Mm-Wave Ethernet 100Mbps Links as Cost-effective FSO Back-up for 99.999% Availability

The Hybrid FSO (HFR) concept: Carrier and enterprise class FSO link back-up
by ELVA 100Mbps wireless LAN bridge with 100Base-T interface

Introduction to FSO market

For last 10-15 years Free Space Optics (FSO) has become from an experimental laser technology to a viable, high-bandwidth wireless alternative to fiber optic cabling. The primary advantages of FSO over fiber cable are its easy installation, rapid deployment time and significant cost savings. The disadvantage of FSO over fiber is variable laser power attenuation through the atmosphere which it is vary dependent on weather. The atmosphere absorption significantly limits the distance at which FSO should be deployed with projected factor of reliability.

The FSO system integrators use historical weather data that usually collected at airports to predict the link availability as a function of distance. These weather data provide a good indication of the reasonable link distances for FSO systems in a particular geographical area. FSO link distances can vary greatly from desert areas in Middle East to heavy-fog cities like London. Another factor in determining FSO distance limitations is the link availability expectation of the application.

-  For enterprise applications, link availability requirements are generally 99% or greater. This allows for relatively longer FSO link ranges, based on the weather prediction. The enterprise market is where the majority of FSO systems have been deployed.

-  For carrier-class the links availability is generally considered to be 99.999% (“5 nines”). Thus for telecommunication applications, they have to meet much higher availability requirements than for enterprise ones. The carriers and ISPs are another potential large user of high-bandwidth FSO systems, either for wireless backhaul or “last-mile” access applications.

The experimental data that are publicly available from research institutions and FSO manufacturers allow evaluating the limits for FSO link atmospheric attenuation. In general, it can vary from 0.2 dB/km in exceptionally clear weather to 350 dB/km in very dense fog. For example, the average attenuation for different bad weather condition is as following:
Fog++ 350 dB/km, Fog+ 310 dB/km, Fog 200 dB/km, Snow 150 dB/km, Rain 45 dB/km. Thus, for commercially available FSO links that atmospheric attenuation means distance ranges from outstanding 4km to as low as 50-100m respectively. Same time an attempt to increase transmit power in FSO systems do not result in a significant increase in the link distance in fog, due to the exponential nature of atmospheric attenuation. More over, an excessive transmit power in FSO systems is strongly prohibited by state certification bodies because of eye safety concern.

Mm-wave links for FSO back-up

The answer for the most cost-effective FSO back-up solution would be to incorporate a millimeter wave back-up, which would not be affected by the same heavy, visibility-limiting weather. Combining radio and laser (or LED transmitter) in tandem works particularly well. This explains because millimeter wave transmission is affected more by rain (because the carrier wavelength is closer to the size of a rain drop) and optical transmission is affected more by fog. Rain drops can vary in size from 0.1mm to 10,0mm, and these will effectively disperse millimeter waves, especially with carrier frequencies greater than 10 GHz (10 GHz = 30,0 mm wavelength; 38 GHz = 8,0 mm wavelength). Fog is typically 1 to 20 micron, and will effectively scatter the FSO wavelengths of 0.785 -0.85 and 1.55 micron. The only weather that could affect the transmission of a hybrid FSO/RF is conditions of simultaneous heavy rain and thick fog. Luckily, these conditions would not occur simultaneously, because as the rain falls, the rain droplets would absorb the suspended fog water droplets, thus diminishing the fog.

Figure 1. FSO hot-swap back-up by mm-wave link. Redundant paths and failover capability are using
a third-party multiplexer on both sides of the link

A real advantage of mm-wave links as FSO back-up is that they have to back-up them at relatively short distances because of FSO nature. Based on FSO technical specs and installation statistics, most of the FSO links are installed on distances no more than 1km, while mm-wave links are designed to work on distances up to 20km. This means that mm-wave links have the very significant gain margin which allows to penetrate 1km distance even at very heavy rain. According to "Rain Attenuation" chart that are available at page 13 of FCC Bulletin 70 (see link below), the maximum attenuation for frequencies 40-100GHz is 50dB/km that much less than max thick fog attenuation of 350dB/km. In other word, on distances of a typical FSO link the mm-wave link will work in ANY rain that could happen in the planet Earth. The only technical issue for mm-wave link is to choose a right antenna diameter to get the guaranteed gain numbers. To illustrate that, we included "distance range vs rain rate' diagrams for 40.5-43.5/59-64/92-95 GHz bands. Click on a thumbnail/link below to see the relevant diagram.

Distance range vs rain rate diagrams for ELVA-1 mm-wave radios
40.5-43.5 GHz	59-64 GHz	92-95 GHz

For a true hot-swap back-up, the connectors from FSO link and mm-wave one has to be intellectually switched over. A commercially available 3-rd level multiplexer (Cisco, 3Com, Nortel or other brand) or a proprietary redundant link controller (RLC) can be used for this purpose. The RLC is generally cheaper unit but not a universal one, so an independent system integrator cannot choose best FSO and mm-wave equipment to combine, but has to be glued to a specific FSO manufacturer. Commercially available switches are more universal, but in case of switching gigabit links they will be as expensive as links itself. Figure 1 shows schematically how to combine FSO and mm-wave links to a hybrid one (HFR). By using two paths, one MMW and one FSO, the HFR link will bring 99.999% availability for weather-independent wireless connection.

ELVA-1 Millimeter Wave Division is leading manufacturer of MMW links, offering true 100Mbps point-to-point LAN bridges with 100Base-T interface and RJ-45 or fiber-optics connectors. With wider ever frequency choice of 40.5-43.5GHz, 59-64GHz, 71-76/81-86GHz, 92-95GHz ELVA links allow an FSO manufacturers and system integrators to build reliable backed-up solutions. For countries with specific frequency licensing policy, ELVA offers custom-designed mm-wave 100Mbps point-to-point LAN bridges that could satisfy local frequency band allocation.

Relevent weblinks:

1. Official FCC Bulletin 70, "Millimeter Wave Propagation" (PDF, 1.7M)
2. THE LAST-MILE SOLUTION: HYBRID FSO RADIO, written by AirFiber (PDF, 1.1M)

Additional information in PDF format:

1. ELVA's 2002 Brochure on PPC-10/100 wireless links  (PDF 800K)
2. ELVA's 100 Mbps Full Duplex Wireless Ethernet Bridge (PDF 140K)
3. An article in St.Petersburg Times about our mm-wave radios and FSO, issue 06 Apr 2004

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