1. A method, comprising:assigning, at an access point (AP) device of a wireless local area network (WLAN), a plurality of different orthogonal frequency division multiplexing (OFDM) sub-channel blocks to a plurality of client stations that are members of the WLAN;
receiving, at the AP device, respective independent data for the plurality of client stations;
generating, at the AP device, an orthogonal frequency division multiple access (OFDMA) data unit that includes a preamble and, for each sub-channel block in the plurality of OFDM sub-channel blocks:
a respective legacy portion of the preamble that spans only the OFDM sub-channel block, the legacy portion including a respective legacy signal field that indicates a duration of the OFDMA data unit,
a respective non-legacy signal field in the preamble, the respective non-legacy signal field spanning only the OFDM sub-channel block, and
respective independent data for a respective client station, the respective independent data included within the OFDM sub-channel block; and
transmitting, by the AP device, the OFDMA data unit.