Various schemes pertaining to encoding and transmit power control for downsized trigger-based (TB) physical-layer protocol data unit (PPDU) transmissions in next-generation WLAN systems are described. A station (STA) receives a trigger frame indicating an allocated resource unit (RU) of a first size. The STA performs channel sensing responsive to receiving the trigger frame. In response to detecting at least one subchannel being busy from the channel sensing, the STA performs a downsized trigger-based (TB) transmission with a downsized RU or multi-RU (MRU) of a second size smaller than the first size by utilizing downsized RU or MRU allocation information while maintaining a value of each of one or more parameters unchanged in an encoding process to perform the downsized TB transmission.