The next generation multibeam satellites open up a new way to design satellite communication channels with the full flexibility in bandwidth, transmit power and beam coverage management. In this paper, we exploit the flexible multibeam satellite capabilities and the geographical distribution of users to improve the performance of satellite-assisted edge caching systems. Our aim is to jointly optimize the bandwidth allocation in multibeam and caching decisions at the edge nodes to address two important problems: i) cache feeding time minimization and ii) cache hits maximization. To tackle the non-convexity of the joint optimization problem, we transform the original problem into a difference-of-convex (DC) form, which is then solved by the proposed iterative algorithm whose convergence to at least a local optimum is theoretically guaranteed. Furthermore, the effectiveness of the proposed design is evaluated under the realistic beams coverage of the satellite SES-14 and Movielens data set. Numerical results show that our proposed joint design can reduce the caching feeding time by 50% and increase the cache hit ratio (CHR) by 10% to 20% compared to existing solutions. Furthermore, we examine the impact of multispot beams and multicarrier wide-beam on the joint design and discuss potential research directions.

This article presents a closed-loop differential phase compensation system for a precoding-enabled multibeam satellite forward link and its validation by live experiments on a GEO satellite scenario. The precoding operation avoids inter-beam interference and maximizes the spectrum efficiency by full frequency reuse as an alternative to the traditional two-color or four-color reuse methods proposed in the DVB-S2 standard. However, the satellite payload introduces differential phase and frequency impairments, which can degrade the precoding performance. This work describes the implementation of the differential phase and frequency tracking and compensation loop in an end-to-end testbed over a multibeam satellite system with independent local oscillators. The developed system performs end-to-end real-time communication over the satellite link, including channel measurements and precompensation. Results are validated by an over-the-air demonstration using two beams of the SES-14 multibeam satellite. Each beam is transmitted by independent transponders, which results in differential frequency and phase offsets due to the transponder undisciplined local oscillators. This phase offset makes it impossible to use precoding without the phase compensation loop. We prove that the implemented system can successfully track and compensate the differential phase and frequency to improve precoding performance.

In this article, we focus on the use of low-Earth orbit (LEO) satellites providing the narrowband Internet of Things (NB-IoT) connectivity to the on-ground user equipments (UEs). Conventional resource allocation algorithms for the NB-IoT systems are particularly designed for terrestrial infrastructures, where devices are under the coverage of a specific base station (BS) and the whole system varies very slowly in time. The existing methods in the literature cannot be applied over LEO satellite-based NB-IoT systems for several reasons. First, with the movement of the LEO satellite, the corresponding channel parameters for each user will quickly change over time. Delaying the scheduling of a certain user would result in a resource allocation based on outdated parameters. Second, the differential Doppler shift, which is a typical impairment in communications over LEO, directly depends on the relative distance among users. Scheduling at the same radio frame users that overcome a certain distance would violate the differential Doppler limit supported by the NB-IoT standard. Third, the propagation delay over an LEO satellite channel is around 4–16 times higher compared to a terrestrial system, imposing the need for message exchange minimization between the users and the BS. In this work, we propose a novel uplink resource allocation strategy that jointly incorporates the new design considerations previously mentioned together with the distinct channel conditions, satellite coverage times, and data demands of various users on Earth. The novel methodology proposed in this article can act as a framework for future works in the field.

Distributed antenna arrays, fractionated payloads and cooperative platforms can provide unprecedented performance in the next generation of spaceborne communications and remote sensing systems. Remote phase synchronization of physically separated oscillators is the first step towards a coherent operation of distributed systems. This work shows the preliminary results of a TDD remote phase synchronization algorithm with a master-follower architecture. Herein, we describe the implementation and validation of the proposed algorithm. The implementation has been conducted in a Cubesat-ready software defined radio and validated at the end-to-end satellite communications testbed available at the University of Luxembourg.

