A preamble, first and second portions of a data payload of a single data unit, and a midamble included between the first and second portions are generated. The midamble includes calibration information, and is based on a maximum number of space-time streams of a communication channel. A network interface of a communication device generates the preamble, the first and second portions of the data payload, and the midamble. A preamble, first and second portions of a data payload of a single data unit, and a midamble are received. The midamble includes calibration information, and is based on a maximum number of space-time streams. A first and/or a second characteristic is updated based on the midamble. A network interface of a communication device receives the preamble, the first and second portions of the data payload, and the midamble; and updates the first or the second characteristic based on the midamble.