Range extension mode for WiFi

A method for generating a physical layer (PHY) data unit for transmission via a communication channel, the PHY data unit conforming to a first communication protocol, the method comprising:

• generating, by a first communication device, orthogonal frequency division multiplexing (OFDM) symbols for a data field of the PHY data unit according to a range extension coding scheme that corresponds to a range extension mode of the first communication protocol;
• generating, by the first communication device, a preamble of the PHY data unit, the preamble having i) a first portion that indicates a duration of the PHY data unit and ii) a second portion that indicates whether at least some OFDM symbols of the data field are generated according to the range extension coding scheme, the first portion of the preamble being formatted such that the first portion of the preamble is decodable by a receiver device that conforms to a second communication protocol, but does not conform to the first communication protocol, to determine the duration of the PHY data unit based on the first portion of the preamble;
• generating, by the first communication device, the PHY data unit to include the preamble and the data field;
• and
• causing the PHY data unit to be transmitted from the first communication device to a second communication device;
• wherein:
• generating the preamble comprises generating OFDM symbols for the second portion of the preamble that include i) a non-legacy signal field, and ii) a copy of the non-legacy signal field, wherein the copy of the non-legacy signal field indicates that the at least some OFDM symbols of the data field are generated according to the range extension coding scheme;
• generating the preamble further comprises i) generating the first portion of the preamble, and ii) generating the second portion of the preamble;
• and
• generating the first portion of the preamble comprises generating a first legacy signal field for the PHY data unit according to the second communication protocol and generating a second legacy signal field as a copy of the first legacy signal field.