In a method for generating a preamble of a data unit for transmission via a communication channel, an indication of a first number of spatial or space-time streams is included in a first field of the preamble. The first number of spatial or space-time streams corresponds to transmission of the data unit to a first receiver. One or more training sequences are included in a second field of the preamble. The preamble is formatted such that the first field of the preamble will be transmitted prior to the second field of the preamble being transmitted.