Low Earth orbit (LEO) satellite communications (SATCOM) with ubiquitous global connectivity is deemed a pivotal catalyst in advancing wireless communication systems for 5G and beyond. LEO SATCOM excels in delivering versatile information services across expansive areas, facilitating both unicast and multicast transmissions via high-speed broadband capability. Nonetheless, given the broadband coverage of LEO SATCOM, traffic demand distribution within the service area is non-uniform, and the time/frequency/power resources available at LEO satellites remain significantly limited. Motivated by these challenges, we propose a rate-matching framework for non-orthogonal unicast and multicast (NOUM) transmission. Our approach aims to minimize the difference between offered rates and traffic demands for both unicast and multicast messages. By multiplexing unicast and multicast transmissions over the same radio resource, rate-splitting multiple access (RSMA) is employed to manage interference between unicast and multicast streams, as well as inter-user interference under imperfect channel state information at the LEO satellite. To address the formulated problem’s non-smoothness and non-convexity, the common rate is approximated using the LogSumExp technique. Thereafter, we represent the common rate portion as the ratio of the approximated function, converting the problem into an unconstrained form. A generalized power iteration (GPI)-based algorithm, coined GPI-RS-NOUM, is proposed upon this reformulation. Through comprehensive numerical analysis across diverse simulation setups, we demonstrate that the proposed framework outperforms various benchmarks for LEO SATCOM with uneven traffic demands.

Satellite networks are emerging as vital solutions for global connectivity beyond 5G. As companies such as SpaceX, OneWeb, and Amazon are poised to launch a large number of satellites in low Earth orbit, the heightened inter-satellite interference caused by mega-constellations has become a significant concern. To address this challenge, recent works have introduced the concept of satellite cluster networks where multiple satellites in a cluster collaborate to enhance the network performance. In order to investigate the performance of these networks, we propose mathematical analyses by modeling the locations of satellites and users using Poisson point processes, building on the success of stochastic geometry-based analyses for satellite networks. In particular, we suggest the lower and upper bounds of the coverage probability as functions of the system parameters, including satellite density, satellite altitude, satellite cluster area, path loss exponent, and the Nakagami parameterm. We validate the analytical expressions by comparing them with simulation results. Our analyses can be used to design reliable satellite cluster networks by effectively estimating the impact of system parameters on the coverage performance.

Due to the high altitudes and large beam sizes of satellites, the curvature of the Earth’s surface can impact system-level performance. To consider this, 3GPP introduces the UV-plane beam mapping for system-level simulations of non-terrestrial networks (NTNs). This paper aims to provide a comprehensive understanding of how beams and user equipments (UEs) are placed on the UV-plane and subsequently mapped to the Earth’s surface. We present a general process of projecting UEs on the UV-plane onto the Earth’s surface. This process could offer a useful guideline for beam and UE deployment when evaluating the system-level performance of NTNs.

Low Earth orbit (LEO) satellite communications with ubiquitous connectivity is deemed a key enabler for 6G. Since LEO satellites provide broadband coverage, traffic demand distribution of users is potentially non-uniform within the coverage area, and the available time/frequency/power resources are considerably limited. Motivated by this, we propose a rate-matching for non-orthogonal unicast and multicast (NOUM) transmission that matches the offered rates to the traffic demands of both unicast and multicast messages under limited power budgets. Also, rate-splitting multiple access (RSMA) is employed to manage interference between unicast and multi-cast streams along with inter-user interference under imperfect channel state information at the LEO satellite. To solve the non-convex and non-smooth formulated problem, we approximate the common rate using the LogSumExp technique; thereafter, we represent the common portion as the ratio of the approximated function, converting the problem into an unconstrained form. A generalized power iteration (GPI)-based algorithm, coined GPI-RS-NOUM, is proposed upon this reformulation. We numerically show that the proposed framework outperforms various benchmarks for LEO networks with uneven traffic demands.

In the pursuit of ubiquitous connectivity, there is a growing interest in satellite systems owing to their large coverage. The deployment of a substantial number of satellites in low Earth orbits necessitates performance analyses and techniques to mitigate inter-satellite interference. Leveraging achievements in stochastic geometry, this paper proposes mathematical analyses for coverage performance by incorporating the concept of a satellite cluster, enabling joint transmission to address the challenges posed by mega-constellations. Furthermore, we investigate the influence of parameters such as cluster region on coverage performance. Validations of derived mathematical results are conducted through comparative analysis with simulation results. Our analyses can facilitate the efficient design of dependable satellite cluster systems by assessing the influence of design parameters on coverage performance.

