This paper presents a comprehensive survey on acquisition, tracking, and pointing (ATP) mechanisms used in free-space optical (FSO) communications systems. ATP mechanisms are a critical component for a wide variety of use cases of mobile FSO communications. ATP mechanisms are used to align FSO transmitter and receiver to attain line-of-sight, which is required for effective operation of FSO communications. Motion is not only associated to mobile stations but also to temporary displacements experienced by stationary FSO terminals, such as in building-to-building FSO communications. The surveyed ATP mechanisms are categorized according to their working principles, use cases, and implementation technology. The advantages and disadvantages of surveyed ATP mechanisms are listed and discussed. We also discuss current challenges and future research directions.

The utilization of electronic voting systems for the election of public offices is becoming widespread globally. This trend can be attributed to the benefits provided by these systems, including remote voting capabilities and accelerated vote counting. Furthermore, electronic voting systems offer improved privacy and enhanced protection against voting bias. Blockchain technology enhances the robustness of the voting process through its immutable vote storage mechanism, thereby reducing the threat of vote tampering and safeguarding the legitimacy of elections. This technology has been adopted by countries such as Germany, Russia, Estonia, and Switzerland for use in their e-voting systems. This study provides a comprehensive overview of the blockchain-based e-voting systems currently being implemented by various countries and companies and proposed for academic research. Additionally, this study analyzes the challenges faced by blockchain e-voting systems and identifies areas for future research to enhance the trustworthiness of such systems.

Supply chains are challenged with providing transparency and trust among participants and stakeholders and ensuring an efficient operation. However, some of these challenges are difficult to resolve because the records of a supply chain can be altered by its participants. The emergence of blockchain technology has been identified as a promising solution to resolve these challenges. Blockchain is an immutable distributed ledger. Here, we introduce blockchain and survey different blockchain frameworks that have been recently proposed to address some of the supply chain challenges. We describe the operation of these blockchain frameworks and their components. We identify the objectives and motivation in each of the surveyed use cases and highlight the advantages and disadvantages of each adopted framework by different industries. We analyze how the reported blockchain frameworks address supply chain challenges. We present a comparative summary of existing literature on blockchain for supply chain. We also summarize the properties of a blockchain framework for its successful adoption in future supply chains and discuss several challenges and opportunities that remain for future research.

In this paper, we propose two different free-space-optics (FSO) coverage models for next-generation high-speed-train communications. To the best of our knowledge, these are the first coverage models proposed for FSO seamless handover. The models provide different coverage areas for performing seamless signal handover and uninterrupted ground-to-train communication. The first model uses two different wavelengths in adjacent covered areas and the second one uses a single wavelength. We find the optimal distance from the train track to a ground base station and the distance between base stations to provide seamless connectivity and handover while minimizing the number of base stations along the track. We base our estimations on a realistic model of an FSO system and provide numerical evaluations demonstrating the performance of the proposed coverage models. We show the different amounts of received power on ground-to-train communications as a function of the location of ground base stations. We also consider the effect of fog on the FSO link as the most attenuating condition for FSO communications. Our results show that communication rates of 1 Gpbs and higher may be achieved with the proposed station positioning and coverage models.

Intersections of flight paths in multidrone missions are indications of a high likelihood of in-flight drone collisions. This likelihood can be proactively minimized during path planning. This article proposes two offline collision-avoidance multidrone path-planning algorithms: 1) DETACH and 2) STEER. Large drone tasks can be divided into smaller ones and carried out by multiple drones. Each drone follows a planned flight path that is optimized to efficiently perform the task. The path planning of the set of drones can then be optimized to complete the task in a short time, with minimum energy expenditure, or with maximum waypoint coverage. Here, we focus on maximizing waypoint coverage. Different from existing schemes, our proposed offline path-planning algorithms detect and remove possible in-flight collisions. They are based on a constrained nearest-neighbor search algorithm that aims to cover a large number of waypoints per flight path. DETACH and STEER perform vector intersection check for flight path analysis, but each at different stages of path planning. We evaluate the waypoint coverage of the proposed algorithms through a novel profit model and compare their performance on a work area with different waypoint densities. Our results show that STEER covers 40% more waypoints and generates 20% more profit than DETACH in high-density waypoint scenarios.

The growing deployment and advanced development of high-speed train (HST) systems, coupled with the reliance on and demand for constant Internet connectivity anytime anywhere, have necessitated the imminent provisioning of broadband Internet services in HSTs. Ground-to-train free space optical (FSO) communications suffer from frequent handovers due to high mobility of HSTs, thus shortening the connection time between the train and ground, and greatly impacting passengers' user experience. To provision high-speed Internet services in HSTs, we propose the rotating transceiver scheme to mitigate the impact of handover processes with steerable FSO transceivers in a ground-to-train communications system. Owing to the rotating feature of the scheme, the on-roof transceiver of a train can maintain a reliable link with a base station (BS) for a longer time, and remarkably reduce the number of handover processes, thus provisioning higher data rates of the system. Meanwhile, since the separation distances between BSs are extended, the total number of BSs required to be deployed along the track is also reduced, thus reducing the deployment cost for service providers. Moreover, a new handover method is designed to mitigate the impact of each handover delay, especially on live streaming applications. Finally, the performance improvement of the proposed scheme over the existing fixed transceiver one is demonstrated via simulation results.