Optical Satellite Networks Research Articles

Research on Wavelength Resource Characteristics of Optical Satellite Network Based on Time-Space Conflict Map

Research on Wavelength Resource Characteristics of Optical Satellite Network Based on Time-Space Conflict Map

When to Crossover From Earth to Space for Lower Latency Data Communications?

For data communications over long distances, optical wireless satellite networks (OWSNs) can offer lower latency than optical fiber terrestrial networks (OFTNs). However, when is it beneficial to switch or crossover from an OFTN to an OWSN for lower latency data communications? In this article, we introduce a crossover function that enables to find the crossover distance, i.e., a distance between two points on the surface of the Earth beyond which switching or crossing over from an OFTN to an OWSN for data communications between these points is useful in terms of latency. Numerical results reveal that a higher refractive index of optical fiber (or $i$) in an OFTN and a lower altitude of satellites (or $h$) in an OWSN result in a shorter crossover distance. To account for the variation in the end-to-end propagation distance that occurs over the OWSN, we examine the crossover function in four different scenarios. Numerical results indicate that the crossover distance varies with the end-to-end propagation distance over an OWSN and is different for different scenarios. We calculate the average crossover distance over all scenarios for different $h$ and $i$ and use it to evaluate the simulation results. Furthermore, for a comparative analysis of OFTNs and OWSNs in terms of latency, we study three different OFTNs having different refractive indexes and three different OWSNs having different satellite altitudes in three different scenarios for long-distance intercontinental data communications, including connections between New York and Dublin, Sao Paulo and London, and Toronto and Sydney. All three OWSNs offer better latency than OFTN2 (with $i_{2}$ = 1.3) and OFTN3 (with $i_{3}$ = 1.4675) in all scenarios. For example, for Toronto–Sydney connection, OWSN1 (with $h_{1}$ = 300 km), OWSN2 (with $h_{2}$ = 550 km), and OWSN3 (with $h_{3}$ = 1100 km) perform better than OFTN2 by 18.11%, 16.08%, and 10.30%, respectively, while they provide an improvement in latency of 27.46%, 25.67%, and 20.54%, respectively, compared to OFTN3. OWSN1 performs better than OFTN1 (with $i_{1}$ = 1.1) for Sao Paulo–London and Toronto–Sydney connections by 2.23% and 3.22%, respectively, while OWSN2 outperforms OFTN1 for Toronto–Sydney connection by 0.82%. For New York–Dublin connection, all OWSNs while for Sao Paulo–London connection, OWSN2 and OWSN3 exhibit higher latency than OFTN1 as the corresponding average crossover distances are greater than the shortest terrestrial distances between cities in these scenarios. Multiple satellites (or laser intersatellite links) on its shortest paths drive up the propagation distance to the extent that OWSN3 ends up with a higher latency than OFTN1 for the Toronto–Sydney intercontinental connection scenario although the related average crossover distance is less than the shortest terrestrial distance between Toronto and Sydney. The challenges related to OWSNs and OFTNs that may arise from this work in future are also highlighted.

Intersatellite Laser Link Planning for Reliable Topology Design in Optical Satellite Networks: A Networking Perspective

The development of reusable rockets makes it possible to launch a massive number of low earth orbit satellites for Internet access. These satellites are expected to be connected by inter-satellite links (ISLs) to provide global Internet service to end-users on the surface of the earth. From the infrastructure’s perspective, the ISLs can be implemented with lasers, forming the optical satellite networks. The design of terrestrial optical networks is usually traffic-driven, meaning more links/bandwidth are deployed to the traffic-intensive areas. But, the orbiting nature of satellites determines that laser ISLs are also moving at high velocities, making the presence of corresponding bandwidth loosed coupled with a specific area. As a result, the traffic-driven network design methodology will be inapplicable to the optical satellite networks. In this work, we model the optical satellite networks and define the laser ISL planning problem, Accordingly, we propose an ISL removal algorithm to remove parts of the ISLs from the grid-mesh topology to improve the average bandwidth utilization ratio (i.e., bandwidth efficiency) while maintaining networks’ reliability and availability. We conduct a simulation study based on the Starlink constellation to evaluate the grid-mesh topology and to investigate the proposed algorithm’s impact on the network performance. Results show that the bandwidth resource in the grid-mesh topology can hardly be used efficiently and removing certain ISLs properly can improve the bandwidth efficiency of optical satellite networks significantly. We also gain another interesting insight that maintaining too many ISLs will not improve, but degrade the network availability.

