A method for performing spatial multiplexing, SM, precoding in a wireless access system supporting a wireless video area network, WVAN, the method performed by a transmitting device including a plurality of antennas and comprising: performing (S20-S25) an SM high-rate physical, HRP, beam searching process for selecting at least one optimized beamforming antenna group with a receiving device that includes a plurality of antennas; transmitting a first packet comprising at least one spatial multiplexing training sequence that is used to calculate a spatial precoding matrix to the receiving device, if channel matrix rank is estimated greater than one after performing the SM HRP beam searching process; receiving a second packet comprising feedback information for the spatial precoding matrix from the receiving device, wherein the spatial precoding matrix is calculated by using the at least one spatial multiplexing training sequence; and performing spatial multiplexing precoding for data streams to be transmitted to the receiving device, by using the spatial precoding matrix, wherein the first packet is transmitted in accordance with an HRP mode and the second packet is received in accordance with a low-rate physical, LRP, mode, wherein the WVAN supports an HRP layer for the HRP mode and an LRP layer for the LRP mode, and wherein during the SM HRP beam searching process, the transmitting device iteratively transmits a first training sequence and receives feedback information for the first training sequence for a number of times equal to a predetermined value, and applies an antenna array weight vector, AWV, to the at least one optimized beamforming antenna group based on the feedback information for the first training sequence. performing (S20-S25) an SM high-rate physical, HRP, beam searching process for selecting at least one optimized beamforming antenna group with a receiving device that includes a plurality of antennas; transmitting a first packet comprising at least one spatial multiplexing training sequence that is used to calculate a spatial precoding matrix to the receiving device, if channel matrix rank is estimated greater than one after performing the SM HRP beam searching process; receiving a second packet comprising feedback information for the spatial precoding matrix from the receiving device, wherein the spatial precoding matrix is calculated by using the at least one spatial multiplexing training sequence; and performing spatial multiplexing precoding for data streams to be transmitted to the receiving device, by using the spatial precoding matrix, wherein the first packet is transmitted in accordance with an HRP mode and the second packet is received in accordance with a low-rate physical, LRP, mode, wherein the WVAN supports an HRP layer for the HRP mode and an LRP layer for the LRP mode, and wherein during the SM HRP beam searching process, the transmitting device iteratively transmits a first training sequence and receives feedback information for the first training sequence for a number of times equal to a predetermined value, and applies an antenna array weight vector, AWV, to the at least one optimized beamforming antenna group based on the feedback information for the first training sequence.