IEEE - Aerospace and Electronic Systems - April 2023 - 5

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destination, and grows to be very significant for long-distance
data communications [6]. The processing delay is
the time taken by a node to process a packet, such as the
time taken to examine the packet header for making routing
and switching decisions, before sending it to the next
hop. It depends upon the number of hops (or nodes or satellites)
between source and destination, and becomes notable
when the data communications has to traverse through
several hops. We define latency (or network latency) in
this work as the total delay incurred from the source
ground station (GS) to the destination GS over the
FSOSN.
To study the effect of LISL range on network latency,
we employ Starlink's Phase I constellation [7], and
assume LISLs between satellites in this constellation to
create an FSOSN. We also assume laser links between
GSs and satellites. In Phase I for building Starlink,
SpaceX has started deploying an LEO constellation of
1584 satellites. The original Phase I constellation consisted
of 1600 LEO satellites [8], and was part of SpaceX's
plan to deploy 4425 satellites in five LEO
constellations. SpaceX proposed the modification to Phase
I in their FCC filing in November 2018 [7]. In a separate
FCC filing, SpaceX has also proposed a plan to deploy
7518 satellites in three VLEO constellations [9].
To examine the effect of LISL range on network
latency, we first study the effect ofLISL range on network
connectivity for six different LISL ranges. The LISL
ranges are considered to vary from 659.5 km to 5016 km.
Then, we investigate the effect of LISL range on network
latency in the FSOSN created by employing LISLs in
Starlink's Phase I constellation for different LISL ranges
in three different scenarios for long-distance intercontinental
data communications. These include connections
between GSs at New York and London, New York and
Istanbul, and New York and Sydney. We find that the connectivity
within the FSOSN increases as the LISL range is
increased from 659.5 to 5016 km. With the increase in
LISL range, more and farther satellites are available to a
satellite for connectivity.
While investigating the effect of a specific LISL range
on network connectivity or network latency within a
APRIL 2023
scenario, the same LISL range is assumed for all satellites
in the FSOSN. To measure network latency between two
cities in a scenario, we calculate the shortest path between
the GSs in these cities over the FSOSN in terms of latency
that encompasses propagation delay and node delay. A
congestion-free FSOSN is considered with LISLs between
satellites having data rates in Gbps; the queueing delay
and transmission delay are thereby assumed as negligible;
and the node delay, which mainly consists of the processing
delay, is assumed to include these delays. We observe
that average network latency (i.e., the average of the latencies
of the shortest paths at all time slots) improves with
the increase in LISL range within a scenario, and the
improvement in average network latency with increase in
LISL range is seen in all scenarios. For a specific LISL
range, it is noticed that the longer the intercontinental distance
between cities, the better the improvement in average
latency. This work-to the best of our knowledge-is
the first to study the effect of LISL range on network
latency (or more precisely, average network latency) in
FSOSNs.
MOTIVATION
LCTs for establishing LISLs are either available or under
development with capabilities to establish LISLs over
ranges (or ISL distances) varying from 4500 to 45,000
km [2], [3], [4], [5]. A LISL range of 45,000 km is more
suitable for establishing an LISL between a LEO and a
geostationary Earth orbit satellite for data relay applications,
such as the European data relay satellite system. For
the upcoming LEO constellations, such as Phase I of Starlink,
4500 or 6000 km could be a more reasonable LISL
range to realize FSOSNs.
A grid-like approach for satellite connectivity provides
each satellite in the constellation with four connections
to its nearby neighbors. These include connections
with two nearest neighbors in the same orbital plane (OP)
and two nearest neighbors in two adjacent OPs [10].In a
generalization of the grid-like approach, repetitive patterns
in the network topology, called motifs, have been
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IEEE - Aerospace and Electronic Systems - April 2023

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