For a first group of devices having a first group size, a set of block allocations is selected from a codebook. A block allocation for the first group of devices is selected from the selected set of block allocations. A corresponding integer number of different orthogonal frequency division multiplexing (OFDM) tone blocks is assigned to each device of the first group of devices according to the allocation. An orthogonal frequency division multiple access (OFDMA) data unit to be transmitted to the first group of devices via the WLAN communication channel is generated using the assigned set of OFDM tone blocks. The OFDMA data unit includes a preamble portion and a data portion, the preamble portion having an index to the codebook that indicates i) the first group size, and ii) the selected block allocation.