For example, for uplink packet 511, it is the last PPDU transmitted by STA₁, which is indicated by (More Data=0). On the other hand, for uplink 512, it indicates that STA₂ has more PPDU to transmit with (More Data=1). When AP learns that the TXOP initiator (STA₁) is to finish its uplink transmission (as indicated by More Data bit=0 in frame control), it signals to the candidate user (e.g., STA₂) to switch over to the primary channel in the ensuing trigger frame (TRIGGER+ACK-P1) and to continue the uplink frame transmission thereby extending the TXOP duration. At the next PPDU transmit time, the TXOP initiator (STA₁) has ended its transmission and other STAs continue to transmit under the direction of AP. Note that other STAs can continue their uplink transmission until the end of TXOP limit.

FIG. 6 illustrates another embodiment of TXOP initiation and sharing for STA-initiated uplink OFDMA transmission. In the embodiment of FIG. 6, if AP cannot find one or more STAs to take over the primary channel, it can also instruct the primary channel STAs to continue with dummy PPDU transmission to retain control over the wireless medium. For example, for uplink packet 611, it is the last PPDU transmitted by STA₁, which is indicated by (More Data=0). AP then instructs STA₁ to continue to transmit dummy PPDUs in the next PPDU transmit time.