Thursday, March 09, 2006


Intel demos a WiMAX-enabled laptop on a scooter that is delivering speeds of 2 Mb/s.

"Support of data rates up to 75 Mb/s" is a widely-bandied-about figure.

I've even seen some people claim 150 Mb/s.

Alas, Shannon-Hartley's not just a good idea, it's the law.

WiMAX supports a number of modulation types and coding rates; the best is 64QAM 3/4, which provides a raw data rate of 4.5 bits per baud, or 4.5 bps/Hz. Let's be generous and assume that a hypothetical WiMAX network has such a dense forest of base stations that everywhere Sean Maloney rides his scooter in San Francisco, his WiMAX card is operating at 64QAM 3/4.

The 2.3 GHz WCS band licenses in the US are for 5 MHz channels; 2.5 GHz BRS/EBS licenses are for 6 MHz channels. So with that 6 MHz channel, the maximum theoretical data rate for WiMAX is 27 Mb/s. Hang Sean's scooter from a crane right in front of the base station antenna, and he won't get any better than 27 Mb/s raw channel capacity.

That's raw bits, though. You have to then factor in the WiMAX PHY overhead (about 15%), MAC overhead (about 20%), and MAC convergence sublayer overhead (about 10%). Now you're down to about 16.2 Mb/s user payload throughput. Remember, we're still talking best case - 6 MHz BRS/EBS channels, and enough base stations that you've got 100% coverage with 64QAM 3/4.

Well, OK, 16 Mb/s is still pretty fast, right? But remember, this is a shared medium, and that 16 Mb/s is combined upstream/downstream. How many WiMAX cards does Intel hope to sell? Because even with only a 256 kb/s upstream, that's only about 7 simultaneous users at 2 Mb/s.

150 Mb/s? Not hardly.

75 Mb/s? Not even close.

2 Mb/s? Sure, if a carrier builds a dense forest of base stations, because you need small cells both to get as much 64QAM coverage as possible and you need a lot of them to provide the capacity needed.