What is Free Space Optics (FSO) ?
History of Free Space Optics
Basic link designs.
Advantages and disadvantages of links
Advanced approaches towards link performance improvement
Techniques based on diversity
Adaptive signal processing in a nondirected los link
Performance Limited Effects.
FSO advantages and Disadvantages.
Free Space Optics communications, also called Free Space Photonics (FSP) or
Optical Wireless, refers to the transmission of visible and infrared (IR) beams
through the atmosphere to obtain optical communications.
Like fiber, Free Space Optics uses lasers and LED to transmit data, but instead
of enclosing the data stream in a glass fiber, it is transmitted through the air
Free space optical communication is an effective means of communication
at high bit rates over short distances.
Wireless optical holds the promise of delivering data rates that can meet the
• In the late nineteenth century, Alexander Graham Bell’s "photo-phone”.
• Late 1950‘s~early 1960's, several scientists theorized and developed
• In the mid-1960's NASA initiated experiments to utilize the laser as a
means of communication between the Goddard Space Flight Center and the
Gemini-7 orbiting space capsule.
• In the early 1980's United States and its military research.
• Germany, France and Japan made significant advancements in free space
optics for satellite communications.
The safety standard recommends that these systems should be located
where the beam cannot be interrupted or viewed inadvertently by a
person or receiver.
The electrical signal is converted to optical power and
transmitted through the air.
After undergoing the influences of the time-dispersive channel
and ambient light, the optical signal is directly translated into a
photocurrent at the detector.
The electrical SNR in optical links depends on the square of
the optical power, which has a deep impact on both design and
performance of OW systems.
One or more laser diodes (LD) or light emitting diodes (LED) are used. The
choice between LED and LD is determined by standard factors that influence price
and performance as known from traditional optical communications
An optical concentrator ( collect and concentrate incoming radiation) and an optical
filter (to reject ambient light), a photo detector (PD, to convert radiation/ optical
power into a photocurrent), and an electrical front-end (performing amplification,
PROPERTIES OF DETECTORS
Large area, low-capacitance Si Photo Diodes(PD) with peak
responsivity in the near IR window are available at low-cost.
PIN Photo Diodes are cheaper but limit the link range due to
their poor sensitivity.
APDs have a better sensitivity than PINs and offer link
performance when the ambient light is weak.
BASIC LINK DESIGNS
There are two basic link designs:
Directed line-of-sight (LOS) :
The beam of a directed link travels from a narrow-beam Tx to a
narrow field-of-view (FOV) receiver(Rx) via the LOS.
Communication between a wide-beam Tx and a wide FOV Rx relies
on numerous signal reflections off the surfaces in the room, instead
of a LOS.
ADVANTAGES AND DISADVANTAGES
OF TWO LINKS
Directed LOS experiences minimal effects of multipath
dispersion, noise and path loss but its disadvantage is
Diffused link is simpler, more robust to shadowing and offers
better mobility than the directed one (no tracking and pointing
is needed) but its speed is less due to multiple reflections.
ADVANCED APPROACHES TOWARDS
LINK PERFORMANCE IMPROVEMENT
It is often desired to combine the mobility of diffuse and the
high speed capability of LOS systems. For this techniques are:
Techniques based on diversity.
Adaptive signal processing in a non-directed LOS link.
TECHNIQUES BASED ON DIVERSITY
Techniques based on diversity is called multi-spot
Tx Sends multiple moderate-width or collimated beams to
ceiling, where they are diffusely reflected to an angle
A way to secure the directed link against connection
Here multiple LOS occur between the diffusing spots and
SIGNAL PROCESSING IN A NON
DIRECTED LOS LINK
• Here LOS and diffuse signal components are simultaneously
present at the receiver.
• These systems exploit the LOS for transmission speed and
reflections for good coverage.
• Here it is difficult to sufficiently maintain the SNR for high bit
rates. So it makes sense to have rate-adaptive links.
• Free space loss due to unguided media
• Clear air absorption
• Inter-Symbol-Interference (ISI)
FREE SPACE LOSS
Free space loss defines the proportion of optical power
arriving at the receiver that is usually captured within the
receiver’s aperture. A typical value for a point-to-point system
that operates with a slight divergent beam would b20 dB.
• It is equivalent of absorption loss in optical fibers.
• It is a wavelength sensitive process that gives rise to low-
loss transmission windows centered on 850nm, 1310nm, and
550nm, essentially the same as that in optical fiber.
This is due to rain, fog, mist, and snow. This attenuation is
continually in the state of flux.
Atmospheric scattering can be Rayleigh due to molecular sized
particles. Mie scattering is due to larger sized particles such as
smoke and fog.
Rayleigh scattering is much less than Mie scattering.
1. Unregulated Spectrum Leads to virtually unlimited use of
spectrum by individual networks.
2. Huge Bandwidth Great Support for high-speed
3. No Strict Laws License-free operation.
4. Optoelectronic Leads to manufacturing
Technology inexpensive components and little
5. Less Interference Facilitates system design and
results in a significant cost savings.
6. Fading Immunity Results in less power loss to
7. Reusability Enables use of same
communication equipments and
wavelengths by nearby systems.
8. Low Power Consumption Leads to less energy requirements
High Launch Power represents eye hazard.
Light Interference negatively affects system performance.
Blockage Leads to design challenges.
Low Power Source requires high sensitive receivers.
Signal Scattering results in multipath impairment.
Alignment Leads to more operation constraints.
FSO laser beams cannot be detected with spectrum analyzers
or RF meters.
FSO laser transmissions are optical and travel along a line of
sight path that cannot be intercepted easily.
The laser beams generated by FSO systems are narrow and
invisible, making them harder to find and even harder to intercept
Data can be transmitted over an encrypted connection adding
to the degree of security available in FSO network transmissions.
For future short-range applications, optical wireless
communications present a viable and promising supplemental
technology to radio wireless systems.
Recent advances in the performance of optical links (in terms
of both transmission speed and spatial coverage) are based
above all on efficient digital signal processing techniques.
 Transparent optical networks, 2007. Icton '07, 9th international conference
 Infrared Data Association: http://www.irda.org.
 O. Bouchet et al.: Free-space optics: Propagation and communication, London: ISTE
 S.T. Jivkova, M. Kavehrad: Multispot diffusing configuration for wireless infrared
access, IEEE Trans. On Communications.
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