IEEE Consumer Electronics Magazine - November/December 2021 - 27

OPTICAL WIRELESS SATELLITE
NETWORKS
In its FCC filings, SpaceX mentioned five 1.5 kg
silicon carbide communication components while
discussing space debris issues when its Starlink
satellites reach end of life and need to be deorbited.14
Silicon carbide is used in mirrors in laser
terminals for laser ISLs. This indicated that SpaceX
would equip its Starlink satellites with five laser
ISLs so that they could connect to other satellites
within a constellation to form an optical wireless
satellite network.
Four of these laser ISLs will likely be used to
connect to nearby satellites in the same and
adjacent orbital planes. The obvious candidates
for these ISLs are the two neighbors of a satellite
in the same orbital plane, one in the front and one
at the back, and the two neighbors in two adjacent
orbital planes, one at the right and one at the left.
At any one region in space above the Earth, half of
the satellites within the constellation are moving
in a northeasterly direction while the other half
are moving in a southeasterly direction. The fifth
laser ISL is likely intended to connect to a satellite
in a crossing orbital plane.7 The satellites in the
Starlink constellation at 550 km altitude travel at
speeds of approximately 7.6 km/s. It is challenging
for the laser ISL of a northeast-bound satellite to
track and connect to a southeast-bound satellite
and then rapidly switch to a new crossing satellite
as the old one moves away.7 In a later FCC filing,
SpaceX revised the number of ISLs per satellite to
four.6 This could be due to the difficulty in engineering
this fifth laser ISL.
Connecting with front and back neighbors
within orbital planes provides southwest $
northeast and northwest $ southeast routing
paths within the two groups of satellites moving
in northeasterly and southeasterly directions,
respectively. Instead of connecting with immediate
left or right neighbors in adjacent orbital
planes, a good idea could be to connect to adjacent
orbital plane neighbors in north-south or
east-west directions. Providing east-west connectivity
may be preferable to north-south connectivity
in phase I of Starlink as it would benefit
most of the population in developed countries
including the United Sates.7
The optical wireless satellite network that is
created by utilizing each satellite's four laser
November/December 2021
ISLs in this manner provides a good mesh network,
as illustrated in Figure 2(b). However,
there are two distinct meshes that exist within
the network, the red colored one between the
group of satellites moving northeast and the
blue colored one between the group moving
southeast. There is no local connectivity
between these meshes. Traffic can be routed
without switching between the two meshes.
Using an additional or fifth laser ISL to create
intermesh links or links between crossing orbital
planes can improve routing options. However,
such links are expected to be down frequently as
they are re-established from one crossing satellite
to another.7
NEXT-GENERATION SATELLITE
NETWORKS-A USE CASE
The refractive index (or index of refraction)
of a medium is the ratio of the speed of light in
vacuum to the speed of light in that medium.16
The higher the refractive index, the slower the
light travels through that medium. If a medium
has a refractive index of 2, the light will travel
through it at 1/2 the speed of light in vacuum.
The speed of light in vacuum is 3 108 m/s
and is generally denoted by c.17
Typical optical fibers are made of glass with
an index of refraction of 1.5. Therefore, the
speed of light in optical fiber is 2/3 of the
speed of light in vacuum or c/1.5. This means
that the speed of light in vacuum is 50% higher
than in optical fiber. This interesting and wellknown
fact gains critical importance in optical
wireless satellite networks. It gives them an
advantage over terrestrial optical fiber networks
in terms of latency or propagation delay when
the data communications take place over long
distances of more than 3000 km.7
A next-generation satellite network that may
come into existence by the mid to late 2020s will
be based on such an optical wireless satellite
network that is created by using laser ISLs in satellite
constellations like phase I of Starlink. Providing
low-latency communications over long
distances may turn out be the primary use case
of this next-generation satellite network. The
cost of creating and maintaining such a network
can easily be recovered by providing low latency
27
IEEE Consumer Electronics Magazine - November/December 2021

Table of Contents for the Digital Edition of IEEE Consumer Electronics Magazine - November/December 2021
