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 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 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, including: generating the first portion of the preamble to include a legacy signal field according to the second communication protocol; generating the second portion of the preamble to include a copy of the legacy signal field; and generating the second portion of the preamble to further include a non-legacy signal field, wherein at least two OFDM symbols of the non-legacy signal field are modulated using binary phase shift keying, BPSK, and one or more OFDM symbols of the non-legacy signal field are modulated using quadrature BPSK, Q-BPSK, wherein inclusion of the copy of the legacy signal field and the modulation of one or more OFDM symbols of the non-legacy signal field is for a receiving device that conforms to the first communication protocol to autodetect that the PHY data unit corresponds to the range extension mode; and generating the PHY data unit to include the preamble and the data field. generating 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 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, including: generating the first portion of the preamble to include a legacy signal field according to the second communication protocol; generating the second portion of the preamble to include a copy of the legacy signal field; and generating the second portion of the preamble to further include a non-legacy signal field, wherein at least two OFDM symbols of the non-legacy signal field are modulated using binary phase shift keying, BPSK, and one or more OFDM symbols of the non-legacy signal field are modulated using quadrature BPSK, Q-BPSK, wherein inclusion of the copy of the legacy signal field and the modulation of one or more OFDM symbols of the non-legacy signal field is for a receiving device that conforms to the first communication protocol to autodetect that the PHY data unit corresponds to the range extension mode; and generating the first portion of the preamble to include a legacy signal field according to the second communication protocol; generating the second portion of the preamble to include a copy of the legacy signal field; and generating the second portion of the preamble to further include a non-legacy signal field, wherein at least two OFDM symbols of the non-legacy signal field are modulated using binary phase shift keying, BPSK, and one or more OFDM symbols of the non-legacy signal field are modulated using quadrature BPSK, Q-BPSK, wherein inclusion of the copy of the legacy signal field and the modulation of one or more OFDM symbols of the non-legacy signal field is for a receiving device that conforms to the first communication protocol to autodetect that the PHY data unit corresponds to the range extension mode; and generating the PHY data unit to include the preamble and the data field.