enabled antennas, cognitive spectrum utilization, and advancements in modulation and coding.
The advancement of IoT applications in smart cities, intelligent vehicular transportation, and entertainment services like augmented and virtual reality is rapidly advancing, driving the development of new technologies to meet the stringent demands of future mobile networks. However, the wireless channel, being uncontrollable, stands as a major obstacle to reach the optimal performance. In addition to being uncontrolled, the wireless propagation channel also inherently hampers precision and adaptability. Such constraints motivate for reconfigurable intelligent surfaces (RISs) to enable applications characterized by low energy consumption, costeffectiveness, and low latency.
Implementing the communication systems with RISs typically entails strategically positioning these surfaces on stationary structures. However, the practical applicability of RIS faces challenges when directly installed at predetermined locations on buildings. To address this constraint, the deployment of RIS onto unmanned aerial vehicles (UAVs), referred to as RISassisted UAVs, is a promising way. The RIS-assisted UAV approach enables comprehensive full-angle (3600) reflection in the sky, effectively expanding the wireless transmission range. The adaptability of UAVs in deployment plays a crucial role in overcoming the limitations associated with fixed installations.
Flexible air-to-ground communication using RIS-assisted UAVs has been identified as a lucrative technology due to the agility of the UAVs, cost-effectiveness, and flexibility in deployment. The RIS-assisted UAVs can be used in mobile networks to improve connectivity for users at events with a temporary high density of users (e.g., sports or social events) or during emergency situations. Besides, RIS-assisted UAVs can be used for the collection of data from sensors or IoT devices in poorly covered areas. Flexible RIS-assisted UAVs are a promising direction, enabling improved effectiveness of mobile networks for communication among users to improve coverage and network throughput.
The goal of this Special Issue (SI) is to encourage researchers from academia and industry to delve into the most recent research opportunities in the areas of innovative architecture, recent advancements, and prospective applications related to flexible RIS for integrated air-to-ground internet of things.
We received 20 high-quality submissions, of which six articles were eventually accepted after a rigorous review process. In the first article, “Empowering Reconfigurable Intelligent Surfaces with Artificial Intelligence to Secure Air-to-Ground Internet-of-Things,” focuses on RIS-assisted anti-jamming UAV communication and radio surveillance. A deep reinforcement learning-based solution has been developed to address the security-related challenges in UAV and RIS design. Deep reinforcement learning enables UAVs to autonomously learn their trajectory and RIS configuration, optimizing their ability to counter jamming signals and maximize communication rates based on received data rates. Additionally, the algorithm proposed in this article facilitates the UAVs in maximizing eavesdropping rates during radio surveillance, even in the absence of explicit knowledge of channel state information by learning through trial and error.
The article, “Toward the Metaverse Realization in 6G: Orchestration of RIS-enabled Smart Wireless Environments via Digital Twins,” explores the complementarity between RIS and digital twins to efficiently realize the metaverse in 6G networks. Specifically, this article examines how a DTassisted RIS-based network architecture can substantially enhance network latency and reliability, which is crucial for the metaverse in 6G. The discussion begins with a brief overview of the current status of RIS and digital twin technologies, followed by an exploration of potential use cases and services achievable through a digital twin-assisted RIS architecture. The advantages of the architecture proposed in this article, particularly in terms of communication latency, are validated through a representative simulation setup.
The next article, “Secure Beamforming for Unmanned Aerial Vehicles Equipped Reconfigurable Intelligent Surfaces,” explores the security aspects of UAV-RIS systems facing challenges in the presence of imperfect channel state information. This is especially pertinent when utilizing beamforming across various propagation channels. This article delineates potential security demands in scenarios involving UAV-RIS beamforming, accounting for practical situations. Subsequently, the authors introduce a beamforming approach based on a deep deterministic policy gradient to enhance the security of UAV-RIS beamforming and improve communication channel performance.
The fourth article, “Deep Learning for Secure (UAV-Assisted RIS Communication Networks,” delves into the intricacies of securing UAV-assisted RIS systems within next-generation communication networks. This article introduces a robust Long Short-Term Memory-based Deep Deterministic Policy Gradient to effectively address security concerns and ensure reliable communication in UAV-assisted RIS networks by thwarting malicious threats.
The fifth article, “Synergizing Airborne Non-Terrestrial Networks and Reconfigurable Intelligent Surfaces-Aided 6G IoT,” discusses why and how non-terrestrial networks provide global connectivity and support the growing number of IoT devices. Despite the potential of non-terrestrial networks, challenges like signal delays and security persist. Therefore, this article explores integrating RIS with non-terrestrial networks to address the above-mentioned issues.
The final accepted article, “Aerial Reconfigurable Intelligent Surface-Assisted Terrestrial Communications,” provides an extensive overview of RIS technology and introduces the concept of an RIS-empowered smart city. Subsequently, the benefits, typical use cases, potential challenges, and corresponding solutions for ARIS-assisted terrestrial communications are discussed in detail, supplemented by a case study.
In conclusion, the articles presented in this special issue high-light the innovative architecture, recent progress, and potential applications associated with flexible RIS for integrated air-to-ground IoT. We hope that this initiative serves as inspiration, encouraging researchers from both academia and industry to explore novel ideas and algorithms, thereby advancing communication assisted by flexible RIS and UAVs. We extend our gratitude to the authors for sharing their pioneering research and to the reviewers for their diligent contributions, which have significantly enhanced the quality of the articles. Moreover, we appreciate Dr. Rath Vannithamby, the Editor-in-Chief, and the dedicated team at IEEE Internet of Things Magazine for providing us with this platform and offering invaluable assistance throughout the production of this special issue. Ultimately, we sincerely believe that readers will find both fascination and practical value in the articles featured in this special issue.