Active geo-location improvements for orthogonal frequency division multiplex wireless local area network devices

1. A method in a first wireless device (WD), the method comprising:

transmitting a ranging signal and recording a time of transmitting the ranging signal;

receiving an orthogonal frequency division multiplexing (OFDM) response signal transmitted from a second WD in response to the ranging signal;

determining an expected bit sequence for a coded and scrambled OFDM response signal for each of a plurality of scrambler seeds;

detecting the presence of an OFDM waveform in a sequence of received samples of the received OFDM response signal;

cross-correlating the received samples with a local copy of a preamble waveform and searching for a known periodicity of the preamble waveform;

demodulating coded bits in the OFDM response signal;

correlating bits of the sequence of received samples of the received OFDM response signal with bits of expected sequences to determine a correlation value for each of the plurality of scrambler seeds to produce a plurality of correlation values, each correlation value being indicative of an extent to which a sequence of received samples matches an expected sequence;

determining a maximum correlation value of the plurality of correlation values;

comparing the maximum correlation value to a predetermined threshold, T; and

when the maximum correlation value exceeds the threshold, T, then:

recording a time of reception of the received OFDM response signal; and

determining a geo-location of the second WD based at least in part on the time of transmission of the ranging signal and the time of reception of the OFDM response signal.

2. The method of claim 1 , wherein the transmitted ranging signal is one of a request-to-send (RTS) signal and a data null signal.

3. The method of claim 1 , further comprising:

using a branched preamble detector and using multiple cross correlators in parallel branches, each cross correlator tuned to a different carrier frequency offset (CFO) to produce the plurality of correlation values, each CFO being within a range of offsets permitted by an IEEE 802.11 Standard;

determining the branch that produces a largest correlation; and

estimating the CFO based upon the branch that produces the largest correlation.

4. The method of claim 1 , further comprising determining parameters of the ranging signal by at least one of:

selecting a WD receiver address (RA) to be used as a medium access control (MAC) address of the second WD;

selecting a MAC address to be used as a wireless transmitter address (TA); and

selecting a duration value.

5. The method of claim 1 , wherein a determination of the bits of the expected sequence includes:

constructing 24 bits of the signal field, the signal field including rate, length and parity;

constructing 16 bits of a service field;

constructing frame control, duration and RA fields;

calculating a 32 bit frame check sequence (FCS) field;

constructing the payload by appending the FCS field to the frame control, duration and RA fields; and

then, for each of the plurality of scrambler seeds:

scrambling the service and payload fields using the scrambler seed;

appending the signal field to the scrambled service and payload fields to form a 152 bit sequence; and

encoding the signal field and the scrambled service and payload fields to form a 304 bit sequence.

6. The method of claim 1 , wherein the threshold, T, depends at least in part on a bit error probability, Pb, the bit error probability, Pb, being based on a signal to noise ratio (SNR).

7. The method of claim 6 , wherein the threshold, T, in percent is given by

T= 100−(1−2 Pb ).

8. The method of claim 1 , further comprising receiving a subsequent spread spectrum signal when the threshold, T, is not exceeded for a predetermined time.

9. A first wireless device (WD), comprising:

a transceiver configured to:

transmit a ranging signal; and

receive an orthogonal frequency division multiplexing (OFDM) 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:

record a time of transmitting the ranging signal

determine an expected bit sequence for a coded and scrambled OFDM response signal for each of a plurality of scrambler seeds;

detect the presence of an OFDM waveform in a sequence of received samples of the received OFDM response signal;

cross-correlate the received samples with a local copy of a preamble waveform and searching for a known periodicity of the preamble waveform;

demodulate coded bits in the OFDM response signal;

correlate bits of the sequence of received samples of the received OFDM response signal with bits of expected sequences to determine a correlation value for each of the plurality of scrambler seeds to produce a plurality of correlation values, each correlation value being indicative of an extent to which a sequence of received samples matches an expected sequence;

determine a maximum correlation value of the plurality of correlation values;

compare the maximum correlation value to a predetermined threshold, T; and

when the maximum correlation value exceeds the threshold, T, then:

record a time of reception of the received OFDM response signal; and

determine a geo-location of the second WD based at least in part on the time of transmission of the ranging signal and the time of reception of the OFDM response signal.

