Which chips support what?
The early chips (AR5210, AR5211) support two antenna configurations, using an external antenna switch.
The AR5212 era chips (all the way to the 11n chips) support the two antenna configuration using an external antenna switch, as well as a "sectored" antenna configuration. This allows for 4 bits to be set for each transmitted packet. Normally that would feed into an external antenna switch and control which of fifteen sectored (narrow focus, non-omni directional) antennas are to be used, with the last being the "omni" antenna.
The AR5416 and later chips implement MIMO _except_ in the case of the 1x1 versions. These don't implement classic diversity by using an external antenna switch. They also implement a 24 bit shift register to control an external phased/sectored antenna array. This can be done per-packet; with the receive configuration being static (eg omni.)
The AR9285/AR9485 implement the classic switch based antenna diversity for receive, as well as LNA mixing. They only support one transmit output, so transmit diversity requires an external antenna switch.