This disclosure provides systems, devices, apparatus and methods, including computer programs encoded on storage media, for null data packet (NDP) based implicit sounding and calibration for a wireless local area network (WLAN). An AP may perform a calibration procedure with one or more stations (STAs) that involves NDP transmissions. The AP may calibrate an effective downlink channel response to an effective uplink channel to compensate for transmit and receive chain differences. The AP may initiate implicit sounding by transmitting a downlink message that includes an uplink NDP announcement frame to the one or more STAs, prompting uplink multi-user (MU) NDP transmission. The AP may estimate the uplink channel based on measuring the received NDPs and mirror the estimated uplink channel for reciprocal downlink channel estimation and precoding generation. The AP may perform MU multiple input multiple output (MIMO) downlink transmission that is precoded based on the channel estimation.