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February 8, 2006

Outdoor N: Speed Gains Not So Much

By Glenn Fleishman

In all the discussion of 802.11n, the notion of using it for outdoor spaces as been little explored: 802.11n is an indoor specification, let’s face it. It achieves its potential rates (in the proposal accepted recently) through improvements to the MAC’s efficiency, RF upgrades, spatial multiplexing, and double-wide channels.

MAC efficiency could be a big improvement, in that the Media Access Control components deal with the fiddly parts of packaging data into frames (or removing them), error correction, and handling source and destination issues. Improvements to the MAC layer will help throughput regardless of other elements. Likewise, fixing radio frequency (RF) deficiencies in 802.11g and adding better characteristics will improve the quality of transmissions, too.

But when you get into the remaining two elements, there’s where the problem creeps in. Spatial multiplexing is one of the key advantages of multiple-in, multiple-out (MIMO) antennas. Using multipath reflection, a MIMO device—such as any future 802.11n radio—can produce multiple data streams that contains different information passing over the same frequencies. (They can also duplicate the same data for redundancy improving the fidelity of reception.)

In outdoor spaces, there isn’t enough multipath reflection close enough to a transmitter to provide that advantage. I’ll be curious whether a concrete canyon would work, say in downtown Manhattan, but wide open spaces won’t.

The double-wide channels, which will be 40 MHz wide instead of the normal 20 MHz for the 802.11 family, have a similar problem. In current thinking, 40 MHz can be used only if the airspace is clear. (Airgo’s early implementation of this is causing it some black eyes for neighborliness, although they have already stated some elements will be changed via firmware upgrades.)

In typical outdoor environments where using Wi-Fi would make sense, it is unlikely that any two adjacent channels would have no signal present.

The conclusion? The 200 Mbps to 600 Mbps possible in 802.11n will probably appear more like 40 to 50 Mbps of actual throughput for outdoor installations; indoors, 100 to 300 Mbps will certainly be achievable, however.

Posted by Glennf at 9:48 AM | Comments (0)