An electronic device (such as an access point) is described. This electronic device transmits an orthogonal frequency division multiple access (OFDMA) frame to a recipient electronic device (such as a client or a station). The OFDMA frame includes multiple predefined resource units (RUs) allocated to the recipient electronic device in a set of predefined RUs having associated frequency bandwidths. Moreover, the multiple predefined RUs include two or more first predefined RUs having a first number of tones less than a predefined amount, or two or more second predefined RUs having a second number of tones greater than or equal to the predefined amount. For example, the predefined amount may include 242 tones. Note that the multiple predefined RUs may have the same or different numbers of tones. Moreover, the electronic may receive an acknowledgment or a block acknowledgment from the recipient electronic device.