10. The first wireless device of claim 9 , wherein the transmitted ranging signal is one of a request-to-send, RTS, signal and a data null signal.

11. The first wireless device of claim 9 , wherein the processing circuitry is further configured to:

use multiple cross correlators in parallel branches, each cross correlator tuned to a different carrier frequency offset (CFO) to produce the plurality of correlation values, each CFO being within a range of offsets permitted by an IEEE 802.11 Standard;

determine the branch that produces a largest correlation; and

estimate the CFO based upon the branch that produces the largest correlation.

12. The first wireless device of claim 9 , wherein the processing circuitry is further configured to determine parameters of the ranging signal by at least one of:

selecting a WD receiver address, RA, to be used as a medium access control (MAC) address of the second WD;

selecting a MAC address to be used as a wireless transmitter address (TA); and

selecting a duration value.

13. The first wireless device of claim 9 , wherein a determination of the bits of the expected sequence includes:

constructing 24 bits of the signal field, the signal field including rate, length and parity;

constructing 16 bits of a service field;

constructing frame control, duration and RA fields;

calculating a 32 bit frame check sequence (FCS) field;

constructing the payload by appending the FCS field to the frame control, duration and RA fields; and

then, for each of the plurality of scrambler seeds:

scrambling the service and payload fields using the scrambler seed;

appending the signal field to the scrambled service and payload fields to form a 152 bit sequence; and

encoding the signal field and the scrambled service and payload fields to form a 304 bit sequence.

14. The first wireless device of claim 9 , wherein the threshold, T, depends at least in part on a bit error probability, Pb, the bit error probability, Pb, being based on a signal to noise ratio (SNR).

15. The first wireless device of claim 14 , wherein the threshold, T, in percent is given by T=100 (1−2 Pb).

16. The first wireless device of claim 9 , wherein the transceiver is further configured to receive a subsequent spread spectrum signal when the threshold, T, is not exceeded for a predetermined time.

17. A first wireless (WD) device for determining a geo-location of a second WD, the first WD comprising:

a transceiver configured to:

transmit a ranging signal; and

receive an orthogonal frequency division multiplexing (OFDM) response signal transmitted from a second WD in response to the ranging signal and record a time of reception of the received OFDM response signal; and

processing circuitry in communication with the transceiver, the processing circuitry configured to:

record a time of transmitting the ranging signal;

determine an expected sequence for a coded and scrambled signal field of the OFDM response signal for each of a plurality of possible scrambler seeds, one of which scrambles the signal field, to produce a plurality of expected sequences;

detect the presence of an OFDM waveform in a sequence of received samples of the received OFDM response signal;

cross-correlate the received samples with a local copy of a preamble waveform and search for a known periodicity of the preamble waveform;

demodulate the coded bits in the OFDM response signal;

correlate a received sequence of samples of the received OFDM response signal with the expected sequences to determine a correlation value for each of the plurality of scrambler seeds to produce a plurality of correlation values;

determine a maximum correlation value among the plurality of correlation values;

compare the maximum correlation value to a predetermined threshold, T; and

when the maximum correlation value exceeds the threshold, T, determine a geo-location of the second WD based at least in part on the time of transmission of the ranging signal and the time of reception of the OFDM response signal.

18. The first wireless device of claim 17 , wherein the threshold, T, depends at least in part on a bit error probability, Pb.

19. The first wireless device of claim 18 , wherein the bit error probability, Pb, is based on a signal to noise ratio (SNR).

20. The first wireless device of claim 17 , wherein the processing circuitry is further configured to receive a subsequent spread spectrum signal when the threshold, T, is not exceeded for a predetermined time.