An access point generates a trigger frame to trigger an uplink multi-user (MU) transmission by multiple client stations. The access point generates the trigger frame to include a common information field and multiple per-station information fields. The access point generates the common information field to include (i) a subfield that indicates a bandwidth of the uplink MU transmission, and (ii) information that indicates a guard interval and a long training field (LTF) mode to be used by the multiple client stations for the uplink MU transmission, and generates each per-station field to include respective frequency allocation information that indicates respective frequency resources to be used by the respective client station for the uplink MU transmission. The access point transmits the trigger frame to the multiple client stations to trigger the uplink MU transmission by the multiple client stations.