A method includes receiving data units each having a preamble with first and second preamble portions, detecting symbol constellation rotations of OFDM symbols in the first preamble portions, and determining, based on the detected rotations, whether the preambles conform to a first format. The method also includes, when it is determined that a preamble conforms to the first format, processing the second preamble portion according to the first format, and, when it is determined that a preamble does not conform to the first format, (i) determining whether information bits in the first preamble portion indicate a single- or multi-user data unit, (ii) when it is determined that the information bits indicate a single-user data unit, processing the second preamble portion according to a second format, and (iii) when it is determined that the information bits indicate a multi-user data unit, processing the second preamble portion according to a third format.