20-09-2009, 03:55 PM
Free Space Optics (FSO) communications, also called Optical Wireless (OW)
Bo Rong, Yi Qian, Member, IEEE, and Kejie Lu, Member, IEEE
Overview
Imagine a technology that offers unsurpassed reliability and
high-speed connectivity. A technology that can be installed globally is
easy to deploy, license-free and offers a fast, high ROI. That
technology is free-space optics (FSO).
Free Space Optics (FSO) communications, also called Optical
Wireless (OW) or Infrared Laser, refers to the transmission of
modulated visible or infrared (IR) beams through the atmosphere to
obtain optical communications. Like fiber, Free Space Optics (FSO)
uses lasers to transmit data, but instead of enclosing the data stream
in a glass fiber, it is transmitted through the air. Free Space Optics
(FSO) works on the same basic principle as Infrared television
remote controls, wireless keyboards or IRDA ports on laptops or
cellular phones
Speed of fiber flexibility of wireless
Optical wireless, based on FSO-technology, is an optical
technology that provides the speed of fiber, with the flexibility of
wireless. It enables optical transmission at speeds of up to 2.5 Gbps
and, in the future, 10 Gbps using WDM. This is not possible with any
fixed wireless/RF technology today. Optical wireless also obviates the
need to buy expensive spectrum, which further distinguishes it from
fixed wireless technologies. Moreover, its narrow beam transmission
is typically two meters versus 20 meters and more for traditional,
even newer radio-based technologies such as millimeter-wave radio.
Optical wireless products similarities with conventional optical
solutions enable the seamless integration of access networks with
optical core networks and help to realize the vision of an all-optical
network.
FSO: Wireless Links at the Speed of Light
Unlike radio and microwave systems, Free Space Optics
(FSO) is an optical technology and no spectrum licensing or
frequency coordination with other users is required, interference from
or to other systems or equipment is not a concern, and the point-topoint
laser signal is extremely difficult to intercept, and therefore
secure. Data rates comparable to optical fiber transmission can be
carried by Free Space Optics (FSO) systems with very low error
rates, while the extremely narrow laser beam widths ensure that there
is almost no practical limit to the number of separate Free Space
Optics (FSO) links that can be installed in a given location.
History
Historically, Free Space Optics (FSO) or optical wireless
communications was first demonstrated by Alexander Graham Bell in
the late nineteenth century (prior to his demonstration of the
telephone!).Free Space Optics (FSO) experiment converted voice
sounds into telephone signals and transmitted them between
receivers through free air space along a beam of light for a distance
of some 600 feet.
Although photophone never became a commercial reality, it
demonstrated the basic principle of optical communications.
Essentially all of the engineering of today, Free Space Optics (FSO)
or free space optical communications systems was done over the
past 40 years or so, mostly for defense application
Originally developed by the military and NASA, FSO has been
used for more than 30 years to provide fast communication links in
remote locations. Cable-Free has extensive experience in this area:
its experts were in the labs developing prototype FSO systems in
Europe mid â„¢90s, before others had even started to think about the
technology.
While fiber-optic communications has gained acceptance in the
telecommunications industry, FSO communications is still relatively
new. FSO technology enables bandwidth transmission capabilities
that are similar to fiber optics, using similar optical transmitters and
receivers and even enabling WDM-like technologies to operate
through free space.
TECHNOLOGY
The technology behind Free Space Optics (FSO) technology
FSO technology requires light, which can be focused by using
either light emitting diodes (LEDs) or lasers (light amplification by
stimulated emission of radiation). The use of lasers is a simple
concept similar to optical transmissions using fiber-optic cables; the
only difference is the medium. Light travels through air faster than it
does through glass, so it is fair to classify FSO technology as optical
communications at the speed of light.
