A communication device determines that an extension field should be included in physical layer (PHY) data unit to provide a receiver with more processing time to process data included in the PHY data unit, wherein the extension field is not required to be processed by the receiver. The communication device generates i) a PHY preamble of the PHY data unit, and ii) a PHY data portion of the PHY data unit, the PHY data unit conforming to a first communication protocol. Each orthogonal frequency division multiplexing (OFDM) symbol in the PHY data portion is generated with a first tone spacing, which is a fraction 1/N of a second tone spacing defined by a second communication protocol, wherein N is a positive integer greater than one. The communication device also generates the extension field of the PHY data unit, which is appended to an end of the PHY data portion.