1,720,962 research outputs found
Communication Frameworks and Architectures: From Radio Interface Evaluation to Constellation Design
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5G New Radio for Non-Terrestrial Networks: Analysis and Comparison of HARQ and RLC ARQ Performance Over Satellite Links
Get PDFIn this paper, we study the extension of 5G New Radio (NR) to Non-Terrestrial Networks (NTN). For terrestrial ones, Hybrid Automatic Repeat reQuest (HARQ) is the main retransmission solution used by 5G NR at the physical and MAC layers, enhancing decoding performance through diversity and coding gain. However, for NTN, its implementation faces challenges due to the significant delays caused by the long distances of satellites. In the first part, we begin by investigating the minimum number of HARQ processes required for various LEO scenarios, as well as the relationship between the number of processes and the coherence time of the satellite link. Next, while the performance of 5G retransmission schemes over AWGN and terrestrial channels is well explored, this is not the case for realistic satellite channel models. To address this, we have developed an open-source simulator that accurately implements all the blocks of the data channel transmission and reception chain, including the retransmission schemes and the Land Mobile Satellite (LMS) channel. We consider the 5G NR Physical Downlink Shared Channel (PDSCH) and we present and discuss a number of results over the LMS channel, which are important to understand the HARQ performance for the NTN satellite scenario. In the second part, we consider the 5G NR alternative retransmission solution, RLC ARQ, which is available at the Radio Link Control (RLC) layer. This method might be interesting for satellite links, because it adds minimal complexity to the receiver side, but it provides less enhancement to signal reception capabilities and more latency. We first present an analytic model to compute its performance over the LMS channel, then we analyze its behavior. Finally, we provide a detailed comparison and discussion of HARQ and RLC ARQ performance in terms of block error rate, spectral efficiency, and latency. This extensive analysis provides valuable insights for researchers and space agencies interested in applying 5G NR to satellite-based Non-Terrestrial Networks
Distributed On-Demand Routing for VLEO Constellations with 3-Terminal Inter-Satellite Links
Get PDFThis paper investigates routing strategies for Very Low Earth Orbit (VLEO) satellite constellations constrained
by a three-terminal (3T) inter-satellite architecture, which is gaining interest as currently adopted by major operators (e.g., Starlink). In the 3T configuration, two terminals enable alongtrack communication, while a third hemispherical terminal can dynamically connect to either the left or right cross-plane neighbor, but not both simultaneously. This contrasts with the traditional four-terminal (4T) architecture, where four directional terminals enable full inter-plane links. The 3T constraint reduces crossplane connectivity, impacting path diversity, reachability, and latency. To address these challenges, we develop Interleaved-Scenario MinHopCount (IS-MHC) and DisCo3T, modified versions of the state-of-the-art routing algorithms Minimum Hop Count (MinHopCount) and Distributed On-Demand Routing for Mega-Constellations (DisCoRoute) for the 3T case. Our new model accurately captures the limitations of 3T connectivity and reduces hop count in this scenario. We conduct a comprehensive evaluation of DisCo3T in terms of latency, hop count, and computational complexity, including a comparative analysis with the original 4T implementation. Results show that the topological constraints of the 3T configuration cause an average hop count increase of 21.7% compared to the 4T baseline; however, DisCo3T retains the processing efficiency of the original 4T algorithm
Distributed Space Traffic Management Solutions with Emerging New Space Industry
Get PDFDay-to-day services, from weather forecast to logistics, rely on space-based infrastructures whose integrity is
crucial to stakeholders and end-users worldwide. Current trends point towards congestion of the near-Earth space
environment increasing at a rate greater than existing systems support, and thus demand novel cost-efficient approaches
to traffic detection, characterization, tracking, and management to ensure space remains a safe, integral part of societies
and economies worldwide. Whereas machine-learning (ML) and artificial intelligence (AI) have been extensively
proposed to address congestion and alleviate big-data problems of the future, little has been done so far to tackle the
need for transnational coordination and conflict-resolution in the context of space traffic management (STM).
In STM, there is an ever-growing need for distributing information and coordinating actions (e.g., avoidance
manoeuvres) to reduce the operational costs borne by individual entities and to decrease the latencies of actionable
responses taken upon the detection of hazardous conditions by one-to-two orders of magnitude. However, these needs
are not exclusive to STM, as evidenced by the widespread adoption of solutions to distributing, coordinating, and
automating actions in other industries such as air traffic management (ATM), where a short-range airborne collision
avoidance system (ACAS) automatically coordinates evasive manoeuvres whenever a conjunction is detected. Within
this context, this paper aims at establishing a roadmap of promising technologies (e.g., blockchain), protocols and
processes that could be adapted from different domains (railway, automotive, aerial, and maritime) to build an
integrated traffic coordination and communication architecture to simplify and harmonise stakeholders’ satellite
operations.
This paper is organised into seven sections. First, Section 1 introduces the problem of STM, highlighting its
complexity. Following this introduction, Section 2 discusses needs and requirements of various stakeholders such as
commercial operators, space situational awareness (SSA) service providers, launch-service providers, satellite and
constellation owners, governmental agencies, regulators, and insurance companies. Then, Section 3 addresses existing
gaps and challenges in STM, focusing on globally coordinated approaches. Next, Section 4 reviews technologies for
distributed, secure, and persistent communications, and proposed solutions to address some of these challenges from
non-space sectors. Thereafter, Section 5 briefly covers the history of STM proposals and presents the state-of-the-art
solution being proposed for modern STM. Following this review, Section 6 devises a step-by-step plan for exploiting
and deploying some of the identified technologies within a five-to-ten-year timeline to close several existing gaps.
Finally, Section 7 concludes the paper
On the use of Pseudo-Noise Ranging with high-rate spectrally-efficient modulations
Get PDFIn this paper, we study the feasibility of coupling the PN ranging with filtered high-order modulations,
and investigate the simultaneous demodulation of a high-rate telemetry stream while tracking the PN ranging
sequence. Accordingly, we design a receiver scheme that is able to perform a parallel cancellation, in closedloop, of the ranging and the telemetry signal reciprocally. From our analysis, we find that the non-constant
envelope property of the modulation causes an additional jitter on the PN ranging timing estimation that,
on the other hand, can be limited by properly sizing the receiver loop bandwidth.
Our study proves that the use of filtered high-order modulations combined with PN ranging outperforms
the state-of-the-art in terms of spectral efficiency and achievable data rate, while having comparable ranging
performanc
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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