A wireless device initiates an Up-Link Trigger Based Multi-User communication by transmitting a trigger frame. The communication may be an Orthogonal Frequency Division Multiple Access communication. The trigger frame includes allocation information for resources of the communication, including Random Access (RA) allocation information. The trigger frame may also include a padding field after the allocation information. A length of the padding field is determined according to an amount of time needed for a station receiving the trigger frame to process the RA allocation information. The amount of time may correspond to a time between an end of a last RA allocation information and an end of a Physical layer Protocol Data Unit including the trigger frame. The amount of time may be determined according to a maximum processing time of stations associated with the wireless device, or may be determined according to a predetermined interval.