IEEE Consumer Electronics Magazine - November/December 2021 - 29

c/1.4675, i.e., 204 287 876 m/s. The parameters of
SpaceX's phase I of its Starlink constellation are
considered for the satellite network in this scenario,
i.e., an altitude of 550 km and a uniform
spacing of 5.45 between the 66 satellites within
an orbital plane. The radius of Earth is 6378 km.
The linear distance between the first two LEO
satellites S and T within the orbital plane is calculated
using the formula for the length of a
chord, i.e., 2 r sin((u/2) (p/180)), where u (the
angular spacing between S and T in degrees) is
5.45, and r is 6378 km þ 550 km. This is the hop
distance between S and T or between any two
consecutive satellites in the orbital plane for
that matter. This distance is multiplied by two to
find the distance between S and its second hop,
by three to find the distance to its third hop, and
so on. To calculate the end-to-end distance for S
and its nth hop, 1100 km is added to their hop
distance to account for the distance between
Earth and S and the distance between the nth
hop of S and Earth.
The distance between the first terrestrial
optical fiber relay station P and its next hop Q
along the surface of the Earth is calculated using
the formula for the length of an arc, i.e.,
2 pR (u/360), where u is 5.45 and R is the radius
of the Earth. To find the distance between P and
its second hop, third hop, and so on, u is 10.90,
16.35, and so on. The hop distance and the endto-end
distance for the terrestrial network are
the same. To calculate latency for satellite and
terrestrial networks, the end-to-end distance is
divided by the speed of light in vacuum and the
speed of light in optical fiber, respectively. The
satellite network operating at an altitude of 550
km begins to outperform the terrestrial optical
fiber network in terms of latency or propagation
delay starting at five hops when the terrestrial
hop distance exceeds 3000 km.
The four components of the end-to-end delay
in the network include transmission delay, processing
delay, queuing delay, and propagation
delay.21 In an optical communication network,
such as an optical wireless satellite network or a
terrestrial optical fiber network, the propagation
delay is the delay introduced by the optical
transmission of the signal along the fiber or vacuum.
It is directly proportional to the end-to-end
distance and becomes very significant for communications
over long distances.22 For the comparison
in Table 5, we define latency (or end-toend
latency) as the propagation delay that is
measured in only one direction, i.e., from the
source to the destination.
SUMMARY
FSO has experienced an immense technological
evolution over the last six decades. It is
emerging as a viable alternative to RF for intersatellite
links. Compared to RF terminals, FSO or
laser terminals require less onboard satellite
resources due to their smaller form factor, and
they are easy to integrate into satellite platforms.
Laser terminals for laser ISLs are still in
their infancy and are expected to offer capacities
of up to 10 Gb/s. Laser ISLs offering capacities in
the hundreds of Gb/s will need to be developed
to establish a true global communications network
in space.
Laser ISLs with satellites in crossing orbital
planes and with satellites in different orbits are
currently considered undesirable. However,
they will be crucial for effective networking
within a satellite constellation and between different
constellations to achieve megaconstellation
satellite networks. Current setup times for
laser ISLs are prohibitive and need to be significantly
reduced through technological advancement.
Efficient ATP systems need to be
developed to support reliable ISLs among crossing
orbital planes and among different orbits.
Four laser ISLs are expected per Starlink satellite,
and SpaceX is looking to equip its satellites
with this capability by late 2020. This number of
laser ISLs restricts the connectivity to two satellites
in the same orbital plane and to two satellites
in adjacent orbital planes. Connecting to
adjacent orbital plane neighbors in east-west
directions is expected as this would benefit the
population in the developed world, especially in
the United States. Six ISLs per satellite are
needed to provide north-south connectivity in
addition to east-west connectivity. A next-generation
satellite will need to be equipped with several
laser ISLs to provide robust connectivity
within the network and this will be vital for
November/December 2021
29
IEEE Consumer Electronics Magazine - November/December 2021

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