A WLAN Access Point (AP) includes a MAC processor and a PHY processor. The MAC processor is configured to generate a trigger frame including user information fields destined to respective STAs, and a padding field including one or more padding bits, to determine a number of padding bits that, after the trigger frame including the padding bats being encoded for transmission, satisfy a processing-time constraint imposed by the STAs, and to insert the determined number of padding bits in the padding field. The PHY processor is configured to generate a packet from the trigger frame, including encoding the trigger frame in accordance with an ECC, into one or more code words whose length depends on a number of padding bits in the padding field, to generate multiple modulated symbols from the one or more code words of the packet, and to transmit the modulated symbols to the STAs.