A motion detection method comprising: receiving, at a wireless sensor device (210, 212) in a space and at a first time, a first received wireless signal based on a first transmission of a transmitted wireless signal, the first transmission being transmitted by a source device; determining, by operation of a processor, a first characteristic of frequency components in a bandwidth of a first signal, the first characteristic comprising a first set of complex values for the frequency components in the bandwidth of the first signal, the first signal being based on the first received wireless signal; receiving, at the wireless sensor device (210, 212) and at a second, later time, a second received wireless signal based on a second transmission of the transmitted wireless signal, the second transmission being transmitted by the source device; determining, by operation of the processor, a second characteristic of frequency components in the bandwidth of a second signal, the second characteristic comprising a second set of complex values for the frequency components in the bandwidth of the second signal, the second signal being based on the second received wireless signal; wherein the source device is a WiFi source device that transmits the first and second transmission according to a WiFi communication standard; wherein determining the first characteristic and the second characteristic comprises transforming, respectively, the first signal and the second signal to a frequency domain using a Fast Fourier Transform algorithm, the first set of complex values represents magnitudes and phases of the frequency components in the bandwidth of the first signal, and the second set of complex values represents magnitudes and phases of the frequency components in the bandwidth of the second signal; and detecting movement of an object (214a, 214b) in the space based on a comparison between the first characteristic and the second characteristic, wherein detecting movement comprises detecting that movement has occurred in response to a difference between the first characteristic and the second characteristic exceeding a threshold. receiving, at a wireless sensor device (210, 212) in a space and at a first time, a first received wireless signal based on a first transmission of a transmitted wireless signal, the first transmission being transmitted by a source device; determining, by operation of a processor, a first characteristic of frequency components in a bandwidth of a first signal, the first characteristic comprising a first set of complex values for the frequency components in the bandwidth of the first signal, the first signal being based on the first received wireless signal; receiving, at the wireless sensor device (210, 212) and at a second, later time, a second received wireless signal based on a second transmission of the transmitted wireless signal, the second transmission being transmitted by the source device; determining, by operation of the processor, a second characteristic of frequency components in the bandwidth of a second signal, the second characteristic comprising a second set of complex values for the frequency components in the bandwidth of the second signal, the second signal being based on the second received wireless signal; wherein the source device is a WiFi source device that transmits the first and second transmission according to a WiFi communication standard; wherein determining the first characteristic and the second characteristic comprises transforming, respectively, the first signal and the second signal to a frequency domain using a Fast Fourier Transform algorithm, the first set of complex values represents magnitudes and phases of the frequency components in the bandwidth of the first signal, and the second set of complex values represents magnitudes and phases of the frequency components in the bandwidth of the second signal; and detecting movement of an object (214a, 214b) in the space based on a comparison between the first characteristic and the second characteristic, wherein detecting movement comprises detecting that movement has occurred in response to a difference between the first characteristic and the second characteristic exceeding a threshold.