A first communication device generates a trigger frame to trigger a contention-based uplink orthogonal frequency multiple access (OFDMA) transmission by multiple second communication devices. The trigger frame may be configured to indicate a predetermined length of the contention-based uplink OFDMA transmission, where the predetermined length corresponds to contention-based uplink OFDMA transmissions. The first communication device transmits the trigger frame to the multiple second communication devices, receives the contention-based uplink OFDMA transmission. The contention-based uplink OFDMA transmission may be of the predetermined length.