The 802.11n standard provides for the use of MIMO (Multiple Input Multiple Output) technology, on the basis of which can be implemented one of the following options:

• SM - Spatial Multiplexing - increase of spatial streams and also transmission speed (implicitly contained in the MCS indicator)

• STBC - Space-Time Block Codes - increase of transmission quality with special coding schemes

The STBC option is also implicitly contained in the MCS index: if the transmitting device has two or more antennas, but the receiving device doesn’t have any, or its channel quality does not allow to use multiple spatial streams, then indices from 0 to 7 imply the use of STBC.

In this case, we can expect a conflict in collision resolution with devices that don’t use MIMO.

When STBC is transmitted (it is a tested AP option in ‘802.11n’), 802.11n provides coexistence mechanisms when non-STBC clients are in the vicinity. The ‘dual beacon’ and ‘dual CTS’ options essentially transmit those frames twice, once with STBC and once without, so non-STBC devices can receive the beacons and set their NAV appropriately: the CTS protection mechanism works in the usual way.

Conclusion: MIMO doesn’t have any impact on multiple access mechanisms.