A first wireless communication device transmits a reference signal at a series of transmission times to transmit a plurality of reference signals over time. The first wireless communication device receives a communication signal from a second device. In response to determining at the first device that an acknowledgment response should be sent to the second device in response to the communication signal, the first wireless communication device applies a code to the reference signal for at least one of the transmission times of the series of transmission times. The resulting coded reference signal indicates an acknowledgment response to the second wireless communication device.