FSO technology is surprisingly simple. Itâ„¢s based on connectivity
between FSO-based optical wireless units, each consisting of an
optical transceiver with a laser transmitter and a receiver to provide
full-duplex (bi-directional) capability. Each optical wireless unit uses a
high-power optical source (i.e. laser), plus a lens or telescope that
transmits light through the atmosphere to another lens receiving the
information. At this point, the receiving lens or telescope connects to
a high-sensitivity receiver via optical fiber
FSO transmits invisible, eye-safe light beams from one
telescope to other using low power infrared lasers in the terahertz
spectrum. The beams of light in Free Space Optics (FSO) systems
are transmitted by laser light focused on highly sensitive photon
detector receivers. These receivers are telescopic lenses able to
collect the photon stream and transmit digital data containing a mix of
Internet messages, video images, radio signals or computer files.
Commercially available systems offer capacities in the range of 100
Mbps to 2.5 Gbps, and demonstration systems report data rates as
high as 160 Gbps.
FSO systems can function over distances of several kilometers.
As long as there is a clear line of sight between the source and the
destination, and enough transmitter power, Free Space Optics (FSO)
communication is possible.
The FSO Laser technology
Laser (Light Amplification by Stimulated Emission of Radiation)
generates light, either visible or infrared, through a process known as
stimulated emission. To understand stimulated emission,
understanding two basic concepts is necessary. The first is
absorption which occurs when an atom absorbs energy or photons.
The second is emission which occurs when an atom emits photons.
Emission occurs when an atom is in an excited or high energy state
and returns to a stable or ground state when this occurs naturally it is
called spontaneous emission because no outside trigger is required.
Stimulated emission occurs when an already excited atom is
bombarded by yet another photon causing it to release that photon
along with the photon which previously excited it. Photons are
particles, or more properly quanta, of light and a light beam is made
up of what can be thought of as a stream of photons.
A basic laser uses a mirrored chamber or cavity to reflect light
waves so they reinforce each other. An excitable substance gas,
liquid, or solid like the original ruby laser is contained within the cavity
and determines the wavelength of the resulting laser beam. Through
a process called pumping, energy is introduced to the cavity exciting
the atoms within and causing a population inversion. A population
inversion is when there are more excited atoms than grounded atoms
which then leads to stimulated emission. The released photons
oscillate back and forth between the mirrors of the cavity, building
energy and causing other atoms to release more photons. One of the
mirrors allows some of the released photons to escape the cavity
resulting in a laser beam emitting from one end of the cavity.
Laser Safety:
Laser communications systems can be designed to be eye-safe,
which means that they pose no danger to people who might happen
to encounter the communications beam. Laser eye safety is classified
by the International Electro-technical Commission (IEC), which is the
international standards body for all fields of electro-technology.
However, that the eye safe limits vary with wavelength Terrestrial
Laser Communications
ADVANTAGES
Applications of Free Space Optics (FSO) technology
A number of advantages of using Free-Space-Optics (FSO) has
been purposed, such as:
1. Enterprise Network
2. Service Providers
3. Broadcast & Cctv
Enterprise Network
A. Campus connectivity solution
B. Disaster recovery and emergency services:
A. Cable-free solves Campus Connectivity Problems:
A number of separate buildings, separated by roads or other
obstacles, between which communications links are frequently in
demand. Traditional connectivity solutions for links between buildings
include leased-line, fiber-optic, copper cable or microwave links but
all of these have associated penalties.
Cable-free systems use optical technology to offer a number of
advantages:
_ No site or frequency licenses unlike radio or microwave
systems
_ No disruption from trench digging - roads, rivers or railways are
no obstacle
_ No risk of interference, signal reflections or airport radar
systems
_ No leased-line rental or connection charges
_ No limitation on bandwidth up to 1500Mbps with current
technology Quick to install
_ On-site upgrades ensure future-proof network provision
_ Permanent or temporary use, medium-term leasing options also
available
_ Cost-effective solution
B. Cable-free connections for Disaster Recovery and Emergency
services:
Natural disasters, terrorist attacks and emergency situations are
by their nature unpredictable and require flexible and innovative
responses.
