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By David Rockwell and G. Stephen Mecherle, fSONA Communications Corp.
Originally Published: June, 2001
By: Lightwave
Access Solutions
Optical free-space wireless solutions provide fast,
low-cost, high-bandwidth access for bridging the last mile.
The global telecommunications network has seen massive
expansion over the last few years, catalyzed by the
telecommunications deregulation in 1996. First came the
tremendous growth of the long-haul WAN, followed by a more
recent emphasis on metropolitan-area networks (MANs).
Meanwhile, LANs and Gigabit Ethernet ports are being
deployed with a comparable growth rate. To exploit this
tremendous capacity and provide users with the broad array
of new services becoming available, network designers must
provide a flexible and cost-effective means of accessing the
telecom network. Currently, however, most local-loop
connections are limited to a T1 line (1.5 Mbits/sec) and, as
a consequence, there is a strong need for a high-bandwidth
bridge between the LANs and MANs or WANs, known as "last
mile" or "first mile."
Optical wireless systems represent one of the most promising
approaches for addressing the emerging broadband access
market and its "last mile" bottleneck. These robust systems,
which establish communication links by transmitting laser
beams directly through the atmosphere, have matured to the
point where mass-produced models are now available. Optical
wireless systems offer many features, principal among them
being low startup and operational costs, rapid deployment,
and high fiber-like bandwidths. Available systems offer
capacities in the range of 100 Mbits/sec to 2.5 Gbits/sec
and demonstration systems report data rates as high as 160
Gbits/sec.
Ideal access attributes
To establish a metric for assessing the attractiveness of
optical wireless, some ideal attributes of a generic
broadband access approach must be considered. One of the
most significant attributes is cost, which includes low
installation cost as well as cost per bit associated with
each subscriber. Low first-in cost, or the cost of launching
access service for the first few subscribers, is also
desirable.
The ideal broadband access approach should also offer rapid
deployment to enable carriers to generate revenue as quickly
as possible. Another important attribute is the capability
to provide a high-bandwidth capacity to each subscriber,
thereby enabling multiple services to be utilized. Moreover,
this capacity should be easily scalable, not only in overall
bandwidth, but also in the total number of subscribers using
the access equipment. The ideal access approach should be
available a high percentage of the time (up to 99.999%
availability) and able to propagate data over relatively
long distances.
Each broadband access approach offers a "zone of advantage."
That is, each approach offers a more optimal performance for
certain specific applications and deployment strategies.
However, no single approach provides all the attributes
listed here. For example, although dedicated fibers offer
massive capacity, they are expensive and carriers miss many
months of potential subscriber revenues while waiting for
fibers to be deployed.
Fiber-based passive optical networks (PONs) represent a
highly attractive approach due to the relatively low cost
per subscriber. However, the inherent high capacity of
fibers is shared among a number of users, thereby reducing
the capacity per user. Also, deployment times can still be
quite long, depending on the location of any particular
subscriber. Radio-frequency (RF) fixed wireless systems are
a credible access option but are limited in data rate,
require Federal Communications Commission licensing, are
subject to rain fading, and are costly relative to other
access schemes.
The logical consequence of this situation is that one must
select an approach that best meets the needs of a specific
deployment. Optical wireless represents an approach with
wide and broadly based applications appeal because of its
many features. Optical wireless complements both RF and
wireline networks, providing fiber-like capacity at data
rates up to 2.5 Gbits/sec and more, with a cost per bit/sec
that is among the lowest available-about $4 per Mbit/sec per
month.
Given that no spectrum license is required, startup costs
are significantly lower than for RF wireless. Optical
wireless systems can be rapidly deployed. Once a suitable
line of sight is identified, a point-to-point link typically
can be installed and brought to operational status in
approximately one hour or less. Well-engineered optical
wireless links, which properly account for the statistical
occurrence of fog, can achieve an availability of 99.9%, or
even full carrier-class availability of 99.999% with a
(lower-capacity) RF link or DSL backup. Finally, optical
wireless systems are "network-friendly" in that they can be:
Engineered to be
protocol-independent.