The extensive coverage offered by satellites makes them effective in enhancing service continuity for users on dynamic airborne and maritime platforms, such as airplanes and ships. In particular, geosynchronous Earth orbit (GEO) satellites ensure stable connectivity for terrestrial users due to their stationary characteristics when observed from Earth. This paper introduces a novel approach to model and analyze GEO satellite networks using stochastic geometry. We model the distribution of GEO satellites in the geostationary orbit according to a binomial point process (BPP) and examine satellite visibility depending on the terminal’s latitude. Then, we identify potential distribution cases for GEO satellites and derive case probabilities based on the properties of the BPP. We also obtain the distance distributions between the terminal and GEO satellites and derive the coverage probability of the network. We further approximate the derived expressions using the Poisson limit theorem. Monte Carlo simulations are performed to validate the analytical findings, demonstrating a strong alignment between the analyses and simulations. The simplified analytical results can be used to estimate the coverage performance of GEO satellite networks by effectively modeling the positions of GEO satellites.

Low-earth orbit (LEO) satellites based low-power Internet of Things (IoT) service has attracted attention to provide the hyper-space communication networks in 6G communication. In this paper, we study the regression based pilot design for estimation and compensation of the Doppler effect caused by the high-speed mobility of the LEO satellite.

In this paper, we consider downlink low Earth orbit (LEO) satellite communication systems where multiple LEO satellites are uniformly distributed over a sphere at a certain altitude according to a homogeneous binomial point process (BPP). Based on the characteristics of the BPP, we analyze the distance distributions and the distribution cases for the serving satellite. We analytically derive the exact outage probability, and its approximated expression is obtained using the Poisson limit theorem. With these derived expressions, the system throughput maximization problem is formulated under the satellite-visibility and outage constraints. To solve this problem, we reformulate it with bounded feasible sets and propose an iterative algorithm to obtain near-optimal solutions. Simulation results perfectly match the derived exact expressions for the outage probability and system throughput. The analytical results of the approximated expressions are fairly close to those of the exact ones. It is also shown that the proposed algorithm for the throughput maximization is very close to the optimal performance obtained by a two-dimensional exhaustive search.

This paper considerssatellite eavesdroppersin uplink satellite communication systems where the eavesdroppers are randomly distributed at arbitrary altitudes according to homogeneous binomial point processes and attempt to overhear signals that a ground terminal transmits to a serving satellite. Non-colluding eavesdropping satellites are assumed, i.e., they do not cooperate with each other, so that their received signals are not combined but are decoded individually. Directional beamforming with two types of antennas: fixed- and steerable-beam antennas, is adopted at the eavesdropping satellites. The possible distribution cases for the eavesdropping satellites and the distributions of the distances between the terminal and the satellites are analyzed. The distributions of the signal-to-noise ratios (SNRs) at both the serving satellite and the most detrimental eavesdropping satellite are derived as closed-form expressions. The ergodic and outage secrecy capacities of the systems are derived with the secrecy outage probability using the SNR distributions. Simpler approximate expressions for the secrecy performance are obtained based on the Poisson limit theorem, and asymptotic analyses are also carried out in the high-SNR regime. Monte-Carlo simulations verify the analytical results for the secrecy performance. The analytical results are expected to be used to evaluate the secrecy performance and design secure satellite constellations by considering the impact of potential threats from malicious satellite eavesdroppers.

In this paper, we analyze the performance impact when the direct sequence spread spectrum aloha scheme in LEO satellite channel for global IoT service is applied. It is expected that satellite communication can play a significant role for mMTC item, the one of 5G representative service models in terms of service coverage extension. Moreover, the usability of hybrid satellite-terrestrial user terminal (or user equipment) with limited power emission capability can be considered in case of the scenario of direct connectivity to LEO satellite where terrestrial signal is not reached. In this paper, we applied the Ingenu-like waveform based on IEEE 802.15.4k standard to LEO satellite channel which has been deployed in unlicensed ISM 2.4GHz and the main elements related with throughput performance are presented.

In this paper, we investigate a shared-band on-board processing (OBP) satellite communication system where a gate-way and a terminal exchange information in the same frequency band via a satellite with OBP capabilities. Over shadowed-Rician fading channels, we derive analytical expressions for the outage probability of the system. The validity of analysis is verified by computer simulations.

This paper presents a multifunction myoelectric hand that is designed with underactuated mechanisms. The finger design allows an adaptive grasp, including adaptation between fingers and phalanges with respect to the shape of an object. In addition, a self-lock is embedded in the metacarpophalangeal joint to prevent back driving when external forces act on the fingers. The thumb design also provides adaptation between phalanges and adds an intermittent rotary motion to the carpometacarpal joint. As a result, the hand can perform versatile grasping motions using only two motors, and is capable of natural and stable grasping without complex sensor and servo systems. Moreover, the adaptive grasping capabilities reduce the requirements of electromyogram pattern recognition, as analogous motions, such as cylindrical and tip grasps, can be classified as one motion.