Wireless trade-off: Capacity vs. distance

EVEN IN THE AIRWAVES, there is no such thing as a free lunch. Wireless links operating in the unlicensed 60 GHz band offer a lot of throughput on a large number of channels with little radio interference. But what you gain in capacity, you can lose in range and reliability.

"Anything above 10 GHz is affected by periods of very heavy rain," said Gregg Levin, chief marketing officer at BridgeWave Communications. "The way you counter that is to shorten up your path distance to get enough signal strength to keep up the link.'

That can be a problem, given the range limitations of the 60 GHz band. In the 5 GHz band, on which weather has little effect, signals can travel five, 10 or even 20 miles, although with less data capacity.

Then there is the fact that 60 GHz is a resonant frequency of oxygen, which means that oxygen molecules absorb energy in that band. Oxygen is always present, cutting the signal range a little more.

The result is that if you want fat wireless pipes, you have to give up some range. The line-of-sight range for 60 GHz data links is from a half-mile to a mile, which means no obstructions. BridgeWave improves reliability of its links by allowing them to automatically drop the throughput from 1 gigabit/ sec to 100 megabits/sec if the signal degrades, maintaining the connection at a lower speed.

If the throughput is good enough, you can also extend a wireless link by using multiple hops. Bud Borja, chief information officer at the Oregon Judicial Department, said this solution requires cooperation between the users and the owners or lease holders of the buildings used to site equipment for the extra hops.

About the Author

William Jackson is a Maryland-based freelance writer.

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