Satellite Communications offer complementary benefits to terrestrial 5G/6G infrastructure, covering a wide range of use cases in need of ubiquitous coverage and reliability. However, to be as competitive as the terrestrial counterpart in terms of supplied throughput, satellite communications require a highly efficient use of the limited available spectrum. Linear precoding has demonstrated the ability to boost the spectral efficiency in the satellite domain, but raising a new issue: the bandwidth requirements of the feeder link. Deployment of several gateways, each of which precoding an independent cluster of beams causes performance degradation. Therefore, in this paper, we investigate the centralized gateway concept, where all digital baseband processes (including precoding) are implemented in a remote server connected via high speed fibers to the distributed remote gateways responsible for the downlink and uplink of the satellite radio frequency signals. In particular, we highlight the main technical challenges and provide a preliminary vision of potential solutions.

The growing demand for broadband applications has driven the satellite communication service providers to investigate High Throughput Satellite (HTS) solutions. While precoding has been identified as the most promising technique to boost the satellite spectral efficiency, new advanced solutions focus on re-configurable demand-driven systems, where throughput delivered aligns with the time and geographical variations of the traffic demand. For such goal, conventional user scheduling algorithms fail to meet the uneven user traffic demand. In this paper, we propose a novel unicast scheduling algorithm that takes into account both the channel orthogonality required for precoding along with the particular user demands. We name such technique as Weighted Semi-Orthogonal Scheduling (WSOS) methodology. Supporting numerical results are provided that validate the effectiveness of the proposed scheduling and quantify the benefits over conventional scheduling techniques.

One of the most dominant applications of Ultra Reliable Low Latency Communication of 5G-NR is V2X communication. For such latency-critical V2X communication, the distributed resource allocation using Semi-Persistent Scheduling (SPS) algorithm designed for out-of-coverage (Mode-4) scenario leads to a high collision probability and requires profuse sensing processes. Therefore, a need for more efficient distributed resource allocation scheme is compelled. In this paper, we investigate the 3GPP proposed SPS scheduling algorithm for its performance and formulate the problem under partial sensing systems. Furthermore, we propose a Foreseeing Semi-Persistent Scheduling (F-SPS) algorithm as an enhancement to the existing methodology, and conclusively, simulations are presented to illustrate the improved performance of the proposed F-SPS scheme in terms of reduced collision probability with an optimised number of sensing processes.

Carrier Aggregation is one of the essential approaches to achieve several orders of magnitude increase in peak data rates. While carrier aggregation benefits have been extensively studied in terrestrial wireless systems, its application to satellite has not been substantially explored. Carrier aggregation can be a prominent solution to address the issue of the spatially-heterogeneous satellite data traffic demand. This paper studies introducing carrier aggregation into satellite systems from a link layer perspective. The proposed modifications at the link layer have been carefully designed to make carrier aggregation transparent to the other layers. However, deployment of carrier aggregation in satellite systems with the combination of multiple carriers that have different characteristics requires effective scheduling schemes for reliable communications. Since channel awareness is indispensable for any efficient resource allocation schemes, we have proposed a perceptive scheduling algorithm that takes into account channel properties along with the instantaneous available resources to ensure that the received data packets are delivered without perturbing the original transmission order. Simulation results are given to validate our analysis and demonstrate the design tradeoffs, and thus, our results provide useful insights to practical scheduler design.

During the last few years, the interest around Machine-Type Communications (MTC) and Internet of Things (IoT) is growing exponentially. The Third Generation Partnership Project (3GPP), acknowledging the importance of this trend, has introduced a number of key features to support IoT. Specifically they provided progressively improved support for Low Power Wide Area Network (LPWAN) introducing the so called Narrowband IoT (NB-IoT) in 2017 for better serving IoT use cases. Furthermore, the integration of Non Terrestrial Networks (NTN) in the New Radio (NR) is playing an important role, enabling many different use cases for the forthcoming 5G system. This work aims at analyzing the feasibility of integrating NB- IoT with satellite communication systems; the main issues are investigated with a particular focus on the impact of doppler shift on NB-IoT waveform. Possible solutions are proposed, empowering the utilization of the NB-IoT terrestrial standard within satellite channel.

