09-04-2014, 03:54 PM
An Introduction to Free-space Optical Communications
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Abstract
Over the last two decades free-space optical
communication (FSO) has become more and more interest-
ing as an adjunct or alternative to radio frequency commu-
nication. This article gives an overview of the challenges
a system designer has to consider while implementing an
FSO system. Typical gains and losses along the path from
the transmitter through the medium to the receiver are in-
troduced in this article. Detailed discussions of these topics
can be found in this special issue of the Radioengineering
Journal.
Introduction
Free-space optical communication (FSO) systems (in
space and inside the atmosphere) have developed in response
to a growing need for high-speed and tap-proof communica-
tion systems. Links involving satellites, deep-space probes,
ground stations, unmanned aerial vehicles (UAVs), high al-
titude platforms (HAPs), aircraft, and other nomadic com-
munication partners are of practical interest. Moreover, all
links can be used in both military and civilian contexts. FSO
is the next frontier for net-centric connectivity, as bandwidth,
spectrum and security issues favor its adoption as an adjunct
to radio frequency (RF) communications [1].
Discussion of Selected Modulation
Schemes
The optical carrier can be modulated in its frequency,
amplitude, phase, and polarization. The most commonly
used schemes because of their relatively simple implemen-
tation are amplitude modulation with direct detection and
phase modulation in combination with a (self-)homodyne or
heterodyne receiver.
The technically simplest digital modulation scheme is
amplitude-shift keying (2 ASK). In optical systems it is re-
ferred to as on-off keying (OOK). OOK is an intensity mod-
ulation scheme where the light source (carrier) is turned on
to transmit a logic ”one” and turned off to transmit a ”zero”.
In its simplest form this modulation scheme is called NRZ
(non-return-to-zero)-OOK. Besides NRZ also other codes
exist. The most common one besides NRZ is RZ (return-
to-zero) coding. The advantages of RZ compared to NRZ
are its higher sensitivity [3] and the fact that the clock fre-
quency lies within the modulation spectrum. Unfortunately,
both NRZ and RZ can lead to loss of clock synchronization
if long strings of ones or zeros are transmitted. This can be
avoided with other coding systems such as Manchester cod-
ing, which is related to RZ but amounts to state changes at
the beginning or in the middle of clock cycles - pulse po-
sition modulation. With such a variant of RZ the clock of
the digital signal can easily be recovered.
The Optical Link Equation
The overall system performance of a link is quantified
using a link margin derived from the link equation. The op-
tical link equation is analogous to the link equation for any
radio frequency (RF) communication link. Starting with the
transmit power the designer identifies all link degradations
and gains to determine the received signal level. The re-
ceived signal level is then compared with the sensitivity of
the receiver, thus giving the link margin.
Conclusion
A brief survey of the fundamentals of FSO has been
presented in this introductory article. This brief survey has
focused on outdoor FSO static optical links and describes
some basic models of the link and some simple models of
the atmosphere.
The advantages of FSO result from the basic charac-
teristics of a laser beam, especially from its high frequency,
coherency and low divergence