Free Space Optics (FSO) Link Considerations PRODUCT SUPPORT DOCUMENT:

Free Space Optics (FSO) Link Considerations PRODUCT SUPPORT DOCUMENT:
PRODUCT SUPPORT DOCUMENT:
Free Space Optics (FSO) Link
Considerations
By Iain Deuchars
ComNet Europe Ltd
Free Space Optics (FSO) Link Considerations
Any signal travelling through the troposphere will be affected by a number of parameters, whose type will be
based on the environmental conditions at that instant in time and at the exact location of the signal propagation.
Those parameters are dynamic and as such it becomes a battle to exactly model a link and dictate its performance
as there are too many changing variables.
Loss of Signal
When light is emitted from a transmitting source, the level of light reaching the intended receiving optic will reduce
based on a number of conditions.
• Increasing separation (link distance) between the transmitter and the receiver
• The divergence of the Laser beam is too great
• Transmit power and receiver sensitivity levels are too low
• Scattering, absorption or refraction of the Laser beam are too high
The first three points can be controlled, or at least influenced, but the scattering, absorption and refraction of
the Laser beam is down to the dynamic environmental conditions. These conditions will change depending on
geographical location, time of year or even time of day. There are equations (Beers-Lambert Law) that can be
used to derive the lost Laser power through the troposphere but we will not expand on those in this document.
What we will say is that different conditions produce different attenuation figures with this number provided in
a dB/Km format.
Fog
The water molecules that make up fog have a physical size close to that
of the wavelength of the Laser light and as a result the light is scattered.
This occurrence is known as Mie Scattering.
Very light fog could create a 10dB/km loss figure with numbers going
up to 400dB/km for very dense fog. The rule of thumb with fog is that if
our eyes can see 100m then the FSO link can see an additional 50% or
150m in this particular example¹.
Airborne Particles
Smog, dust and sand storm conditions could all be thought of as similar
to fog with respect to attenuation levels they create and the impact they
have on the transmitted Laser beam through the sky. A point to note with
smog, dust and sand is that they can deposit particles on the front face
of the FSO device and any build up will cause attenuation. The vertical
and smooth face of the FSO along with wind help particles to slide
off but users should carry out regular maintenance checks to keep the
front surfaces of the FSO clear. Comnet studies in dry regions show that
dust accumulation has little negative effect on the performance but we
suggest a maintenance schedule of between three to six months would
be acceptable. Dynamic power measurements within the management
platform of the SkyLaser device can be used to detect foreign bodies preventing clear transmission and reception
from the front aperture of the FSO. A continued reduction in received power levels would indicate a foreign body
effecting the system and alarms can be triggered if this particular scenario develops.
[1] T
he Comnet FSO1GLR multi-laser system will improve transmission performance and reduce the overal attenuation that the system
experiences. This will help system performance in fog and other noted problem conditions.
1
ComNet Communication Networks
Rain
Unlike fog the rain droplets look and act like pieces of glass to the
Laser beam and so refraction takes place through the rain drop but
that will not dramatically change the received Laser power². If the rain
is monsoon conditions then you may get scattering due to the high
volume of rain drops and this will increase the attenuation figure. A
figure of 5dB/km could be given for light to medium rain and 10dB/km
for heavy rain, with heavy rain equating to 30mm/hr.
One point to note is that microwave systems (especially in the 60 –
80GHz) region are heavily effected by rain and FSO provide the lowest
attenuation figures in rain for any wireless system.
Snow
Snow will have a similar effect to fog with attenuation per Km increasing as the density or physical size of the
snowflakes increase. If the snow fall is such that you cannot see the remote end of the FSO link then the likelihood
of error free communications will be low, however this does not mean that the link will drop out².
Scintillation
Scintillation is a phenomena caused by air thermals emitted from bodies as a result of solar radiation. The thermals
create pockets of air with different refractive indexes where these pockets are changing in real time. Different
refractive indexes will cause light to change path (refraction) but this does not mean a negative effect as light can
be brought back towards the receiver rather being diverted away. The scintillation issue will be greater in hotter
countries based on the solar radiation. For example bad scintillation could equate to 3dB/km in the UK and 10dB/
km in the Middle East region. Obviously scintillation is present if all the previous environmental conditions are not
present, so scintillation and fog would never have a constructive effect, one will always be present at the detriment
of the other.
Comnet have measured FSO operational performance in scintillation conditions and found that the four Laser
system will provide better performance than the single Laser system due to the multi-beam propagation. With all
testing the link was never found to drop, rather the throughput was slightly reduced but still provided >90% of the
total capacity of the gigabit connection.
Availability
Availability tends to be expressed as a percentage that states the time that a piece of equipment or a particular link
will be operational. If the term “five nines” is used this equates to 99.999% availability, which over a 1 year period
would define that the link was inoperable for less than 5 minutes and 15 seconds. To achieve this level of availability
with the Comnet SkyLaser system the following criteria need to be observed.
• The FSO1GLR four Laser system must be used
• Link length should not exceed 600m, depending on environment
• FSOAAL alignment software must be operational
• NWK9 Netwave link provided for backup
SkyLaser Redundant Link
FSO1GLR
FSO1GLR
Primary FSO Transmission Path
CNGE2+2SMS Switch
in RSFP Mode
NW9 Client
CNGE2+2SMS Switch
in RSFP Mode
NW9 Access Point
Secondary Microwave NetWave® Transmission Path
[2] The Comnet FSO1GLR multi-laser system will improve transmission performance and reduce the overal attenuation that the system
experiences.
2
Free Space Optics (FSO) Link Considerations
Link Performance
• Link Budget = Power output – Receive sensitivity
• Link losses = Geometric attenuation + In system loss + TX/RX losses + environmental effect losses
• Link Margin = Link Budget – Link Losses
If the link budget of the SkyLaser system is greater than the total link losses then we should be in an operating
position. For the sake of Bit Error Rates (BER) we would like a link margin of greater than 3dB.
The composition of the various losses is:
• In System losses: Digital losses, eye patterns and system inefficiencies
• TX/RX losses: Pointing errors and manufacturing tolerances
• Geometric attenuation: based on the transmit aperture, beam divergence and link distance.
• Environmental effect losses: Those previously discussed
From our measured figures in the diagram below you can see that at a 3dB/km environmental effect loss figure
we can achieve a 1500m link distance based on our FSO1GLR product. As the attenuation figure increases for
environmental effects the operational distance reduces accordingly.
SkyLaser operation range vs Environmental losses
As the environmental effect losses increase our operating distance must reduce to maintain the Link Margin.
The following graph shows the approximate operating distance for the FSO1GLR four Laser system against varying
environmental loss figures. This graph cannot be taken as a guarantee of operational performance but simply to
offer an indication.
3
ComNet offers an extensive line of environmentally hardened fiber optic video and data transmission equipment as well as a line of Ethernet networking
equipment that is uniquely designed to meet the needs of the Industrial Security, Intelligent Transportation, and industrial control markets.
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© 2016 Communication Networks. All Rights Reserved. “ComNet” and the “ComNet Logo” are registered trademarks of Communication Networks.
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