Design and Implementation of Semi-Physical Platform for Label Based Frame Switching in Integrated Satellite Terrestrial Networks

With the explosion of traffic demand in recent years, the integration of satellite optical networks and terrestrial networks (ISTN) creates a promising networking solution for future low-latency, high-rate, and high-capacity communications. Owing to the high cost of deploying and maintaining a satellite optical network, it is critical to carefully design and plan the network to ensure the performance of the network. Thus, a semi-physical simulation platform based on software-defined networks (SDNs) is developed to simulate a satellite optical network and evaluate the performance of the proposed label-based advanced orbiting system (AOS) frame switching method that adheres to the Consultative Committee for Space Data Systems’ recommended standard (CCSDS). The semi-physical simulation platform has two major innovations: (1) adapting and integrating network protocols between the CCSDS and open system interconnect (OSI) reference models, particularly at the data link layer, and (2) the foundation for an SDN-based satellite optical network. In the control plane, real-time VxWorks Simulators serve as controllers to establish and manage various network protocols and the link manager protocol (LMP). Here, network protocols include open shortest path first (OSPF) for routing managing and controlling messages, constraint shortest path first–traffic engineering (CSPF-TE), and constraint-label distribution protocol (CR-LDP) for routing data services. LMP is used to assign and reserve satellite optical link resources. The performance of the architecture and protocols is evaluated via a semi-physical simulation platform.

Distributed competitive satellite optical burst switching mechanism for satellite networks with ultra-high link delays

The ultra-high link latency of the conventional central reservation switching method in satellite optical networks limits link utilization. In this study, a novel fully distributed competitive satellite optical burst switching (DC-SOBS) mechanism was proposed to overcome the drawbacks of the optical circuit switching method and achieve link multiplexing. Furthermore, the proposed method addressed the low link utilization problem caused by the high link latency of the central-control-related methods. The DC-SOBS mechanism in the ring network of six Geostationary Orbit (GEO) satellites improved link utilization by three orders of magnitude compared with the reservation method. A data conflict processing method was proposed to ensure that the same information was provided to various optical switching nodes, and the same algorithm was used to achieve a consistent scheduling result in each node.

Link assignment based on vertex deleting using adjacency matrix for LEO optical satellite network

In the LEO optical satellite network (OSN), each satellite has more than a dozen potential links, but the number of onboard terminals is limited. The beam divergence angle of optical satellite communication is extremely small, so only one bidirectional communication link can be established between a pair of communication terminals, therefore how to use limited onboard communication terminals to establish inter-satellite links (ISLs) for each satellite to achieve high-efficiency communication is an urgent problem to be solved. However, previous studies on link assignment schemes cannot take into account the terminal utilization ratio and the small end-to-end delay of the network. To solve the above-mentioned link assignment problem, this paper proposes the link assignment scheme based on vertex deleting using adjacency matrix (VDAM-LAS) for the first time. This scheme can ensure that any number of communication terminals on each satellite can be used to establish ISLs and the smaller network delay and ISLs hops can be obtained. In addition, it solves the link assignment problem of the OSN with the limit of the maximum allowable link hops of a single satellite, which is a problem that cannot be optimized using the previous LASs. The simulation results show that when assigning incessant ISLs, compared with the Manhattan Street Network (MSN)-LAS, greedy-LAS and perfect match model (PMM)-LAS, the average hops are reduced by 8.87%, 3.13%, 0.49%, respectively, and the average propagation delay is reduced by 17.24%, 2.86%, 0.8%, respectively. When assigning intermittent ISLs, the average hops are reduced by 10.45% and 6.89%, respectively, compared with the greedy-LAS and PMM-LAS.

Effect of Doppler shift on preamplifier DPSK receivers using balanced detection for optical satellite networks.

