Two pass detection technique for non-echo pulsed ranging
A technique for non-echo pulsed ranging of a mobile device within an environment includes a plurality of emitters within the environment transmitting signal pulses of a predefined frequency to a mobile device that can receive a signal pulse, convert the signal pulse into a digital waveform, store the digital waveform into a buffer having a predetermined length of time, and analyze the digital waveform to detect a signal pulse at the predefined frequency. If a signal pulse is detected, the mobile device can re-analyze the digital waveform that was stored in the buffer to see if another signal pulse can be detected within the waveform, indicating the existence of a reflected signal. If another signal pulse is detected, the mobile device can select the signal pulse that was received first in time, i.e. the direct signal, for use in ranging of the mobile device.
1. A system for non-echo pulsed ranging, the system comprising:
a plurality of emitters at fixed locations within an environment, the emitters operable to transmit signal pulses of a predefined frequency; and
a mobile device operable to:
receive at least one of the signal pulses from the plurality of emitters,
convert the at least one of the signal pulses into a digital waveform,
store the digital waveform into a buffer having a predetermined length of time, and
perform a first analysis to analyze the digital waveform to detect a detected signal pulse at the predefined frequency;
wherein if the detected signal pulse is detected, the mobile device is further operable to perform a second analysis to re-analyze the digital waveform to attempt to detect another signal pulse at the predefined frequency, a presence of the another signal pulse indicating an existence of a reflected signal pulse, and
wherein if the another signal pulse is detected, the mobile device is further operable to select a selected signal pulse that was received first in time for use in ranging of the mobile device, the selected signal pulse being one of the detected signal pulse or the another signal pulse.
2. The system of claim 1 , wherein the predetermined length of time is an amount of time needed for a reflected signal to travel a maximum distance within the environment.
3. The system of claim 1 , wherein the mobile device uses a less selective technique for the first analysis and a more selective technique for the second analysis.
4. The system of claim 3 , wherein the first analysis uses a band pass amplifier.
5. The system of claim 3 , wherein the first analysis uses a Goertzel algorithm.
6. The system of claim 3 , wherein the second analysis uses a higher order filtering operation than the first analysis.
7. The system of claim 3 , wherein the second analysis uses a cross-correlation using a transmit mask.
8. The system of claim 3 , wherein the second analysis uses a Fast Fourier Transform operation.
9. The system of claim 1 , wherein a detection of the detected signal pulse at the first analysis directs the mobile device to wakeup circuitry needed to perform the second analysis.
10. The system of claim 1 , wherein the emitters transmit ultrasonic bursts having a frequency between 19 kHz and 22.05 kHz and the mobile device utilizes existing, unmodified audio circuitry to receive the ultrasonic bursts.