A communication device generates a physical layer (PHY) preamble of a multi-user (MU) PHY data unit. The PHY preamble includes a first signal field, and a second signal field. The second signal field includes: a common field that includes a value, selected from a codebook, that indicates i) an allocation of frequency resources to one or more resource units, wherein each resource unit corresponds to a respective block of consecutive orthogonal frequency division multiplexing (OFDM) tones, and ii) a respective number of receiving devices assigned to each resource unit. The second signal field also includes a plurality of user-specific fields that indicate an allocation of the one or more resource units to the multiple receiving devices, wherein each user-specific field corresponds to a respective receiving device and includes i) an identifier of the respective receiving device, and ii) an indication of which one or more spatial streams are allocated to the respective receiving device.