IEEE - Aerospace and Electronic Systems - April 2023 - 6

Laser Intersatellite Link Range in Free-Space Optical Satellite Networks: Impact on Latency
proposed, where a satellite's connectivity is also limited to
four neighbors [11]. Within a motif, the same connectivity
pattern is adopted for all satellites, and different motifs
provide different connectivity patterns. The performance
of different motifs was found to be widely different, and
to pick the motif with the best performance out of all possible
motifs involved exhaustively evaluating all possible
motifs. The performance of the motif-based approach for
connectivity to four neighbors was evaluated using
Starlink's Phase I constellation consisting of 1584 LEO
satellites at 550 km altitude and two LISL ranges including
2006 and 5014 km.
Using Starlink's original Phase I constellation of 1600
LEO satellites at 1150 km altitude and LISLs between satellites,
the latency of the resulting FSOSN is investigated
[12]. The satellite connectivity was limited to five
neighbors in this study. This included two nearest neighbors
of a satellite in the same OP, i.e., one in the front and
one at the rear. Instead of connecting to nearest left or
right neighbors in adjacent OPs, connectivity to adjacent
OP neighbors in east-west directions was employed as
such connectivity would benefit most of the population in
developed countries including the United States. A nearby
satellite in a crossing OP was considered as the fifth neighbor,
however, an LISL range to establish this crossing OP
LISL was not specified.
In a previous work [13], we presented a comprehensive
classification of LISLs, which was based on the location
of satellites in different OPs within a constellation
and the different durations of existence of LISLs between
satellites. Furthermore, we employed the satellite constellation
for Phase I of Starlink to study the effect of varying
a satellite's LISL range on the number of different types
of LISLs it can form with other satellites in the constellation.
Unlike previous work [10], [11], [12], we place no
limitation on the connectivity of a satellite. While studying
the effect of LISL range on latency and in calculating
the shortest path between a pair of cities at a certain LISL
range, we consider all satellites that are within the LISL
range of a satellite as neighbors of that satellite, and do
not limit a satellite's connectivity to nearest four or five
neighbors. To the best of our knowledge, a study that
investigates the effect of LISL range on network latency
in FSOSNs without any constraint on satellite connectivity
does not exist in the literature.
EFFECT OF LISL RANGE ON NETWORK CONNECTIVITY
To understand the impact of LISL range on network
latency, it is necessary to first study the effect of LISL
range on network connectivity. We use the satellite constellation
for Phase I of Starlink to study this effect. In this
constellation, SpaceX is deploying 1584 LEO satellites at
an altitude of 550 km at an inclination of 53 with respect
6
Table 1.
Satellite Connectivity for Satellite x10101 for Different
LISL Ranges at the Equator
LISL range (km)
659.5
1319
1500
1700
2500
5016
8
12
22
38
180
# Satellites in LISL range
4
to the equator [7]. It will have 24 OPs and each OP will
have 66 satellites. We assume this constellation to be uniform,
and consider the spacing between OPs to be 15 and
the spacing between satellites within an OP to be 5.45.
While examining the effect of a specific LISL range, all
satellites in the constellation are considered to have the
same LISL range. Furthermore, all satellites that are
within the LISL range of a satellite are considered as its
connectivity neighbors and thereby potential options for
establishing LISLs.
We consider six different LISL ranges to study the effect
of LISL range on network connectivity. To illustrate this
effect, we employ one satellite in the constellation as an
example, i.e., satellite x10101, which is the first satellite in
the first OP. The satellite connectivity for this satellite at different
LISL ranges is shown in Table 1.As observed from
Table 1 the satellite connectivity increases with the increase
in LISL range. The number of neighbors within the LISL
range ofx10101 is 4, 8, 12, 22, 38, and 180 at LISL range of
659.5, 1319, 1500, 1700, 2500, and 5016 km, respectively.
Figure 1 illustrates the satellite connectivity for x10101 at
2500 km LISL range. Similar to x10101,an increase in
LISL range causes an increase in connectivity for all satellites
within the network, which translates into increased network
connectivity.
At the LISL range of 659.5 km, which can be considered
as the minimum LISL range for this constellation,
x10101 has four neighbors. These include only two intraOP
neighbors (i.e., two neighbors in the same OP) and
two crossing OP neighbors. The maximum LISL range is
the LISL range of a satellite that is constrained only by
visibility. For this constellation, the maximum LISL range
can be calculated as 5016 km [13]. At this range, x10101
has 180 neighbors, including 14 intra-OP neighbors, 30
adjacent OP neighbors, 44 nearby OP neighbors, and 92
crossing OP neighbors. An LISL range of 1700 km
appears to be a more reasonable minimum LISL range for
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APRIL 2023

IEEE - Aerospace and Electronic Systems - April 2023

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