FIG. 3 illustrates one example of an uplink OFDMA transmission that is initiated by STA1. STA1 only transmits over the primary channel and lets AP take over TXOP control after first UL PPDU. The AP initiates and controls UL OFDMA over both primary and detected idle secondary channels using trigger frames. As illustrated in FIG. 3, STA1 starts an uplink access and transmits an uplink packet 301 over the primary channel. However, AP detects some secondary channel idle (PIFS prior to STA1 transmission) and desires to enable UL OFDMA with other STAB. For example, AP initiates and controls UL OFDMA for STA1, STAP_2, . . . , STAP_N over primary channel using trigger frame TRIGGER+ACK-P1 and STAS2_1-STAS2_N, STAS3_1, STAS3_N, STAS4_1, STAS4_N over idle secondary channels #2 to #4 using trigger frames TRIGGER-S2 to TRIGGER-S4, respectively. As a result, STA1, STAP_2, . . . , STAP_N transmits uplink packet over the primary channel, and STAS2_1-STAS2_N, STAS3_1-STAS3_N, STAS4_1-STAS4_N transmit uplink packet 312, 313, and 314 over secondary channels #2, #3, and #4 respectively, each padded to the same length as uplink packet 311 transmitted by STA1. The TRIGGER+ACK-P1, TRIGGER+ACK-S2, TRIGGER+ACK-S3, TRIGGER+ACK-S4 frame serves as a trigger frame and ACK frame for the UL MU transmission (i.e., synchronize uplink transmission timing and PPDU TxTime and allocate UL OFDMA resource units to STAs), performs uplink ACK, performs uplink power control, and regulates transmission opportunity (TXOP) sharing and usage.
