1. A method for generating a data unit for transmission via a communication channel, the method comprising: generating a preamble of the data unit, including i) generating a set of training fields, and ii) mapping each training field in the set of training fields to a plurality of space-time streams, including when the set of training fields consist of four training fields, mapping each training field in the set of training fields to four space-time streams according to a first space-time stream mapping matrix, and when the set of training fields consists of six training fields, mapping each training field in the set of training fields to six space-time streams according to a second space-time stream mapping matrix, wherein the first space-time stream mapping matrix is not a submatrix of the second space-time stream mapping matrix; and generating a data portion of the data unit so that a receiver device can receive the data portion via a corresponding number of space-time streams using channel information derived from the set of training fields.