In another aspect, a method for quickly adapting to congestion is provided via including the OFDMA based contention window parameters in the trigger frame for a non-periodic channel access so that the SOFMDA capable stations can update their backoff counters for the next channel access period accordingly. A trigger frame may be transmitted as soon as needed, without having to wait for a next scheduled transmission of other frames such as a beacon frame.

In another aspect, a method is provided for SOFDMA capable stations to start transmitting a trigger response frame earlier when no transmission is detected in the previous channel access slot. In this way, it can further help improving the medium utilization and spectrum efficiency.

In another aspect, a legacy preamble is included in the trigger response frame to allow other stations to sense the medium busy via clear channel assessment (CCA) detection mechanism and to allow the SOFDMA capable AP to detect the trigger response transmission frame. Due to identical values used in the legacy preamble field of trigger response frame, the SOFDMA mechanism allows legacy preambles transmitted by multiple stations to be detected by others stations. In addition, the legacy preamble of trigger response frame could be used to reduce the probability of false detection of trigger response frame in TXOP.

In another aspect, a method of using OFDMA sub-channel distinguishable multiple trigger response frames from different stations is provided. With the orthogonal characteristics of OFDMA sub-channels, the AP could be able to distinguish the trigger response frames sent from different stations.