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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.
Airgo’s True G gear will be less than $100; sports two antennas: True G is Airgo’s alternative to True MIMO, which employs multiple simultaneous data streams over different paths when endpoints are using Airgo’s technology. True G only has two antennas to True MIMO’s three—as sold by Linksys, Belkin, and others. A company spokesperson explained that the reduction in cost was achieved through fewer antennas and a lower component count.
Airgo has been frustrated in its attempts to capitalize on MIMO as a term that they “own”: True MIMO is a trademark, but Airgo also wants MIMO to refer only to those devices that use spatial multiplexing. (Some dispute Airgo’s founders position as the inventors of MIMO, too, citing earlier papers, but the founders’ early work in the field is what’s been instantiated as product by them.)
True AG chipsets will also be available for 802.11a and g range extension. Both True G and AG will have two receive and two transmit antennas; the True MIMO devices have three receive and two transmit antennas.
According to this article, Airgo expects street prices of gear based on their chips to drop to $129 to $149 for True MIMO and $69 to $99 for a router for True G. True AG will be slightly more expensive.