Mobile control centres are often brought in, with vehicles that
need to be connected to a number of other centres, observation
points or services, but local infrastructure may be damaged,
inadequate or unreliable.
In urban environments, Microwave or radio solutions require
licensing and often suffer from interference, multipath and signal
reflections and public health fears in urban areas are voiced with
increasing frequency.
Computer and telecommunications networks may need to be set
up in a short space of time in Disaster Recovery and Emergency
situations, and operators cannot depend on local telecommunications
providers to help especially with response teams that may be sent a
considerable distance to operate.
Cable-free offers state-of-the-art communications systems ideally
suited for rapid provision of network connections, with industrystandard
data interfaces from 2Mbps to 622Mbps. Installations are
quick and costs competitive with limited-bandwidth radio and
microwave solutions. With no need for frequency allocation or
licensing, Cable-free can be deployed to expand networks rapidly
anywhere in the world.
Cable-free offers an innovative solution offering full-bandwidth
network connectivity:
_ Data rates up to 622Mbps
_ Line-of-sight connections in excess of 2,000m
_ No site or frequency licenses required
_ No leased-line rental or connection charges
_ Trench digging and long runs of fiber-optic or copper cable are
not needed
_ No risk of interference, signal reflections or airport radar
systems
_ No ignition hazard in flammable environments
_ Suited for permanent or temporary use
_ Plug-in modules allow on-site upgradeability of data bandwidth
or format
_ Cost-effective, safe and reliable solution
Service Providers
A. Cable-free solves network problem
B. virtual point to multi point
A. Cable-free solves network problems:
Telecommunication operators have made huge investment in
fixed infrastructure, providing coverage of urban and industrial areas
with voice, data and video services connected usually from a
backbone SDH fiber network using copper, fiber or wireless
technologies.
Cable digging, increasingly unpopular in cities, is regulated by
the local authority that may restrict re-digging frequency of roads -
and the cost may be prohibitive in any case, especially if a river or
railway is in the way. Using copper telephone lines only provides
2Mbps access per pair - too slow for power users requesting Ethernet
at 10/100Mbps or ATM services at 155 or 622Mbps. Cable Modems
and xDSL are often not reliable on all installations, depending on the
age and condition of the copper cables.
Point-to-point Microwave or radio solutions for Ëœwireless local
loopâ„¢ may require licensing and often suffer from interference,
multipath and signal reflections - and public health fears in urban
areas are voiced with increasing frequency.
Cable-free offers an innovative solution offering full-bandwidth
connectivity:
_ Data rates up to 622Mbps
_ Line-of-sight connections in excess of 1,000m
_ No site or frequency licenses required
_ No leased-line rental or connection charges
_ Trench digging and long runs of fiber-optic or copper cable are
not needed
_ No risk of interference, signal reflections or airport radar
systems
_ No ignition hazard in flammable environments
_ Suited for permanent or temporary use
_ Plug-in modules allow on-site upgradeability of data bandwidth
or format
_ Cost-effective, safe and reliable solution
B. Cable-free Virtual-Point-to-Multipoint:
Cable-free V-PMP systems enable implementation of Licensefree
Local Multipoint Distribution Service (LMDS) networks using
Optical Wireless technology. Services using LMDS technology
include high-speed Internet access, real-time multimedia file transfer,
corporate local area network extensions, interactive video, video-on-
demand, video conferencing, and telephony amongst many other
potential applications.
Cable-free Solutions has applied its proven expertise in Optical
Wireless to create License-free Next-Generation LMDS solutions
using Virtual-Point-to-Multipoint technology. Key benefits of this
technology are:
_ License-free operation
_ Non-shared bandwidth for each subscriber
_ High symmetric bandwidths up to 1.5Gbps per subscriber
_ Rapid network deployment and fast new-user connection
_ Low cost of start-up
_ No frequency planning
_ Data Security against interference and interception
_ Redundant 1+1 connection options including equipment and
path diversity
Cable-free V-PMP combines Point-to-Point Optical Wireless
technology and Intelligent Routers to create high-performance cellular
networks offering high symmetrical bandwidths up to 1.5Gbps to endusers
within the coverage area.
