A first portion of a physical layer (PHY) preamble of a PHY data unit is generated for transmission via a communication channel that comprises a plurality of sub-channels. A first portion of the PHY preamble is generated to include a legacy portion and a plurality of first non-legacy signal fields spanning the respective sub-channels. A second portion of a PHY preamble that immediately follows the first portion of the PHY preamble of the PHY data unit is generated to include: a non-legacy short training field spanning all sub-channels in the plurality of sub-channels, a plurality of non-legacy long training fields immediately following the non-legacy short training field, each non-legacy training field spanning all sub-channels in the plurality of sub-channels, and a second non-legacy signal field immediately following the plurality of non-legacy long training fields, the second-legacy signal field spanning all sub-channels in the plurality of sub-channels.