Carrier Aggregation (CA) is an integral part of current terrestrial networks. Its ability to enhance the peak data rate, to efficiently utilize the limited available spectrum resources and to satisfy the demand for data-hungry applications has drawn large attention from different wireless network communities. Given the benefits of CA in the terrestrial wireless environment, it is of great interest to analyze and evaluate the potential impact of CA in the satellite domain. In this paper, we study CA in multibeam high throughput satellite systems. We consider both inter-transponder and intra-transponder CA at the satellite payload level of the communication stack, and we address the problem of carrier-user assignment assuming that multiple users can be multiplexed in each carrier. The transmission parameters of different carriers are generated considering the transmission characteristics of carriers in different transponders. In particular, we propose a flexible carrier allocation approach for a CA-enabled multibeam satellite system targeting a proportionally fair user demand satisfaction. Simulation results and analysis shed some light on this rather unexplored scenario and demonstrate the feasibility of the CA in satellite communication systems.

Beam-Hopping (BH) and precoding are two trending technologies for the satellite community. While BH enables flexibility to adapt the offered capacity to the heterogeneous demand, precoding aims at boosting the spectral efficiency. In this paper, we consider a high throughput satellite (HTS) system that employs BH in conjunction with precoding. In particular, we propose the concept of Cluster-Hopping (CH) that seamlessly combines the BH and precoding paradigms and utilize their individual competencies. The cluster is defined as a set of adjacent beams that are simultaneously illuminated. In addition, we propose an efficient time-space illumination pattern design, where we determine the set of clusters that can be illuminated simultaneously at each hopping event along with the illumination duration. We model the CH time-space illumination pattern design as an integer programming problem which can be efficiently solved. Supporting results based on numerical simulations are provided which validate the effectiveness of the proposed CH concept and time-space illumination pattern design.

We present a method for estimating the undetected error rate when a cyclic redundancy check (CRC) is performed on the output of the decoder of short low-density parity-check (LDPC) codes. This system is of interest for telecommand links, where new LDPC codes have been designed for updating the current standard. We show that these new LDPC codes combined with CRC are adequate for complying with the stringent requirements of this kind of transmissions in terms of error detection.

We consider the framework of the McEliece cryptosystem based on low-density parity-check (LDPC) codes, which is a promising post-quantum alternative to classical public key cryptosystems. The use of LDPC codes in this context allows to achieve good security levels with very compact keys, which is an important advantage over the classical McEliece cryptosystem based on Goppa codes. However, only regular LDPC codes have been considered up to now, while some further improvement can be achieved by using irregular LDPC codes, which are known to achieve better error correction performance than regular LDPC codes. This is shown in this paper, for the first time at our knowledge. The possible use of irregular transformation matrices is also investigated, which further increases the efficiency of the system, especially in regard to the public key size.

In this paper, we carry out a practical assessment of the performance of finite-length LDPC codes over the wiretap channel with fast Rayleigh fading. Classical metrics for physical layer security, like the secrecy capacity, are based on information theoretic arguments, and provide the ultimate security bounds for these schemes. However, it is difficult to design practical schemes, using some specific finite-length code, able to approach such a performance. Then we use a more practical metric, based on the error probability, which allows assessing the performance achieved in terms of both reliability and security over the fast Rayleigh fading wire-tap channel.

In this paper, we provide a tight estimation of the security gap for the wiretap channel with fast Rayleigh fading. The error rate has already been used as a practical physical layer security metric, and the security gap has been defined to exploit such a metric over the wiretap channel with additive white Gaussian noise. We study two different approaches for estimating the security gap also over a fast fading wiretap channel. The first approach is quite conservative, and tends to overestimate the security gap. The second approach instead allows to perform a more realistic evaluation, and to highlight the gain achievable through information scrambling and coding, even when both the legitimate receiver and the eavesdropper channels are known only in statistical terms.