IEEE Consumer Electronics Magazine - November/December 2021 - 22

Communications Technology
space optics (FSO) or free space optical communications.
FSO can be further divided into four
link subtypes: terrestrial, nonterrestrial (or
aerial), space, and deep-space. Examples of terrestrial
FSO are building-to-building FSO links.
Nonterrestrial FSO links include ground-tounmanned
aerial vehicles (UAVs), ground-to-high
altitude platform systems (HAPSs), and HAPS-toHAPS.
Ground-to-satellite, satellite-to-ground,
and satellite-to-satellite FSO links are instances of
space FSO. Deep-space FSO can be an FSO link
between the Earth and a spacecraft in deep-space
like Galileo. Satellite-to-satellite FSO links, which
are also referred to as laser intersatellite links
(ISLs), are the focus of this article.
The first significant breakthrough that laid
the foundation for today's FSO came with the
discovery of the first working laser in 1960. The
first optical or laser ISL was demonstrated in
2008 between two low Earth orbit (LEO) satellites,
Terra SAR-X, and NFIRE. The capacity of
this laser ISL was 5.5 Gb/s, and it was established
at an intersatellite distance of 5500 km at a speed
of 25 000 km/h. A brief history of the developments
in FSO, mainly with respect to space, is
provided by Kaushal and Kaddoum.1
In an FSO link, an optical signal is transmitted
from an optical transmitter to an optical receiver
over the atmosphere or the vacuum in space.
Unobstructed line of sight is required between a
transmitter and receiver for FSO communications.
A basic FSO system is described by Mikolajczyk
et al.2 At the transmitter, the input data
are converted into a modulated electrical signal
that controls the intensity of light coming out of
the laser by controlling the laser current in the
driver. The telescope at the transmitter focuses
this modulated laser beam at the receiver's telescope.
This optical signal travels through the
medium and is received at the receiver's telescope.
The photodetector at the receiver converts
this optical signal to an electrical signal,
which is then demodulated to retrieve the transmitted
data. An optical filter at the receiver eliminates
the background solar radiation.
This article aims to provide an analysis of the
vital role that FSO is envisioned to play in nextgeneration
satellite networks. After introducing
the types of FSO links and functionalities of a
basic FSO system, a comparison of FSO and
22
radio frequency (RF) technologies for ISLs is provided,
including a comparison between FSO and
RF links for LEO satellites. Next, the types and
properties of FSO or laser ISLs are considered,
and the challenges in establishing laser ISLs and
the capabilities of terminals for laser ISLs are
explored. After that, the basic parameters of a
satellite constellation and various aspects of
SpaceX's upcoming mega-constellation Starlink
are highlighted. Then, the optical wireless satellite
network formed from employing laser ISLs is
examined. Finally, a use case for next-generation
optical wireless satellite networks as providers
of low-latency communications over long distances
is investigated.
Although the mentioned aspects have been
analyzed in the literature individually,1-7 to the
best of our knowledge, this is the first article to
analyze all these aspects of the next-generation
optical wireless satellite networks together at one
place within a framework. Furthermore, a use case
analyzing the suitability of next-generation optical
wireless satellite networks for low-latency communications
does not exist in the literature.
COMPARISON OF FSO AND RF FOR
INTERSATELLITE LINKS
Both FSO and RF signals are instances of electromagnetic
waves. RF is a well-established wireless
communications technology that is currently
in its fifth generation for mobile communications.8
However, FSO is emerging as a viable
alternative to RF.9 For point-to-point wireless
communication in space, especially satellite-tosatellite
links or ISLs, the benefits of an FSO link
compared to an RF link heavily outweigh the
drawbacks, and a comparison of the two is presented
in Table 1. 3 Typically, the beam spread or
beam divergence of an RF signal is 1000 times
greater than that of an FSO signal.10
Compared to RF links, higher frequency and
higher bandwidth of FSO links as well as the
characteristics of their laser beam present the
following significant advantages: higher data
rates; smaller antenna sizes leading to lesser
weight and lesser volume; narrower beams
resulting in no interference and higher security;
and lower transmit power due to lower beam
spread and higher directivity. Due to their
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