Wireless Systems Design

The Optical Option

Not all wireless is RF. As a wireless system designer, when seeking a wireless solution you should always keep in mind that you can use free-space optical (FSO) tools to get you from here to there without wires. Your first thought of an optical solution is probably a TV remote control or the short range infrared (IR) like those IrDA links used in some of the older PDAs, laptops, and other devices.

IrDA is still a good option for some short-range problems. But what I am reminding you of here is the long range optical equipment that some of you probably didn't even know existed. The general term for such equipment is free-space optical. That equipment has been around for years but it has slowly grown and proved itself over and over again as a truly viable wireless solution when you need a high-data-rate connection. All this came back to me recently at a couple of conferences I attended. I had a chance to talk with some of the key suppliers of this technology. It is far better than I suspected.

First, lets talk about range. Most of us think of really "short range" when considering optical. But "short" is relative. For available FSO equipment today, "short" is more like a few hundred meters. Maximum range is about 2 km, but you can extend that by connecting the transceivers back-to-back just like the microwave relay towers used to carry long distance phone calls over long ranges back in the olden days.

If you need to transmit data from one building to another on a campus, in some downtown area, or from downtown to suburb, a FSO solution may be just the right thing. It sure beats the hassle of figuring out how to run fiber or other cables to do the same thing. Remember, laying fiber is horrendously expensive because you have to tear up streets, dig trenches, and otherwise mess with the environment, not to mention the need for municipal building and land permits. It takes months to get this done and the overall cost can run two to three times as much as just bolting on a couple of FSO transceivers on the tops of buildings. Installation times may be no more than a few hours to a couple of days.

That does bring up one really key point. You absolutely positively must have a direct line of sight (LOS) path from one location to the other. Most of the time you can get this by just mounting the transceivers on top of an available building. Other times you may need a tower. The LOS problem also exists for most microwave radios but is not as stringent when techniques such as OFDM and MIMO are used to help mitigate the multipath problems that typically crop up.

You have probably heard that optical is no good because rain, snow, and fog wipe out the link. Well, that can happen. Yet, optical links are still more reliable than you think and all of them are tolerant to some rain, snow, and fog. Despite the scattering and attenuation of the IR light, with some built-in extra laser power margin, the data does get through. And if you are using TCP/IP, any temporary loss of data is usually recovered by a retransmission anyway. And of course if you are really paranoid, you can use multiple back up links to overcome most problems and to ensure reliability.

Usually, FSO suppliers offer several models designed for different ranges, small low-power versions for short links of say 20 to 500 meters and big honking models for those 2 km links. Most use lasers in the 785- to 850-nm IR range.

One feature you should look for is auto tracking. This is a feature in most transceivers that keeps the two transceivers pointed at one another under changing conditions. What can change? When you are mounting your transceiver on tall buildings, the optical beam can easily get out of line just because of building sway. Tall buildings are designed to sway in high wind, in some cases several feet. Even a variation of several inches is often enough to take the beam out of alignment. Remember we are talking about a thin coherent light laser beam, further narrowed by lenses, that is only a few degrees wide even kilometers away. Precise alignment during installation is critical. It is not unlike adjusting a satellite receiver dish antenna to hit the satellite. In any case, building sway, wind, temperature, vibration, or other aberration is adjusted for automatically with auto-tracking features. As for speed, you also usually have your choice of transceivers. Most units will easily handle standard Ethernet and Fast Ethernet speeds (10/100 Mbits/s). A popular speed option is the OC-3 Sonet rate of 155 Mbits/s. But you can get transceivers that can hit rates as high as 1.25 Gbits/s. You can't get that with RF wireless unless you opt for the rare millimeter (mm) wave radios that are becoming available. In any case, these units are protocol agnostic so you can do with them what you will.

A more recent addition to the FSO bag of tricks is mesh networking. Mesh is big these days with wireless mesh sensor networks and Wi-Fi municipal mesh networks. Now you can do it optically. Omnilux has a three-transceiver node that lets you mesh LANs and things like closed-circuit TV cameras over a short range. Mesh also lets you extend the range of a node since you can hop data from one node to another. Mesh usually brings some network redundancy and reliability. One more great wireless option.

Finally, FSO units are truly fast deployment devices. You can set them up in minutes to hours with the right hardware, such as a tripod or pipe clamps. If you need to set up a high-speed link fast, this is the way to go. FSO has also been used as a temporary link while a permanent fiber link is being installed. Some users have said later, "what did we need the fixed fiber link for anyway?"

I am a big fan of this technology. Check it out as an option next time the need arises. The FSO vendors I spoke with were Canon (www.canonbeam.com), Lightpointe (www.freespaceoptics.com), and Omnilux (www.omnilux.com). If I missed any one, drop me a line at [email protected].

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