A physical layer (PHY) preamble of a multi-user (MU) PHY data unit is generated. The PHY preamble is generated to include formatting information regarding the MU PHY data unit, including a field to be transmitted within a first individual communication channel among a plurality of individual communication channels that collectively span a bandwidth of a composite channel, each individual communication channel including a plurality of basic resource allocation (RA) units that collectively span a bandwidth of the individual communication channel. The field is generated to indicate one or more groupings of basic RA units, wherein at least one of the one or more groupings of basic RA which includes basic RA units of one or more other individual communication channels. A data portion of the MU PHY data unit is generated according to the formatting information in the PHY preamble, and the MU PHY data unit is generated to include the PHY preamble and the data portion.