Abstract: A device comprising: a receive antenna configured to receive radio-frequency (RF) signals having a first center frequency; a transmit antenna configured to transmit radio-frequency (RF) signals having a second center frequency that is a harmonic of the first center frequency; and a processor. The processor is configured to: generate a plurality of wake-up signals at a respective plurality of random times; and for each one of the plurality of wake-up signals, in response to generating the each one wake-up signal, cause the transmit antenna to transmit an RF signal having the second center frequency and indicating a code associated with the device to an interrogator device different from the device.