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.  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  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. 3 CORPORATE DRIVE | DANBURY, CT 06810 | USA | WWW.COMNET.NET T: 203.796.5300 | F: 203.796.5303 | TECH SUPPORT: 1.888.678.9427 | INFO@COMNET.NET 8 TURNBERRY PARK ROAD | GILDERSOME | MORLEY | LEEDS, UK LS27 7LE T: +44 (0)113 307 6400 | F: +44 (0)113 253 7462 | INFO-EUROPE@COMNET.NET © 2016 Communication Networks. All Rights Reserved. “ComNet” and the “ComNet Logo” are registered trademarks of Communication Networks.
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