A first communication device generates a physical layer (PHY) preamble of a PHY data unit. A signal field of the PHY preamble includes i) a common block of information bits having information for multiple second communication devices and ii) a plurality of user blocks of information bits, each user block having information for a respective one of the multiple second communication devices. The common block includes a frequency resource unit allocation field that defines a plurality of frequency resource units corresponding to a PHY payload of the PHY data unit. The user blocks respectively correspond to frequency resource units defined by the frequency resource unit allocation field. Each user block includes an indication of a modulation and coding scheme (MCS) used in the corresponding frequency resource unit. Each group of two user blocks is encoded as a respective second encoded block to allow the multiple second communication devices to efficiently decode the signal field.