A plurality of training fields of a preamble of a physical layer data unit are generated, and training fields of the plurality of training fields are mapped to respective signal streams of a plurality of streams using a mapping matrix. A signal field of the preamble of the data unit is generated, and the signal field is mapped to the plurality of signal streams using a column of the mapping matrix. The plurality of signal streams are mapped to a plurality of spatial streams, and at least i) the plurality of training fields and ii) the signal field are transmitted via the plurality of spatial streams.