Active geo-location range for WLAN devices
A method and devices are disclosed that increase the range of active geo-location from the airborne measuring station as compared with known methods by increasing the effective receive sensitivity of the airborne measuring station. In one embodiment this may be accomplished by transmitting a predetermined ranging packet and correlating the raw received bit stream of the response packet with the predetermined bit stream. In one embodiment, the disclosed method applies to the reception of IEEE 802.11 direct sequence spread spectrum DSSS ACK and DSSS CTS packets in response to DSSS data null and DSSS RTS packets respectively, in the 2.4 GHz band.
1. A method for a first wireless device (WD), the method comprising:
transmitting a ranging signal;
receiving a response signal transmitted from a second WD in response to the ranging signal;
extracting a received sequence from the response signal;
correlating the received sequence with an expected sequence to determine a correlation value;
comparing the correlation value to a threshold; and
when the correlation value exceeds the threshold, then determining a geo-location of the second WD based at least in part on a time of transmitting the ranging signal and a time of receiving the response signal.
2. The method of claim 1 , wherein, when the correlation value does not exceed the threshold for a first period of time, then transmitting another ranging signal to the second WD without first determining a geo-location of the second WD.
3. The method of claim 2 , wherein, when the correlation value does not exceed the threshold for a second period of time, then transmitting another ranging signal to a third WD without first determining a geo-location of the second WD.
4. The method of claim 1 , wherein, when the correlation value does not exceed the threshold for a first period of time, then transmitting another ranging signal to a third WD without first determining a geo-location of the second WD.
5. The method of claim 1 , wherein the threshold is based at least in part on a correlation percent selected to achieve a desired bit error probability, Pb.
6. The method of claim 1 , wherein the threshold is set to between 20% and 30% to achieve greater than 97% detection of packets of 304 bits.
7. The method of claim 1 , wherein the transmitted ranging signal is one of a request-to-send (RTS) signal and a data null signal.
8. The method of claim 1 , wherein the threshold depends at least in part on a bit error probability (Pb), Pb being based on a signal to noise ratio (SNR).
9. The method of claim 8 , wherein the threshold is set to correspond to a selected probability that the response signal is correctly detected.
10. A first wireless device (WD), comprising:
a transceiver configured to:
transmit a ranging signal; and
receive a response signal transmitted from a second WD in response to the ranging signal; and
processing circuitry in communication with the transceiver, the processing circuitry configured to:
extract a received sequence from the response signal;
correlate the received sequence with an expected sequence to determine a correlation value;
compare the correlation value to a threshold; and
when the correlation value exceeds the threshold, then determine a geo-location of the second WD based at least in part on a time of transmitting the ranging signal and a time of receiving the response signal.
11. The WD of claim 10 , wherein, when the correlation value does not exceed the threshold for a first period of time, the transceiver is configured to transmit another ranging signal to the second WD without first determining a geo-location of the second WD.
12. The WD of claim 11 , wherein, when the correlation value does not exceed the threshold for a second period of time, then the transceiver is configured to transmit another ranging signal to a third WD without first determining a geo-location of the second WD.
13. The WD of claim 10 , wherein, when the correlation value does not exceed the threshold for a first period of time, then the transceiver is configured to transmit another ranging signal to a third WD.
14. The WD of claim 10 , wherein the threshold is based at least in part on a correlation percent selected to achieve a desired bit error probability, Pb.
15. The WD of claim 14 , wherein the threshold is set to between 20% and 30% to achieve greater than 97% detection of packets of 304 bits.
16. The WD of claim 10 , wherein the transmitted ranging signal is one of a request-to-send (RTS) signal and a data null signal.
17. The WD of claim 10 , wherein the threshold depends at least in part on a bit error probability (Pb), Pb being based on a signal to noise ratio (SNR).
18. A method for a first wireless device (WD), the method comprising:
transmitting a ranging signal and recording a time of transmitting the ranging signal;
receiving a response signal transmitted from a second WD in response to the ranging signal;
extracting a received sequence from the response signal, extracting the received sequence from the response signal including de-spreading the response signal by a known sequence;
correlating the received sequence with an expected sequence to determine a correlation value;
comparing the correlation value to a threshold; and
when the correlation value exceeds the threshold, then determining a geo-location of the second WD based at least in part on the time of transmitting the ranging signal and a time of receiving the response signal, determining the geolocation of the second WD including determining a time between an ending of the transmitted ranging signal and an ending of a reception of the transmitted ranging signal.
19. The method of claim 18 , wherein the threshold depends at least in part on a bit error probability (Pb), Pb being based on a signal to noise ratio (SNR).