Broadcast & Cctv
A. Outside broadcast application
B. satellite uplink connections
A. Outside Broadcast Applications:
Outside Broadcast and Television News reporting demand
innovative solutions to communications problems and with todays alldigital
environments, high integrity feeds and very high data rates
such as 270Mbps SDI are often required.
Connecting remote cameras to OB vans, mixing/edit facilities or
hopping feeds between difficult-to-access areas and tall buildings
may not be suited to traditional solutions such as leased-line, fiberoptic,
copper cable or microwave links all of which have associated
penalties.
With Cable-free many of these problems are solved:
_ Microwave links are limited to 34Mbps, Cable-free easily
achieves 270Mbps at 2,000m
_ Copper 270Mbps SDI connections limited to 300m, Cable-free
offers 2,000m
_ High quality analogue Composite Video option offers 9MHz with
67dB SNR
_ No site or frequency licenses required
_ No leased-line rental or connection charges
_ Long runs of fragile fiber-optic and expensive Tri-axial cable are
not needed
_ Deployment time is reduced compared to running long fiber or
copper cables
_ No risk of interference, signal reflections or airport radar
systems
_ No ignition hazard in flammable environments
_ Mains or +12V battery power options
_ Low manpower costs and rapid response capability
_ Suited for permanent or temporary use, leasing options
available
_ Cost-effective solution
B. Cable-free connects to Satellite Uplinks:
Satellite uplinks and downlinks are frequently used for
telecommunications and broadcast television applications where
temporary connection has to be made such as television news
reporting and coverage of live sporting event.
In such applications the uplink may be a suitably equipped vehicle
with line-of-sight coverage of the satellite orbit, or a permanently
installed antenna which cannot be moved. The terrestrial problem is
then connecting the source to the uplink terminal, and the downlink
terminal to the recipient this is traditionally performed with leased-line,
fiber-optic, copper cable or microwave links all of which have
associated penalties.
Cable-free offers a new, high-performance alternative for the
terrestrial links:
_ Cable-free offers 622Mbps, compared with Microwave links
typically 34Mbps
_ Copper 270Mbps SDI connections limited to 300m, Cablefree
offers 2,000m
_ No site or frequency licenses required
_ No leased-line rental or connection charges
_ Long runs of fragile fiber-optic and expensive Tri-axial video
cable are not needed
_ Deployment time is reduced compared to running long fiber or
copper cables
_ No risk of interference, signal reflections or airport radar
systems
_ No ignition hazard in flammable environments
_ Mains or +12V battery power options
_ Low manpower costs and rapid response capability
_ Suited for permanent or temporary use, leasing options
available
_ Cost-effective solution
Cable-Free links to Satellite Television feeds:
High-quality television signals need to be relayed from a camera to
the satellite up-link, often when high-bandwidth network connections
are unavailable locally or will take too long to organize.
Cable-free offers an ideal solution which carries high-quality
analogue or uncompressed digital pictures with a system that can be
rapidly deployed on tripods and powered from 12-volt batteries. With
no need for frequency allocation or licensing, Cable-free can be
deployed anywhere in the world and even used in war zones with no
risk of interference with military communication systems.
HOW FREE SPACE OPTICS (FSO) BENEFITS
FSO is free from licensing and regulation which translates into
ease, speed and low cost of deployment. Since Free Space Optics
(FSO) transceivers can transmit and receive through windows, it is
possible to mount Free Space Optics (FSO) systems inside buildings,
reducing the need to compete for roof space, simplifying wiring and
cabling, and permitting Free Space Optics (FSO) equipment to
operate in a very favorable environment. The only essential
requirement for Free Space Optics (FSO) or optical wireless
transmission is line of sight between the two ends of the link.
Why FSO?