Optical satellite networks using laser intersatellite links are recognized as being capable of satisfying the increasing broadband communication demands. However, the Doppler shift due to relative movement between satellites must be taken into consideration. A convenient method that does not require complicated mathematical calculation is adopted to derive the relationship between the residual Doppler shift and the bit error rate (BER) for the balanced differential phase-shift keying (DPSK) detection system. It is shown that the DPSK receiver is sensitive to the residual Doppler shift; when the residual Doppler shift Δfres/Rb is 0.05, 0.10, and 0.15, the BER is reduced from a level of 10-17 to a level of 10-14, 10-11, and 10-6, respectively. The BER of a certain node pair under different link assignment schemes is calculated in a typical multi-hop LEO constellation. The results show that the communication BER of the same node pair is not only related to the step change of the Doppler but also related to the accumulation of the Doppler shift in the optical path. The results obtained here will be helpful in routing selection, link assignment, topology design, and system compensation.

An Elastic Resource Allocation Algorithm Based on Dispersion Degree for Hybrid Requests in Satellite Optical Networks

The satellite-assisted Internet of Things (IoT) communication is considered a key component of the 6G network, and low Earth orbit (LEO) satellite is the leading choice of IoT-related satellites due to its minimum delay. Hybrid requests, including immediate reservation (IR) and advanced reservation (AR) services in LEO satellite netoworks cause the occurrence of resource fragments (RFrags), which adversely affect the network performance. To alleviate the degradation of network performance caused by resource fragmentation, a routing, wavelength, and time-slots assignment algorithm, which is named elastic resource allocation algorithm based on the dispersion degree (ERA-DD), is proposed in this article. Possible resource fragmentation types are analyzed and a fragmentation description named dispersion degree (DD) is designed. In the DD, the number of free resource blocks is accurately described by the state jumps (SJs) of adjacent resource slots, and the numerical relationship between SJs and free resource blocks is proposed and proved. Besides, restrictions on the selection priority of candidate schemes in the ERA-DD algorithm are analyzed. Finally, the traffic blocking rate, the wavelength utilization, the average communication delay, and the average initial delay are evaluated by simulation. The results demonstrate that RFrags are more fully utilized compared with the maximum total link spectrum consecutiveness (MTLSC) algorithm. The traffic blocking rate can be reduced by 32.5% and wavelength utilization can be increased by 1.6%.

Temporary Laser Inter-Satellite Links in Free-Space Optical Satellite Networks

Laser inter-satellite links (LISLs) between satellites in a free-space optical satellite network (FSOSN) can be divided into two classes: permanent LISLs (PLs) and temporary LISLs (TLs). TLs are not desirable in next-generation FSOSNs (NG-FSOSNs) due to high LISL setup time, but they may become feasible in next-next-generation FSOSNs (NNG-FSOSNs). Using the satellite constellation for Phase I of Starlink, we study the impact of TLs on network latency in an NG-FSOSN (which has only PLs) versus an NNG-FSOSN (which has PLs and TLs) under different long-distance inter-continental data communications scenarios, including Sydney–Sao Paulo, Toronto–Istanbul, Madrid–Tokyo, and New York–Jakarta, and different LISL ranges for satellites, including 659.5 km, 1,319 km, 1,500 km, 1,700 km, 2,500 km, 3,500 km, and 5,016 km. It is observed from the results that TLs provide higher satellite connectivity and thereby higher network connectivity, and they lead to lower average network latency for the NNG-FSOSN compared to the NG-FSOSN in all scenarios at all LISL ranges. In comparison with the NG-FSOSN, the improvement in latency with the NNG-FSOSN is significant at LISL ranges of 1,500 km, 1,700 km, and 2,500 km, where the improvement is 16.83 ms, 23.43 ms, and 18.20 ms, respectively, for the Sydney–Sao Paulo inter-continental connection. For the Toronto–Istanbul, Madrid–Tokyo, and New York–Jakarta inter-continental connections, the improvement is 14.58 ms, 23.35 ms, and 23.52 ms, respectively, at the 1,700 km LISL range.

