In order to save the power, a SOFDMA station with its backoff counter>0 should detect the trigger response frame or a new trigger frame in aSlotTime in the CA slot. If it detects a trigger response frame sent by other SOFDMA station(s), it may go to the sleep for about CA slot time (aSlotTime) and wake up just before the current CA slot ends to continue monitoring the channel. If the SOFDMA station detects an empty CA slot, it may continue monitoring the channel for a short period of time. If a new trigger frame after aSlotTime of the beginning of empty CA slot is detected, it will decode the trigger frame and may respond the trigger frame earlier than original scheduled slot time according to the information in the new trigger frame.

FIG. 8 shows an example timeline of transmissions in HE-TXOP of successful trigger response stations. As seen from top to bottom of the drawing, with time increasing from left to right, the following sequence of events may take place:

801. The SOFDMA AP transmits a trigger frame indicating channel access after detecting that the medium is idle and its NAV is at “0.” The SOFDMA AP sets the Duration field of the trigger frame to the time required by SOFDMA process. Legacy stations and SOFDMA stations in OBSS may use the Duration information to update their NAV values. Non-AP SOFDMA stations in the BSS use the training sequence of trigger frame to perform timing synchronization with the SOFDMA AP.

802. After xIFS time, the SOFDMA capable stations, STA1 and STA2, check their NAV values (=0) and contend the medium via SOFDMA channel access mechanism. STA1 and STA2 transmit a trigger response frame respectively at same time.