Active geo-location for orthogonal frequency division multiplex wireless local area network devices
A method in a first wireless device (WD) is described. The method includes determining a plurality of expected response signals, each having a scrambling seed number and an expected sequence of time domain symbols; receiving a plurality of response signals; determining a plurality of frequency shifted time domain samples, cross-correlating each frequency shifted time domain sample with the expected sequence of time domain symbols, for each scrambling seed number of each expected response signal; determining a maximum correlation value for each frequency shift and each frequency shifted time domain sample; summing the maximum correlation value for each frequency shift, each frequency shifted time domain sample, and each response signal; determining a peak correlation value based on the summed maximum correlation value; and determining at least the round trip time associated with the plurality of ranging signals based at least in part on the peak correlation value.
1. A method in a first wireless device (WD) for determining at least a round trip time (RTT) associated with a plurality of ranging signals transmitted by the first WD to a second WD and a plurality of response signals received from the second WD in response to the plurality of ranging signals, the method comprising:
determining a plurality of expected response signals, each expected response signal having an expected sequence of time domain symbols;
for each response signal of the plurality of response signals:
determining a plurality of time domain samples from each response signal;
cross-correlating each time domain sample of the plurality of time domain samples with the expected sequence of time domain symbols of each expected response signal of the plurality of expected response signals;
determining a maximum correlation value at least for each time domain sample;
determining a peak correlation value based at least in part on the determined maximum correlation value; and
determining at least the RTT associated with the plurality of ranging signals based at least in part on the peak correlation value.
2. The method of claim 1 , wherein the method further includes:
transmitting the plurality of ranging signals, each ranging signal being transmitted at a first time value, the peak correlation value being associated with a second time value, the determining the at least RTT associated with the plurality of ranging signals being further based at least in part on the first and second time values; and
receiving a plurality of response signals from the second WD in response to the plurality of ranging signals.
3. The method of claim 1 , wherein the method further includes:
summing the maximum correlation value for each frequency shift, each time domain sample, and each response signal, the peak correlation value being determined further based on the summed maximum correlation value.
4. The method of claim 1 , wherein the cross-correlating uses a branched correlator including a plurality of cross-correlators and a plurality of parallel branches, and
each cross correlator corresponds to one parallel branch, each cross-correlator being tuned to a different frequency offset to cover a range of carrier frequency offsets, CFO.
5. The method of claim 4 , wherein time domain sample of the plurality of time domain samples from each response signal is saved in one matched filter corresponding to one cross-correlator of the plurality of cross-correlators.
6. The method of claim 1 , wherein the method further includes:
opening a reception window after each ranging signal is transmitted by the first WD to the second WD, at least one response signal being received after the reception window is opened, the determining the plurality of time domain samples being performed after the reception window is opened; and
closing the reception window, the cross-correlating being performed after the reception window is closed.
7. The method of claim 6 , wherein the method further includes:
determining a maximum correlation output for each time domain sample associated with the maximum correlation value;
calculating a rolling sum value for each frequency shifted time domain sample; and
determining a peak rolling sum correlation value based on the rolling sum value, the rolling sum value (RS) being defined as:
For AC={ac 1 ,ac 2 ,ac 3 , . . . ac K }
where additive correlation, AC, is a sum of all reception windows in a burst, and K=number of samples in the reception window,
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where s=samples in a rolling sum window, and,
for k=1 to K, ac k =Σ n=1 N ac nk
where N is the number of ranging signals in a burst.
8. The method of claim 1 , wherein the plurality of ranging signals is a burst of N transmissions of a series of bursts, there being a time interval (Tp) between a transmission of each ranging signal of the plurality of ranging signals and another ranging signal of the plurality of ranging signals that is immediately subsequent, there being a wait time (Tw) between each burst of N transmission, each ranging signal of the plurality of ranging signals being an orthogonal frequency division multiplex (OFDM) ranging signal.
9. The method of claim 1 , wherein one or more of:
each expected response signal further has a scrambling seed number, and the expected sequence of time domain symbols corresponds to the scrambling seed number;
the cross-correlating is performed for each scrambling seed number of each expected response signal;
the plurality of time domain samples is a plurality of frequency shifted time domain samples;
each frequency shifted time domain sample corresponding to a frequency shift of a plurality of frequency shifts; and
the maximum correlation value is determined for each frequency shift and each frequency shifted time domain sample.
