Terahertz (THz) band will play an important role in enabling sixth generation (6G) envisioned applications. Compared with lower frequency signals, THz waves are severely attenuated by the atmosphere temperature, pressure, and humidity. Thus, designing a THz communication system must take into account how to circumvent or diminish those issues to achieve a sufficient quality of service. Different solutions are being analyzed: intelligent communication environments, ubiquitous artificial intelligence, extensive network automation, and dynamic spectrum access, among others. This survey focuses on the benefits of integrating intelligent surfaces (ISs) and THz communication systems by providing an overview of IS in wireless communications with the scanning of the recent developments, a description of the architecture, and an explanation of the operation. The survey also covers THz channel models, differentiating them based on deterministic and statistical channel modeling. The IS-aided THz channels are elucidated at the end of the survey. Finally, discussions and research directions are given to help enrich the IS field of research and guide the reader through open issues.

The integration of low Earth orbit (LEO) satellites with Internet of Things (IoT) devices has driven the need for efficient, low-complexity multiple access techniques. However, most approaches based on the Slotted Aloha protocol rely on the ON-OFF channel model, which oversimplifies the physical channel phenomena and neglects critical fading effects. To overcome this limitation, the present work develops a precise closed-form analytical model for instantaneous throughput, outage, and data collision probabilities of a direct-to-satellite IoT (DtS-IoT) network operating over Nakagami-m fading channels while employing the Slotted Aloha technique. The analysis accounts for the presence of multiple satellites in orbit, each with different channel loads and elevation angles, thus providing a more realistic representation of fading conditions and inter-satellite variability. The simulation results confirm the validity of the proposed analytical expressions and show that they exhibit excellent agreement with theoretical predictions under Nakagami-m fading. Furthermore, the results indicate that dynamically adjusting the transmit power and channel load, especially when satellite position information is available, can significantly increase system throughput and provide actionable insights for system optimization in terms of energy efficiency and overall performance.

The energy and time efficiency of wireless sensor networks (WSNs) is frequently affected by the low reliability of their links. To mitigate the outage probability, cooperation topologies are used. However, these topologies have particular challenges since the relay consumes energy in assisting a foreign communication, and the successful transmission in each direction is conditioned to the availability of the two segments involved in the communication. To overcome the temporary unavailability of a link, the use of buffers in the relay has been proposed, but energy and time efficiency remain a challenge for basic configurations. We propose two-way buffer-aided relaying strategies that exploit the presence of buffers in the different nodes that participate in the communication, as well as the efficient use of buffer capacity. The proposed strategies make the decision to forward the messages in one of the communication directions or broadcast coded messages based on buffer and channel state information. Firstly, we evaluate the impact of considering the use of buffers in the transmitter nodes. Then, we propose and evaluate the impact of the full and joint use of the entire buffer capacity to assist communication in both directions. Finally, we evaluate the performance of a system that exploits both the use of full and joint buffering and the presence of buffering in the transmitter. The results show that better performance, in terms of outage probability, is obtained by the third strategy; since it allows the buffer capacity to be exploited to a greater extent in the most urgent direction at each moment, as well as to select the start of each transmission to a viable communication opportunity. This represents a notable benefit in terms of energy and time efficiency for WSNs since unnecessary transmission of information is avoided, the number of idle slots decreases, and the amount of information per unit of time and energy increases.

The task of offloading packets in LEO satellite networks in an IoT cluster context is often affected by excessive energy consumption, frequent loss of data packets in the medium access, limited visibility time with the ground station, and limited resources available to the satellites. Inter-satellite communication (ISC) techniques have been introduced to address these challenges. However, these techniques require high technical and strategic development to meet the requirements of the end users. In this regard, four strategies are proposed: Brief (which eliminates all redundancy); Balanced (which eliminates the redundancy but keeps the amount of information for each satellite balanced); Redundancy (which includes all the information collected on each satellite); and Coded (which provides redundancy through information coding). Our techniques are to exchange signaling or data packets between satellites via ISC to download the collected data. At the same time, one of these goals is prioritized: the lowest energy consumption, the highest reliability of information delivery, or the balance between the previous two. Consequently, the system performance is evaluated from each evolved stage and the overall system. The results show that Redundancy and Coded techniques increase the offloading rate, while Brief and Balanced techniques increase the system's energy efficiency. Since Brief and Balanced techniques avoid redundant information transmission, decreasing the slot time. The Coded technique presents a better tradeoff between energy efficiency and reliability than the Brief , Balanced , and Redundancy techniques. Finally, the results show the positive impact of ISC on satellite networks that collect information from ground IoT devices to download it to an Earth station.

In the Internet of Things (IoT) environment, a vast number of IoT devices connect to the Internet, enabling real-time access worldwide. Satellite-based solutions are crucial for the implementation of ubiquitous communication and integrated networks in such environments. This paper addresses two optimization problems aimed at enhancing system performance in the context of direct-to-satellite-IoT (DtS-IoT), specifically focusing on maximizing system throughput while simultaneously minimizing collision and outage probabilities. The first optimization problem targets system capacity as a function of the satellite's orbital position. The second optimization problem presents a comprehensive multi-objective approach through a cost function that penalizes collisions and outages while rewarding throughput improvements. In addition, it incorporates constraints on essential parameters to guarantee solution feasibility within operational limits, enhancing robustness and real-world applicability. Numerical and analytical results rigorously evaluate the system model's performance under generic conditions, with simulations based on Monte Carlo methods. The results obtained show that the proposed approach significantly increases system throughput in certain scenarios and achieves up to an 80% reduction in collision probability compared to traditional configurations, depending on system parameters. The proposed simulation approach is validated by close agreement with the derived analytical expressions.