Link Assignment Based on Vertex Deleting Using Adjacency Matrix for Leo Optical Satellite Network

Link Assignment Based on Vertex Deleting Using Adjacency Matrix for Leo Optical Satellite Network

Review on Free-Space Optical Communications for Delay and Disruption Tolerant Networks

The increase of data-rates that are provided by free-space optical (FSO) communications is essential in our data-driven society. When used in satellite and interplanetary networks, these optical links can ensure fast connections, yet they are susceptible to atmospheric disruptions and long orbital delays. The Delay and Disruption Tolerant Networking (DTN) architecture ensures a reliable connection between two end nodes, without the need for a direct connection. This can be an asset when used with FSO links, providing protocols that can handle the intermittent nature of the connection. This paper provides a review on the theoretical and state-of-the-art studies on FSO and DTN. The aim of this review is to provide motivation for the research of an optical wireless satellite network, with focus on the use of the Licklider Transmission Protocol. The assessment presented establishes the viability of these networks, providing many examples to rely on, and summarizing the most recent stage of the development of the technologies addressed.

Laser Intersatellite Links in a Starlink Constellation: A Classification and Analysis

Laser intersatellite links (LISLs) are envisioned between satellites in upcoming satellite constellations, such as the one in phase I of SpaceX's Starlink. Within a constellation, satellites can establish LISLs with other satellites in the same orbital plane (OP) or in different OPs. We present a classification of LISLs based on the location of satellites within a constellation and of LISLs' duration. Then, using the satellite constellation for phase I of Starlink, we study the effect that varying a satellite's LISL range has on the number of different types of LISLs it can establish with other satellites. In addition to permanent LISLs, we observe a significant number of temporary LISLs between satellites in crossing OPs. Such LISLs can play a vital role in achieving low-latency paths within next-generation optical wireless satellite networks.

Congestion avoidance based on artificial bee colony algorithm for routing and wavelength assignment in optical satellite networks

Massive data traffic demand for satellite networks has been increased with the emergence of global broadband services. Due to limited onboard processing capacity and imbalanced distribution of global traffic, severe link congestion may occur in hot spots, deteriorating the performance of the whole network. A link cost modification applied in load balancing algorithm based on artificial bee colony is proposed for routing and wavelength assignment problem to distribute traffic in advance. Moreover, congestion avoidance based on wavelength utilization is designed for unexpected congestion to minimize the adverse impact of congestion on network performance. Experiment results show that the method we proposed realizes high performance in communication success rate and average transmission delay and also achieves the maximum utilization of wavelength utilization under the condition of low blocking rate and the equalization of traffic load.

Site Diversity in Downlink Optical Satellite Networks Through Ground Station Selection

Recent advances have shown that satellite communication (SatCom) will be an important enabler for next generation terrestrial networks as it can provide numerous advantages, including global coverage, high speed connectivity, reliability, and instant deployment. An ideal alternative for radio frequency (RF) satellites is its free-space optical (FSO) counterpart. FSO or laser SatCom can mitigate the problems occurring in RF SatCom, while providing important advantages, including reduced mass, lower consumption, better throughput, and lower costs. Furthermore, laser SatCom is inherently resistant to jamming, interception, and interference. Owing to these benefits, this paper focuses on downlink laser SatCom, where the best ground station (GS) is selected among numerous candidates to provide reliable connectivity and site diversity. To quantify the performance of the proposed scheme, we derive closed-form outage probability and ergodic capacity expressions for two different practical GS deployment scenarios. Thereafter, asymptotic analysis is conducted to obtain the overall site diversity order, and aperture averaging is studied to illustrate the impact of aperture diameter on the overall performance. Furthermore, we investigate the site diversity order for a constellation of satellites that are communicating with the best GS by using opportunistic scheduling. Finally, important design guidelines that can be useful in the design of practical laser SatComs are outlined.

Free Space Optics for Next-Generation Satellite Networks

Free space optics (FSO) refers to optical wireless communications in outdoor environments. The aim of this article is to analyze the role that FSO is envisaged to play in the creation of next-generation satellite networks. To begin with, the reader is introduced to the types of FSO links and functionalities of a basic FSO system. Next, a comparison of FSO and radio frequency (RF) technologies for intersatellite links (ISLs) is provided, including a comparison between FSO and RF links when employed between low Earth orbit satellites. After that, the types of FSO or laser ISLs are considered, and the challenges in establishing laser ISLs, the properties of laser ISLs, and the capabilities of terminals for laser ISLs are discussed. Then, the parameters of a satellite constellation are highlighted, and different aspects of SpaceX's upcoming megaconstellation Starlink are explored. In addition, the optical wireless satellite network that is created by utilizing laser ISLs is examined. Finally, a use case is investigated for next-generation optical wireless satellite networks that are envisioned by the mid to late 2020s.

