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: generating a set of more than four training fields, and applying a different spatial mapping to each training field in the set of training fields; and generating a data portion of the data unit, wherein the data portion is transmitted using more than four space-time streams, so that a receiver device can receive the data portion via the more than four space-time streams using channel information derived from the set of training fields.