The global telecommunications network has seen massive
expansion over the last few years. First came the tremendous growth
of the optical fiber long-haul, wide-area network (WAN), followed by a
more recent emphasis on metropolitan area networks (MANs). In
order for this tremendous network capacity to be exploited, and for
the users to be able to utilize the broad array of new services
becoming available, network designers must provide a flexible and
cost-effective means for the users to access the telecommunications
network. Presently, however, most local loop network connections
are limited to 1.5 Mbps. As a consequence, there is a strong need for
a high-bandwidth bridge between the LANs and the MANs or WANs.
FSO systems represent one of the most promising approaches
for addressing the emerging broadband access market. Free Space
Optics (FSO) systems offer many features, principal among them
being low start-up and operational costs, rapid deployment, and high
fiber-like bandwidths due to the optical nature of the technology.
CHARACTERISTICS
Performance
Free Space Optics (FSO) products performance can be
characterized by four main parameters (for a given data rate):
1. Total transmitted power:
High transmitted power may be achieved by using erbium doped
fiber amplifiers, or by non-coherently combining multiple lower cost
semiconductor lasers.
2. Transmitting beamwidth:
Narrow transmitting beamwidth can be achieved on a limited
basis for fixed-pointed units, with the minimum beamwidth large
enough to accommodate building sway and wind loading.
3. Receiving optics collecting area:
Larger receiving optics captures a larger fraction of the total
transmitted power, up to terminal cost, volume and weight limitations.
This allow links to travel over longer distance, penetrate lower
visibility fog, or both.
4. Receiver sensitivity:
Free Space Optics (FSO) receivers must be designed to be
tolerant to scintillation, i.e. have rapid response to changing signal
levels and high dynamic range in the front end, so that the
fluctuations can be removed in the later stage limiting amplifier or
AGC.
Reliability
Every customer wants to know the expected failure rate of the
equipment they are investing in, for outdoor or industrial applications
the ruggedness of a system becomes even more important. A system
can be engineered and designed for exceptional reliability.
Engineering a product for long-life includes selecting top-quality,
long-life components from reliable vendors. Telecom grade
components are preferred, as are low-stress electronics. The system
must also be designed to maintain an optimum operating
environment for the selected components and sub-systems. A
rugged, environmentally-sealed housing is the first defence of a
system against the elements. Appropriate heating and cooling
mechanisms should be also in place in order to maintain optimum
temperature and humidity within the device. In addition, a system
design that incorporates a mechanism for reducing laser power
during clear weather will extend the life of the laser drivers and the
product itself. Active cooling of each laser will further enhance the
lifespan of these relatively expensive sub-systems. If these
considerations are taken into account, the system should have an
impressive MTBF (mean time before failure).
Figure Of Merit (FOM):
A figure of merit (FOM) can be used to compare competing
systems, based on
the basic physics of this equation:
Figure of Merit = (Power*Diameter2)/(Divergence2*Sensitivity); where
Power = Laser power in milliwatts
Diameter = effective diameter in cm (excluding any obscuration
losses)
Divergence = beam divergence in millirad
Sensitivity = receiver sensitivity in nanowatts
Cost
While cost is always a consideration when procuring telecom
products, many buyers are interested in obtaining the best value
proposition in the medium to low cost range. For example, higher
performance, with little extra cost penalty, often provides the best
value. The key factors that affect cost are system design (i.e. choice
of components and their configuration), minimization of manual labor
(especially for optical alignment), and volume manufacturing to
reduce procurement costs and amortize non-recurring costs
Wavelength
Currently available Free Space Optics (FSO) hardware can be
classified into two categories depending on the operating wavelength:
systems that operate near 800 nm and those that operate near 1550
nm. Each vendor manages to make huge claims that their own
chosen wavelength is best. But as we point out below, itâ„¢s actually
real world page link margin, not market-eering that matters.
