Orthogonal frequency division multiplex data unit generation and decoding

A communication device for decoding an orthogonal frequency division multiplex (OFDM) data unit, the communication device comprising:

• a network interface device having one or more integrated circuits configured to
• receive OFDM symbols of the OFDM data unit that include a first OFDM symbol that corresponds to a legacy signal field followed by a second OFDM symbol that corresponds to a candidate signal field,
• decode i) the first OFDM symbol to obtain a signal field bit sequence that corresponds to the legacy signal field, and ii) the second OFDM symbol to obtain a candidate bit sequence that corresponds to the candidate signal field,
• unscramble the candidate bit sequence with a scrambling bit sequence that corresponds to the first communication protocol to obtain an unscrambled bit sequence that corresponds to the candidate signal field,
• determine whether the candidate signal field corresponds to a repetition of the legacy signal field, including determining whether the unscrambled bit sequence is a repetition of the signal field bit sequence, and
• decode at least some of the OFDM symbols of the OFDM data unit according to i) a first communication protocol in response to a determination that the unscrambled bit sequence is the repetition of the signal field bit sequence, or ii) a second communication protocol in response to a determination that the unscrambled bit sequence is not the repetition of the signal field bit sequence.

A method for generating an orthogonal frequency division multiplex (OFDM) data unit, the method comprising:

• generating a first OFDM symbol that corresponds to a legacy signal field, the legacy signal field corresponding to a first communication protocol and having a signal field bit sequence;
• generating a second OFDM symbol that corresponds to a repetition of the legacy signal field to indicate a second communication protocol that is different from the first communication protocol, including
• scrambling the signal field bit sequence with a scrambling bit sequence to obtain a scrambled bit sequence, and
• generating the second OFDM symbol based on the scrambled bit sequence;
• generating a third OFDM symbol that corresponds to a non-legacy signal field, the non-legacy signal field corresponding to the second communication protocol;
• generating the OFDM data unit to include the first OFDM symbol, followed by the second OFDM symbol, followed by the third OFDM symbol.