The legacy preamble field of trigger response frame would be used by the AP and other STAs for the timing and frequency synchronization and CCA detection so as to protect the CA-TXOP contended from legacy EDCA stations during the SOFDMA channel access, when legacy stations and HE stations were deployed in the same area and on the same frequency channel. In addition, the legacy preamble field of trigger response frame could be used to identify the current contention window for UL MU SOFDMA transmissions.

The example illustrated in FIG. 5B shows fields like L-STF, L-LTF and L-SIG, which may be used for backward compatibility to the legacy STAs. When the SOFDMA stations share the same frequency channel with legacy stations, the first two sub-fields are the same as the legacy preamble so that legacy stations could detect medium occupancy with legacy CCA detection mechanism. If stations miss detecting the first two sub-fields on legacy preamble, they could apply the energy detection mechanism in the middle packet transmission to detect the medium usage status. The L-SIG field in the trigger response frame is also used for compatibility to legacy STAs, but it may be set to identical values by all SOFDMA responding stations which contend the same CA slot.

With reference to FIG. 5B which shows an example setting of various fields of the L-SIG.

a) Rate bits of L-SIG may be set to “BPSK r=?” for SOFDMA trigger response frame.

b) Reserved bit of L-SIG field may be set to “1” to indicate the SOFDMA trigger response frame.

c) Tail bits of L-SIG field may be set to “0.” Parity bit P may be set to achieve parity consistency for the L-SIG field.