A communication device determines an estimate of a communication channel, and determines, based on the estimate of the communication channel, a plurality of bit rates to be used for a data unit, including: determining a first bit rate for a first set of one or more orthogonal frequency division multiplexing (OFDM) subcarriers, and determining a second bit rate for a second set of one or more OFDM subcarriers, the second bit rate being different than the first bit rate. The communication device generates the data unit for transmission, wherein i) all data modulated on the first set of one or more OFDM subcarriers corresponds to the first bit rate and ii) all data modulated on the second set of one or more OFDM subcarriers corresponds to the second bit rate.