10. A first wireless device (WD) configured to determine at least a round trip time (RTT) associated with a plurality of ranging signals transmitted by the first WD to a second WD and a plurality of response signals received from the second WD in response to the plurality of ranging signals, the WD comprising processing circuitry configured to:
determine a plurality of expected response signals, each expected response signal having an expected sequence of time domain symbols;
for each response signal of the plurality of response signals:
determine a plurality of time domain samples from each response signal;
cross-correlate each time domain sample of the plurality of time domain samples with the expected sequence of time domain symbols of each expected response signal of the plurality of expected response signals;
determine a maximum correlation value at least for each time domain sample;
determine a peak correlation value based at least in part on the determined maximum correlation value; and
determine at least the RTT associated with the plurality of ranging signals based at least in part on the peak correlation value.
11. The first WD of claim 10 , wherein the processing circuitry is further configured to:
cause transmission of the plurality of ranging signals, each ranging signal being transmitted at a first time value, the peak correlation value being associated with a second time value, the determining the at least RTT associated with the plurality of ranging signals being further based at least in part on the first and second time values; and
receive a plurality of response signals from the second WD in response to the plurality of ranging signals.
12. The first WD of claim 10 , wherein the processing circuitry is further configured to:
sum the maximum correlation value for each frequency shift, each time domain sample, and each response signal, the peak correlation value being determined further based on the summed maximum correlation value.
13. The first WD of claim 10 , wherein the cross-correlating uses a branched correlator including a plurality of cross-correlators and a plurality of parallel branches, and
each cross correlator corresponds to one parallel branch, each cross-correlator being tuned to a different frequency offset to cover a range of carrier frequency offsets, CFO.
14. The first WD of claim 13 , wherein time domain sample of the plurality of time domain samples from each response signal is saved in one matched filter corresponding to one cross-correlator of the plurality of cross-correlators.
15. The first WD of claim 10 , wherein the processing circuitry is further configured to:
open a reception window after each ranging signal is transmitted by the first WD to the second WD, at least one response signal being received after the reception window is opened, the determining the plurality of time domain samples being performed after the reception window is opened; and
close the reception window, the cross-correlating being performed after the reception window is closed.
16. The first WD of claim 15 , wherein the processing circuitry is further configured to:
determine a maximum correlation output for each time domain sample associated with the maximum correlation value;
calculate a rolling sum value for each frequency shifted time domain sample; and
determine a peak rolling sum correlation value based on the rolling sum value, the rolling sum value (RS) being defined as:
For AC={ac 1 ,ac 2 ,ac 3 , . . . ac K }
where additive correlation, AC, is a sum of all reception windows in a burst, and K=number of samples in the reception window,
For
k
=
(
s
-
1
)
2
to
K
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where s=samples in a rolling sum window, and,
for k=1 to K, ac k =Σ k =Σ n=1 N ac nk
where N is the number of ranging signals in a burst.
17. The first WD of claim 10 , wherein the plurality of ranging signals is a burst of N transmissions of a series of bursts, there being a time interval (Tp) between a transmission of each ranging signal of the plurality of ranging signals and another ranging signal of the plurality of ranging signals that is immediately subsequent, there being a wait time (Tw) between each burst of N transmission, each ranging signal of the plurality of ranging signals being an orthogonal frequency division multiplex (OFDM) ranging signal.
18. The first WD of claim 10 , wherein one or more of:
each expected response signal further has a scrambling seed number, and the expected sequence of time domain symbols corresponds to the scrambling seed number;
the cross-correlating is performed for each scrambling seed number of each expected response signal;
the plurality of time domain samples is a plurality of time domain samples;
each time domain sample corresponding to a frequency shift of a plurality of frequency shifts; and
the maximum correlation value is determined for each frequency shift and each time domain sample.
19. An airborne station comprising a first WD and being configured to determine at least a round trip time (RTT) associated with a plurality of ranging signals transmitted by the first WD to a second WD and a plurality of response signals received from the second WD in response to the plurality of ranging signals, the first WD comprising processing circuitry configured to:
determine a plurality of expected response signals, each expected response signal having an expected sequence of time domain symbols;
for each response signal of the plurality of response signals:
determine a plurality of time domain samples from each response signal;
cross-correlate each time domain sample of the plurality of time domain samples with the expected sequence of time domain symbols of each expected response signal of the plurality of expected response signals;
determine a maximum correlation value at least for each time domain sample;
sum the maximum correlation value for each time domain sample and each response signal;
determine a peak correlation value based on the summed maximum correlation value; and
determine at least the RTT associated with the plurality of ranging signals based at least in part on the peak correlation value.
20. The airborne station of claim 19 , wherein the cross-correlating uses a branched correlator including a plurality of cross-correlators and a plurality of parallel branches, each cross correlator corresponding to one parallel branch, each cross-correlator tuned to a different frequency offset to cover a range of carrier frequency offsets, CFO, each time domain sample of the plurality of time domain samples from each response signal being saved in one matched filter corresponding to one cross-correlator of the plurality of cross-correlators.