Implemented in cellular or mesh architectures as well as
point-to-point links. Sent from roof-top to roof-top or
through office windows.
Designed to be compatible with common monitoring protocols
to ensure the highest level of successful implementation.
Redeployed to a different subscriber location if desired,
for example, if an existing subscriber no longer requires an
optical wireless connection due to receiving a direct
fiber-optic connection.
Given these attributes, it becomes clear why optical
wireless systems are being implemented in a broad range of
applications and markets, including local-exchange carriers
(LECs), Internet service providers (ISPs), network service
integrators, and businesses ranging in size from small ISPs
to major carriers and incumbent local-exchange carriers
(ILECs). Market-analysis studies predict healthy growth of
optical wireless systems, with annual growth rates in the
range of 80% to 90% and projected total global sales of more
than $2 billion in 2005.
Cost factor
Because cost is such an important factor in the broadband
access market segment, a cost comparison of optical wireless
and a number of established broadband access technologies
are summarized in the Table. The final figure of merit
defined for this comparison is cost per Mbit/sec per month.
Optical wireless, at $4 per Mbit/sec per month, is half as
expensive as the next lowest-cost alternative.
The most expensive technologies cost more than 80 times as
much as optical wireless. This cost advantage arises from
the combination of high fiber-like data rates and a low
implementation cost. The cost advantage is so compelling
that the individual numbers in the Table can vary somewhat
without affecting the conclusion that optical wireless is
the lowest-cost access approach. An even more compelling
case for optical wireless cost-effectiveness applies for
higher data rates of 622 Mbits/sec and up.
Engineering maturity
The engineering maturity of optical wireless is often
underestimated, due to a misunderstanding of how long such
systems have been under development. Historically, optical
wireless was first demonstrated by Alexander Graham Bell in
1880. Nevertheless, essentially all the engineering for
these systems was done over the past 40 years or so, mostly
for defense applications. By addressing the principal
engineering challenges, this aerospace/defense activity
established a strong foundation on which today's commercial
optical wireless systems are based.
First, these systems can be designed to be eye-safe, posing
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
electrotechnology. While the IEC is an advisory agency, its
guidelines are adopted by the regulatory agencies in most of
the world's countries.
A laser transmitter that is completely safe when viewed by
the unaided eye is designated IEC Class 1M. In the United
States, laser eye safety is controlled by the Center for
Devices and Radiological Health (CDRH), a division of the
Food and Drug Administration (FDA). Currently, the CDRH is
in the process of adopting the safety classifications of the
IEC.
Note, however, that the eye safety limits vary with
wavelength. The optical wireless hardware currently on the
market can be classified into two broad categories: systems
that operate at a wavelength near 800 nm and those that
operate near 1,550 nm. Laser beams at 800 nm are near
infrared and therefore invisible. Yet, like visible
wavelengths, the light passes through the cornea and lens
and is focused onto a tiny spot on the retina. That is shown
in Figure 1a, which applies for visible and near-infrared
wavelengths in the range of 400 to 1,400 nm.
The collimated light beam entering the eye in this
retinal-hazard wavelength region is concentrated by a factor
of 100,000 times when it strikes the retina. Because the
retina has no pain sensors, and the invisible light does not
induce a blink reflex, the retina could be permanently
damaged by some commercially available optical wireless
products operating at 800 nm before the victim is aware that
hazardous illumination has occurred.
In contrast, Figure 1b shows that laser beams at wavelengths
greater than 1,400 nm are absorbed by the cornea and lens
and do not focus on the retina. Because of these biophysical
properties of the eye, wavelengths greater than 1,400 nm are
allowed approximately 50 times greater intensities than
wavelengths near 800 nm. This fact can be exploited by
specifying a wavelength in the 1,550-nm range, where the
factor of 50 additional laser power allows the system to
propagate over longer distances and/or support higher data
rates.
