A communication device generates a physical layer (PHY) data unit that includes a PHY preamble and one or more PHY midambles. The communication generates the PHY preamble of the PHY data unit to include i) a signal field having a subfield that indicates that the PHY data unit includes one or more PHY midambles, ii) a short training field (STF) for automatic gain control (AGC) training and synchronization at a receiver, and iii) one or more long training fields (LTFs) for determining a channel estimate at the receiver. The communication generates a data payload of the PHY data unit having i) a set of orthogonal frequency division multiplexing (OFDM) symbols, and ii) one or more PHY midambles. Each of the one or more PHY midambles includes one or more LTFs for determining an updated channel estimate. The communication device transmits the PHY data unit via a wireless communication channel.