A first set of orthogonal frequency domain multiplexing (OFDM) symbols for a first portion of a PHY data unit and a second set of OFDM symbols for a second portion of the PHY data unit are generated. OFDM symbols of the first set are generated with a first OFDM tone spacing. At least some OFDM symbols of the second set are generated with a second tone spacing different from the first tone spacing. A value for a length indicator indicative of a duration of the PHY data unit is determined based on the first tone spacing and the second tone spacing. The first portion of the PHY data unit is generated to include (i) the first set of OFDM symbols and (ii) the length indicator set to the determined value. The second portion of the PHY data unit is generated to include the second set of OFDM symbols.