Abstract: A method transmits training frames in a MIMO system. A long sequence of training frames is transmitted from a station B to a station A via a channel of the MIMO system, in which the station A includes NA antennas and NA—SS RF chains and the station B includes NB antennas and NB—SS RF chains, and a number of frames in the long sequence of training frames is at least equal to where is a lower bound operator. A short sequence of training frames is transmitted from the station A to the station B via the channel in response to receiving the long sequence of training frames.