A method performed in a base station for frequency calibration of a receiver is disclosed. The base station generates a first signal with periodic ASK modulation and transmits the first signal as a preamble or frequency marker signal to facilitate frequency calibration of the receiver. The receiver sweeps its oscillator frequency to detect if there is a preamble or frequency marker signal present. When it is determined that there is a signal present, the oscillator frequency of the receiver is calibrated to the transmitting frequency of the base station. The base station then transmits a second signal as a wakeup signal to the receiver.