Embodiments of an access point (AP) may comprise memory and processing circuitry coupled to the memory, and transceiver circuitry coupled to the processing circuitry. The processing circuitry of the AP may be configured to generate a trigger frame to allocate a center 26 tone RU for an HE-trigger-based PPDU, the center 26 tone RU having an adjacent upper 20 MHz subchannel and an adjacent lower 20 MHz subchannel. In an embodiment, the center 26 tone RU is for a station (STA) and wherein the trigger frame indicates to the STA to transmit a pre-HE-STF preamble on one of the adjacent upper 20 MHz subchannel, the adjacent lower 20 MHz subchannel, or both the adjacent upper and lower 20 MHz subchannels, and configure the wireless device to transmit the trigger frame to the station.