A method in a wireless local area network, WLAN, system, the method comprising: transmitting, by a first station, STA, beam refinement protocol, BRP, frames for a same direction to a second STA; wherein: the BRP frames are transmitted during a service period, SP, based on time division duplex, TDD, in order to perform multiple-input multiple-output, MIMO, beamforming training; the SP comprise a first TDD slot and a second TDD slot for the same direction allocated to the first STA and the second STA; if the first TDD slot is insufficient to transmit all of the BRP frames necessary for the MIMO beamforming training, as many BRP frames as possible which are first BRP frames are transmitted in the first TDD slot, and remaining BRP frames necessary to complete the MIMO beamforming training which are second BRP frames are transmitted in the second TDD slot; the first BRP frames are transmitted at an interval of short inter-frame space, SIFS, in the first TDD slot, and the second BRP frames are transmitted at an interval of SIFS in the second TDD slot; the second TDD slot is an earliest TDD slot allocated after the first TDD slot; an interval between a last BRP frame of the first BRP frames and an initial BRP frame of the second BRP frames is not SIFS; and the initial BRP frame of the second BRP frames is characterized as a subsequent BRP frame to the last BRP frame of the first BRP frames based on a code field value. transmitting, by a first station, STA, beam refinement protocol, BRP, frames for a same direction to a second STA; wherein: the BRP frames are transmitted during a service period, SP, based on time division duplex, TDD, in order to perform multiple-input multiple-output, MIMO, beamforming training; the SP comprise a first TDD slot and a second TDD slot for the same direction allocated to the first STA and the second STA; if the first TDD slot is insufficient to transmit all of the BRP frames necessary for the MIMO beamforming training, as many BRP frames as possible which are first BRP frames are transmitted in the first TDD slot, and remaining BRP frames necessary to complete the MIMO beamforming training which are second BRP frames are transmitted in the second TDD slot; the first BRP frames are transmitted at an interval of short inter-frame space, SIFS, in the first TDD slot, and the second BRP frames are transmitted at an interval of SIFS in the second TDD slot; the second TDD slot is an earliest TDD slot allocated after the first TDD slot; an interval between a last BRP frame of the first BRP frames and an initial BRP frame of the second BRP frames is not SIFS; and the initial BRP frame of the second BRP frames is characterized as a subsequent BRP frame to the last BRP frame of the first BRP frames based on a code field value. the BRP frames are transmitted during a service period, SP, based on time division duplex, TDD, in order to perform multiple-input multiple-output, MIMO, beamforming training; the SP comprise a first TDD slot and a second TDD slot for the same direction allocated to the first STA and the second STA; if the first TDD slot is insufficient to transmit all of the BRP frames necessary for the MIMO beamforming training, as many BRP frames as possible which are first BRP frames are transmitted in the first TDD slot, and remaining BRP frames necessary to complete the MIMO beamforming training which are second BRP frames are transmitted in the second TDD slot; the first BRP frames are transmitted at an interval of short inter-frame space, SIFS, in the first TDD slot, and the second BRP frames are transmitted at an interval of SIFS in the second TDD slot; the second TDD slot is an earliest TDD slot allocated after the first TDD slot; an interval between a last BRP frame of the first BRP frames and an initial BRP frame of the second BRP frames is not SIFS; and the initial BRP frame of the second BRP frames is characterized as a subsequent BRP frame to the last BRP frame of the first BRP frames based on a code field value.