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Xirrus have been doing this for years - see http://en.wikipedia.org/wiki/Xirrus or http://www.xirrus.com/

They put 8 (or more) access points into a single unit, each with a directional antenna covering a segment of the room or venue. I looked at their product at a trade show or conference once (don't remember which) but it was way overkill for the spaces we had at the time which were separated with heavy reinforced concrete walls and floors, so needed an access point for each area.

That's why I emphasized the word like...Although the antenna performance is better, the rocket plus traditional sector antenna might be an overwhelming size for the typical street light, the NS and LOCO are a size where neighbors might not complain about the "junk" on the pole....I did see someone present once on a saucer shaped access point unit that was actually 12 (?) access points with tight sectors...If they have an outdoor version that could sit on the pole top...

If the original poster is responsible for the convention center Wi-Fi, what he/she can do is crank down the transmit and receive gain and add more access points. If he/she is a vendor on the trade floor, that might not be enough; if the convention center doesn't set up such high density Wi-Fi for everybody, then every other vendor is going to leave their base station hardware set at the default setting, which, while appropriate for a house in the suburbs, or maybe an apartment complex, is WAY TOO LOUD in a crowded convention hall. The more devices you get in close proximity, the more interference issues you're going to see. Of course, all the other non-Wi-Fi devices aren't affected by such changes, so this only works if most of the interference is actually coming from other Wi-Fi sources.

Alternatively, as you suggested, move to the 5 GHz band. (I assume that's why you were recommending 802.11a, but it isn't limited to "a"; you can also run 802.11n up there with some hardware.)

Finally, you can use a high-gain external directional antenna to provide coverage for just your section of the floor at high enough gain to drown out everybody else (e.g. a 25 dBi yagi pointed down from the ceiling or something). Bear in mind that this is the Wi-Fi equivalent of a tactical nuke, so you should expect retaliation from other vendors the following year....

Alternatively, try renting one of these guys and see how you fare. That said, this is probably the Wi-Fi equivalent of a fuel-air bomb, so again, talk to the nearby vendors and try to convince them to share your connection rather than making a further mess of the spectrum.

Xirrus 'Arrays' are designed for what you're doing. I've used 2 4-radio Xirrus arrays to serve 240 users in a single ballroom. http://store.xirrus.com/SearchResults.asp?Cat=4

most of your fixes are here already - visit http://www.xirrus.com/

- voip is already being done successfully over wi-fi. there's plenty of non overlapping channels and multiple phones can run off a single xirrus AP

- very high user density - 20k users supported at interop

- rf management built in

- plays nice with other wireless networks

- 1/4 the power usage of other wi-fi solutions

- fast roaming built in between radios and accses points

wimax is irrelavent. wimax is used for long haul point to point typically. don't expect consumer wimax for wireless lan to become a reality.

I interned for a company named "Xirrus" this past summer; their primary product is similar to what you describe. It's an "Array" of directional access points in a radial pattern, each access point running on a different channel in the 5GHz band (802.11a). These things are pretty expensive and mostly sold to universities, airports, etc, but they work pretty well... Actually I just noticed that Xirrus was mentioned in TFA. Anyway, if you want a look at the inside of one of these things, see here: http://xirrus.com/products/arrays.php