Wireless vibrometer with antenna array
A wireless vibrometer employs an antenna array to significantly boost the signal-to-noise ratio of faint received signals twin small objects vibrating at acoustic frequencies. This technique may be used to provide an improved physiological monitor (such as a pulse detector) or for long-range eavesdropping using the emitted power from a cell phone or the like.
1. A wireless vibrometer comprising:
an antenna array having antennas distributed over at least one dimension;
a transmitter connectable to given antennas of the array, the transmitter transmitting a transmitter radiofrequency signal and shifting at least one of a relative phase and amplitude of the transmitter radiofrequency signal transmitted from each given antenna according to a transmission weight associated with each given antenna, the shifting being manifest in the transmitter radiofrequency signal as it is transmitted;
a receiver connectable to given antennas of the array, the receiver receiving a reflection of the radiofrequency signal from the array and shifting at least one of a relative phase and amplitude of a reflection of the radiofrequency signal after receipt of the radiofrequency signal from each given antenna according to a reception weight associated with each given antenna before combining the reflection radiofrequency signals to a received signal; and
an electronic computer executing a program stored in a non-transitive medium to:
(a) extract an audio signal from the received signal;
(b) evaluate the audio signal to adjust the transmission weights and reception weights to provide a processed audio signal with improved signal-to-noise ratio; and
(c) output a measure of the processed audio signal.
2. The wireless vibrometer of claim 1 further including a housing for supporting the antenna array adjacent to a person's skin to direct transmitted radiofrequency signals into the skin and to receive reflection radio signals reflected out of the skin.
3. The wireless vibrometer of claim 2 wherein the housing provides an adjustable band for passing around a limb of a human to retain the housing against the limb.
4. The wireless vibrometer of claim 3 wherein the electronic computer extracts a dominant frequency of the processed audio signal within a pulse rate range of the human heart and the measure of the processed audio signal is a pulse rate.
5. The wireless vibrometer of claim 4 wherein the array provides antennas dispersed in two dimensions.
6. The wireless vibrometer of claim 5 wherein the antenna array extends over an area of less than 2.5 square inches.
7. The wireless vibrometer of claim 1 wherein the electronic computer selects the transmission weights and the reception weights by cycling through a limited set of discrete transmission weights and reception weights to select transmission weights and reception weights according to a maximization of the audio range of the received signal provided by the selected transmission weights and reception weights.
8. The wireless vibrometer of claim 7 wherein the limited set of discrete transmission weights and reception weights provides for a range of amplitude weighting of no less than 2 to 1 in no more than 100 weight increments.
9. The wireless vibrometer of claim 7 wherein the limited set of discrete transmission weights and reception weights provides for a range of phase weighting of no less than 180 degrees in no more than 100 weight increments.
10. The wireless vibrometer of claim 1 wherein the electronic computer:
(i) transmits a radio signal from an antenna while cycling through a limited set of discrete transmission weights to select first transmission weights according to a maximization of the audio range of the received signal; and
(ii) uses the first transmission weights as reception weights while cycling through the limited set of discrete transmission weights to select second transmission weights according to a maximization of a measure of the audio signal of the received signal.
11. The wireless vibrometer of claim 10 wherein the measure of the audio signal is a measure of signal-to-noise ratio.
12. The wireless vibrometer of claim 1 wherein the electronic computer further controls a frequency of the transmitter and receiver and cycles through a discrete set of transmission frequencies to select a transmission frequency for obtaining the extracted audio signal according to a maximization of a measure of the audio signal of the received signal.
13. The wireless vibrometer of claim 12 wherein the measure of the audio signal is a measure of signal-to noise ratio.
14. The wireless vibrometer of claim 1 wherein the frequency of the transmitter and receiver is selected from a frequency range of 500 megahertz to five gigahertz.