18-05-2012, 03:48 PM
Free-space Optical Communications
free space comunication.pdf (Size: 448.56 KB / Downloads: 171)
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 communication
systems. Links involving satellites, deep-space probes,
ground stations, unmanned aerial vehicles (UAVs), high altitude
platforms (HAPs), aircraft, and other nomadic communication
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 implementation
are amplitude modulation with direct detection and
phase modulation in combination with a (self-)homodyne or
heterodyne receiver.
The Optical Link Equation
The overall system performance of a link is quantified
using a link margin derived from the link equation. The optical
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 received
signal level is then compared with the sensitivity of
the receiver, thus giving the link margin.
Channel Without Atmospheric Disturbance
In this section optical links unaffected by the atmosphere
are discussed. In the basic free-space channel the optical
field generated at the transmitter propagates only with
an associated beam spreading loss. For this system the performance
can be determined directly from the power flow.
The signal power received PRx [W] depends on the transmit
power PTx [W], transmit antenna gain GTx, receive antenna
gain GRx, the range loss Gr, and system-dependent losses
Asystem;lin.
free space comunication.pdf (Size: 448.56 KB / Downloads: 171)
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 communication
systems. Links involving satellites, deep-space probes,
ground stations, unmanned aerial vehicles (UAVs), high altitude
platforms (HAPs), aircraft, and other nomadic communication
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 implementation
are amplitude modulation with direct detection and
phase modulation in combination with a (self-)homodyne or
heterodyne receiver.
The Optical Link Equation
The overall system performance of a link is quantified
using a link margin derived from the link equation. The optical
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 received
signal level is then compared with the sensitivity of
the receiver, thus giving the link margin.
Channel Without Atmospheric Disturbance
In this section optical links unaffected by the atmosphere
are discussed. In the basic free-space channel the optical
field generated at the transmitter propagates only with
an associated beam spreading loss. For this system the performance
can be determined directly from the power flow.
The signal power received PRx [W] depends on the transmit
power PTx [W], transmit antenna gain GTx, receive antenna
gain GRx, the range loss Gr, and system-dependent losses
Asystem;lin.