A second example of the engineering maturity of optical
wireless systems is how well they address the challenges
associated with fog. As is well known from common
experience, fog substantially attenuates visible radiation,
and it has a similar affect on the near-infrared wavelengths
employed in optical wireless systems. Note that the effect
of fog on optical wireless radiation is entirely analogous
to the attenuation-and fades-suffered by RF wireless systems
due to rainfall. Similar to the case of rain attenuation
with RF wireless, fog attenuation is not a "show-stopper"
for optical wireless because the optical link can be
engineered such that, for a large fraction of the time, a
sufficient power level will be received even in the presence
of heavy fog.
This important element of link engineering begins with the
collection of fog-statistics data, which show what
percentage of the time the fog attenuation will be greater
than a certain value. Then, the fog statistics for the
subscriber's location are used to determine how much fog
attenuation (in dB/km) must be accommodated to guarantee a
given value of availability, such as 99.9%. Next, the link
design calculations are consulted to determine how much link
margin is allocated to fog attenuation. Finally, the maximum
link length is calculated according to the simple equation:
Length (km) = Link margin (dB) / fog attenuation (dB/km)
When this simple analysis is applied to actual deployments,
locations having frequent and heavy fog will have shorter
allowable links for a given availability. Alternatively, a
relatively fog-free site might be able to accommodate link
lengths of several kilometers using identical optical
wireless equipment.
Optical wireless-based communication systems can be enhanced
to yield even greater availabilities. In particular, by
including an RF wireless link or DSL as a backup, one can
offer availabilities of 99.999%.
Another example where practical engineering yields a
reliable, low-cost optical wireless approach is the ability
to maintain sufficiently accurate pointing stability without
invoking the cost, complexity, and reliability issues
associated with the use of an active pointing-stabilization
approach. This preferred low-cost, fixed-pointed approach is
schematically shown in Figure 2, where the transmitted beam
is broadened significantly beyond its near-perfect minimum
beam divergence angle, and the receiver field of view is
broadened to a comparable extent.
The broadening of the transmitted beam and receiver field of
view leads to large pointing/alignment tolerances and a very
low probability of building motion being of sufficient
magnitude to take the link down. Well-engineered hardware
exploits this approach of designing for loose alignment
tolerances. Therefore, it is possible to perform initial
alignment of the transceivers at opposite ends of the link
during installation, then leave them unattended for many
years of reliable service. Note that this approach is
facilitated for systems operating at wavelengths greater
than 1,400 nm because the higher allowable eye-safe powers
allow the transmitted beam to be significantly broadened
spatially while still maintaining an adequate intensity at
the receiver.
Other features of optical wireless systems that have accrued
from the long engineering history include systems designed
to allow simple, rapid installations (an hour or less),
assuming a suitable line of sight has been identified
already. Also, systems can feature scalability to multiple
types of architectures, protocols, and higher data
throughputs.
Field-test results
The final proof of the viability of any broadband access
approach, including optical wireless, is the successful
conclusion of rigorous field tests. In this context, it is
appropriate to summarize some recent results that were
achieved for a 0.5-km link near Vancouver, British Columbia.
In October and November of last year, this link was operated
24 hours a day, seven days a week, for a total of 60 days.
Weather conditions varied widely during the tests and
included periods of steady drizzle, heavy driving rain, and
multiple occasions of moderate fog. The only time the link
dropped out was a 20-minute period when fog reduced the
visibility to about 50 m. These results demonstrate a total
link availability of 99.97% in the Pacific Northwest,
arguably a challenging climate for optical wireless.
Mature technology
Optical wireless represents a mature, reliable approach for
broadband access. Such systems have been engineered to
provide robust performance that is highly competitive with
other access approaches, offering high capacity, excellent
availability of 99.9% (99.999% with an RF wireless or DSL
backup), lowest cost per bit per second, and rapid
deployment in about one hour.
These systems are compatible with a wide range of
applications and markets and sufficiently flexible for easy
implementation using a variety of different architectures.
Because of these features, market projections indicate
healthy growth for optical wireless sales. This market
potential will be met with well-engineered systems, designed
for high-volume manufacturing, with immediate availability.
David Rockwell is director of advanced technology and G.
Stephen Mecherle is chief technology officer at fSONA
Com-munications Corp. (Richmond, British Columbia). They can
be reached via the company's Website, www.fsona.com.
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