Contrary to claims, there are only a few compelling reasons for
selecting 1550 nm Free Space Optics (FSO) systems, and many
against. One argument in favour is about laser eye safety, but ignores
the effect of increased transmit aperture used by a competing 980nm
solution from a vendor such as CableFree. There is reduced solar
background radiation at 1550nm, but the receiver devices are much
less sensitive than the enhanced silicon at 800-980nm, completely
negating any advantage. The true argument for 1550nm is about the
existence of EDFAs, optical amplifiers which can boost transmit
signals to whole watts of power, and the existence of DWDM
components which enable multi-channel multi-gigabit systems for the
future. But the cost penalty associated with 1550nm makes it
inappropriate.
Security
The common perception of wireless is that it offers less security
than wireline connections. In fact, Free Space Optics (FSO) is far
more secure than RF or other wireless-based transmission
technologies for several reasons:
1. FSO laser beams cannot be detected with spectrum analyzers or
RF meters
2. FSO laser transmissions are optical and travel along a line of sight
path that cannot be intercepted easily. It requires a matching Free
Space Optics (FSO) transceiver carefully aligned to complete the
transmission. Interception is very difficult and extremely unlikely
3. Data can be transmitted over an encrypted connection adding to
the degree of security available in Free Space Optics (FSO) network
transmissions.
CHALLENGES
The advantages of free space optical wireless or Free Space
Optics (FSO) do not come without some cost. When light is
transmitted through optical fiber, transmission integrity is quite
predictable; barring unforseen events such as backhoes or animal
interference. When light is transmitted through the air, as with Free
Space Optics (FSO) optical wireless systems, it must contend with a
a complex and not always quantifiable subject - the atmosphere.
Attenuation, Fog:
Fog substantially attenuates visible radiation, and it has a similar
affect on the near-infrared wavelengths that are employed in Free
Space Optics (FSO) systems. Similar to the case of rain attenuation
with RF wireless, fog attenuation is not a show-stopper for Free
Space Optics (FSO) optical wireless, because the optical page link can be
engineered such that, for a large fraction of the time, an acceptable
power will be received even in the presence of heavy fog.
Physical Obstructions:
Free Space Optics (FSO) products which have widely spaced
redundant transmitters and large receive optics will all but eliminate
interference concerns from objects such as birds. On a typical day,
an object covering 98% of the receive aperture and all but 1
transmitter; will not cause an Free Space Optics (FSO) page link to drop
out. Thus birds are unlikely to have any impact on Free Space Optics
(FSO) transmission.
Pointing Stability, Building Sway, Tower Movement
Only wide-beamwidth fixed pointed Free Space Optics (FSO)
systems are capable of handling the vast majority of movement found
in deployments on buildings. Narrow beam systems are unreliable,
requiring manual re-alignment on a regular basis, due to building
movement. ËœWide beamâ„¢ means more than 5milliradians. Narrow
systems (1-2mRad) are not reliable without a tracking system
The combination of effective beam divergence and a well
matched receive Field-of-View (FOV) provide for an extremely robust
fixed pointed Free Space Optics (FSO) system suitable for most
deployments. Fixed-pointed Free Space Optics (FSO) systems are
generally preferred over actively-tracked Free Space Optics (FSO)
systems due to their lower cost.
Scintillation:
Performance of many Free Space Optics (FSO) optical wireless
systems is adversely affected by scintillation on bright sunny days.
Some optical wireless products have a unique combination of large
aperture receiver, widely spaced transmitters, finely tuned receive
filtering, and automatic gain control characteristics. In addition,
certain optical wireless systems also apply a clock recovery phaselock-
loop time constant that all but eliminate the affects of
atmospheric scintillation and jitter transference.
Solar Interference:
Solar interference in Free Space Optics (FSO) free space optical
systems can be combated in two ways. Optical narrowband filter
proceeding the receive detector used to filter all but the wavelength
actually used for intersystem communications. To handle off-axis
solar energy, sophisticated spatial filters have been implemented in
CableFree systems, allowing them to operate unaffected by solar
interference that is more than 1 degree off-axis.