Overview on routing and resource allocation based machine learning in optical networks

For optical networks, routing and resource allocation which considerably determines the resource efficiency and network capacity is one of the most important works. It has been widely studied and many excellent algorithms have been developed. However, theoretical analysis shows that routing and resource allocation belongs to the Nondeterministic Polynomial Complete (NP-C) problem no matter in wavelength division multiplexing (WDM) optical networks, elastic optical networks (EONs), or space division multiplexing (SDM) optical networks. At presents, there doesn't exist a polynomial-time algorithm for routing and resource allocation. In recent years, machine learning which shows great advantages in solving complex problems has been widely concerned and researched. Using machine learning to conduct routing and resource allocation has aroused a great interest of researchers. This paper provides an overview on routing and resource allocation based on machine learning in optical networks. At first, we briefly introduce the routing and wavelength allocation (RWA) problem in WDM optical networks, the routing and spectrum allocation (RSA) problem in EONs, and the routing, core, spectrum allocation (RCSA) problem in SDM optical networks respectively. Commonly used machine learning techniques in optical networks are briefly elaborated. Then, the problems of quality of transmission (QoT) estimation, traffic estimation, and crosstalk prediction which can help to routing and resource allocation are also elaborated. The machine learning enabled RWA algorithms, RSA algorithms, and RCSA algorithms are elaborated, analyzed and compared in detail. In addition, the applications of machine learning in the QoT estimation, traffic estimation, and crosstalk prediction, etc., are also elaborated. Based on the existing research results, we present future research directions about how to use machine learning techniques to conduct routing and resource allocation in multidimensional time–space-frequency optical networks and satellite optical networks.

An Analytic Method of Wavelength Requirements in Dynamic Optical Satellite Networks

Optical satellite network (OSN) plays an important role in broadband applications of space-based information. For OSNs, the premise of data transmission and routing is allocating wavelength resource. However, the dynamic characteristics of OSNs, i.e., intermittent link connection and time-varying topology make it difficult to estimate wavelength requirements in OSNs. In this letter, we study wavelength multiplexing in intermittent optical inter-satellite links and present a method based on space-time evolving graph to estimate required wavelengths in dynamic OSNs. Some indicators are proposed to represent topology variation and numerical simulations have been conducted to investigate impacts of topology variation on wavelength requirements.

Separated traffic grooming for Low Earth Orbit (LEO) optical satellite networks with wavelength dimensioning

SummaryIntersatellite laser links have the advantages of high bandwidth, low delay, and low power consumption, which makes wavelength division multiplexing (WDM) with wavelength routing a promising alternative to accommodate the tremendous increasing traffics from terrestrial users. To make better use of the finite wavelengths in optical satellite networks (OSNs), traffic grooming is necessary for the sake of its inherent superiority in partitioning wavelength channel into multiple low‐rate channels to satisfy the traffic requests. However, in most occasions, wavelength dimensioning is taken as a part of traffic grooming, which increases the complexity of application. Therefore, this paper investigates a novel separated traffic grooming method to minimize the number of wavelengths while satisfying the traffic requests, of which the process can be divided into two parts. Firstly, the basic connection requests are bundled to construct aggregation units (AUs). And then, the AUs are groomed onto wavelengths. In the process, an AUs construction method with multihop lightpath bypass grooming (MHLBG) is designed for uniform traffics, and a bandwidth reorganization algorithm (BRA) with MHLBG is customized for nonuniform traffics. Finally, we evaluate performance in simulation, and the results show that the proposed method has realized a good compromise between numbers of wavelengths to be used with the average end‐to‐end delay. Also, the separated traffic grooming method yields better performance than the integrated one.

Research on Wavelength Routing Algorithm Based on Multi-service in Satellite Optical Networks

Research on Wavelength Routing Algorithm Based on Multi-service in Satellite Optical Networks

Bee Colony Algorithm Optimization Based on Link Cost for Routing and Wavelength Assignment in Satellite Optical Networks

Bee Colony Algorithm Optimization Based on Link Cost for Routing and Wavelength Assignment in Satellite Optical Networks