ap(config-if)#speed ?
1.0 Allow 1 Mb/s rate
11.0 Allow 11 Mb/s rate
2.0 Allow 2 Mb/s rate
5.5 Allow 5.5 Mb/s rate
basic-1.0 Require 1 Mb/s rate
basic-11.0 Require 11 Mb/s rate
basic-2.0 Require 2 Mb/s rate
basic-5.5 Require 5.5 Mb/s rate
range Set rates for best range
throughput Set rates for best throughput
This can be done from the GUI interface too.
refer http://www.cisco.com/en/US/products/hw/wireless/ps4570/products_configuration_guide_chapter09186a0080184ac8.html#1035986