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

Abstract

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 work, 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 indices 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 = 1,100 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. The 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 inter-satellite 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 inter-continental 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.

On Crossover Distance for Optical Wireless Satellite Networks and Optical Fiber Terrestrial Networks

“…In [14], crossover functions were proposed to enable the calculation of the crossover distance in four different scenarios. It was concluded that the crossover distance depended upon the altitude of satellites in the OWSN, the optical fiber refractive index in the OFTN, and the end-to-end propagation distance over the OWSN, i.e., the propagation distance between source and destination points (or satellite ground stations) in different cities over the OWSN.…”

“…However, a direct LISL was assumed to exist between the ingress and egress satellites, which is usually not the case in OWSNs arising from real satellite constellations. In this work, we extend the crossover function for the first scenario proposed in [14] to conform it to a more realistic scenario to enable a more realistic determination of the crossover distance by incorporating intermediate satellites between ingress and egress.…”

“…However, when should the data traffic switch or crossover from an OFTN on Earth to an OWSN in space for lower latency data communications between two points, such as optical fiber relay stations or satellite ground stations, on the surface of the Earth? Crossover functions have been proposed to calculate the crossover distance, i.e., the distance between two points on Earth beyond which switching or crossing over from an OFTN to an OWSN leads to lower latency data communications between these points, in different scenarios [14]. In this work, we extend one of these crossover functions.…”

“…More specifically, the contribution of this work is as follows. We enhance the crossover function for the first scenario in [14] to incorporate intermediate satellites or hops between the ingress and egress satellites in the OWSN for a more realistic determination of the crossover distance in this scenario.…”

Link Budget Analysis for Free-Space Optical Satellite Networks

“…Free-space optical communication is receiving more and more attention these days as it is a promising and rapidly developing technology for wireless communication between satellites due to its larger link bandwidth, license free spectrum, higher link data rate, better security, smaller antenna size, lower terminal mass, and lower terminal power consumption compared to radio frequency-based satellite communication [1]. The free-space optical satellites networks (FSOSNs) based on upcoming low Earth orbit (LEO) satellite constellations, such as SpaceX's Starlink [2], and Telesat's Lightspeed [3], are expected to employ optical or laser inter-satellite links while optical or laser uplink and downlink communications are envisioned for future FSOSNs [4], [5].…”