1. A method for generating a data unit for transmission via a communication channel, the method comprising: generating, at a communication device, a preamble of the data unit, including i) generating a set of training fields, and ii) mapping the set of training fields to a plurality of space-time streams consisting of a number of space-time streams, including when the number of space-time streams is four or less, mapping the set of training fields to the plurality of space-time streams according to a first space-time stream mapping matrix, and when the number of space-time streams is greater than four, mapping each training field in the set of training fields to the plurality of space-time streams according to one of one or more second space-time stream mapping matrices, wherein the first space-time stream mapping matrix is not a submatrix of at least one of the one or more second space-time stream mapping matrices; and transmitting, with the communication device, the data unit so that a receiver device can streams using channel information derived from use the set of training fields to estimate the communication channel.