Active geo-location range for wireless local area network 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 burst of predetermined ranging packets and recording the correlation values of each received bit stream of the response packets with the corresponding predetermined bit streams for each of the transmitted ranging packets within a preset reception window. A rolling maximum is then performed on the correlation values in each reception window. At the end of the burst, the rolling maximum results are summed and the time of the peak value is determined.
1. A method in a first wireless device (WD), the method comprising:
transmitting a plurality of ranging signals in a burst of N transmissions, with a time Tp between each transmission, and with a wait time Tw between each burst;
recording times of transmission for each of the ranging signals in a burst;
setting a duration and start and end times of a reception window that starts at time Ts after the transmission of each ranging signal and ends at time it after the transmission of each ranging signal;
receiving direct sequence spread spectrum (DSSS) response signals transmitted from a second WD, each DSS response signal being in response to one of the plurality of ranging signals;
demodulating and de-spreading each of the received DSSS response signals to produce a received sequence of bits;
determining an expected sequence of bits for the received response signals;
correlating the received sequence of bits with the expected sequence of bits to determine a correlation value, the correlation value being indicative of an extent to which the received sequence matches the expected sequence;
recording a plurality of correlation values for a duration of a reception window for each of the plurality of ranging signal transmissions;
calculating a rolling maximum value, M, of the recorded correlation values for the duration of the reception window for each of the plurality of ranging signal transmissions;
detecting a completion of a burst of ranging signals;
calculating a sum of the rolling maximums;
determining a peak value of the sum of the rolling maximums;
recording a time of the peak value referenced to an end of the plurality of ranging signa transmissions; and
determining a geo-location of the second WI) based at least in part on a time of transmission of a ranging signal and the time of the peak value of the sum of the rolling maximums.
2. The method of claim 1 , wherein each 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 determining parameters of the plurality of ranging signals by one of selecting a WI) 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 for the duration of the reception window.
4. The method of claim 1 , wherein the determination of the expected sequence of bits includes:
constructing medium access control (MAC) header bits based on using a transmitter address (TA) of the ranging signal as a receiver address (RA) of the DSSS response signal;
adding preamble and header bits to the MAC header bits to produce a packet; and
scrambling the packet using a predetermined seed to produce the bits of the expected sequence.
5. The method of claim 1 , wherein the rolling maximum value M is defined as:
For
k
=
0
to
(
W-s
)
,
M
k
+
s
2
=
max
{
C
}
k
k
+
s
where W=number of samples in the reception window, C are correlator values, s=number of samples in the assumed maximum jitter of the response packet, and
max
{
C
}
k
k
+
s
is a maximum correlation value between k and k+s.
6. The method of claim 1 , further comprising applying soft bit detection of the received DSSS response signals.
7. A first wireless device (WD), comprising:
a transmitter configured to transmit a plurality of ranging signals in a burst of N transmissions, with a time Tp between each transmission, and a wait time Tw between each burst;
processing circuitry in communication with the transmitter, the processing circuitry configured to:
record a time of each transmission;
receive a plurality of direct sequence spread spectrum (DSSS) response signals, each DSSS response signal being responsive to a different one of the plurality of ranging signals;
demodulate and de-spread each of the plurality of received DSSS response signals to produce a received sequence of bits;
determine an expected sequence of bits for the received DSSS response signals;
a correlator in communication with the processing circuitry, the correlator configured to:
correlate the bits of the received sequence with the bits of the expected sequence to determine a correlation value, the correlation value being indicative of an extent to which the received sequence matches the expected sequence;
record the correlation values for a duration of a reception window for each of the plurality of ranging signal transmissions;
the processing circuitry being further configured to:
calculate a rolling maximum value, M, of the recorded correlation values for the duration of the reception window for each of the plurality of ranging signal transmissions;
detect a completion of a burst of ranging signals;
calculate a sum of the rolling maximums;
determine a peak value of the sum of the rolling maximums;
record a time of the peak value referenced to an end of the plurality of ranging signal transmissions; and
determine a geo-location of the second WI) based at least in part on a time of transmission of a ranging signal and the time of the peak value of the sum of the rolling maximums.
8. The first WD of claim 7 , wherein each transmitted ranging signal is one of a request-to-send, RTS, signal and a data null signal.
9. The first WD of claim 7 , wherein the processing circuitry is further configured to determine parameters of the plurality of ranging signals by 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 for the duration of the reception window.
10. The first WD of claim 7 , wherein the determination of the expected sequence of bits includes:
constructing medium access control (MAC) header bits based on using a transmitter address (TA) of the ranging signal as a receiver address (RA) of the DSSS response signal;
adding preamble and header bits to the MAC header bits to produce a packet; and
scrambling the packet using a predetermined seed to produce the bits of the expected sequence.
11. The first WD of claim 7 , wherein the rolling maximum value M is defined as:
For
k
=
0
to
(
W-s
)
,
M
k
+
s
2
=
max
{
C
}
k
k
+
s
where W=number of samples in the reception window, C are correlator values, s=number of samples in the assumed maximum jitter of the response packet, and
max
{
C
}
k
k
+
s
is a maximum correlation value between k and k+s.
12. The first WD of claim 7 , wherein the processing circuitry is further configured to apply soft bit detection of the received DSSS response signals.
13. An airborne station, comprising:
a transmitter configured to:
transmit a plurality of ranging signals in a burst of N transmissions, with a time Tp between each transmission, and a wait time Tw between each burst, a ranging signal being one of a request-to-send, RTS, signal and a data null signal;
record the time of each transmission, and
a receiver configured to:
receive a direct sequence spread spectrum, DSSS, response signal from a ground station, the DSSS response signal being spread by an 11 bit barker code, each DSSS response signal responsive to a different one of the plurality of ranging signals;
processing circuitry in communication with the transmitter and receiver, the processing circuitry configured to:
demodulate and de-spread each of the received DSSS response signals to produce a received sequence of bits;
obtain an expected sequence of bits for the received response signals;
a correlator in communication with the processing circuitry, the correlator configured to:
correlate the bits of the received sequence with bits of the expected sequence to determine a correlation value; the correlation value being indicative of an extent to which the received sequence matches the expected sequence;
record correlation values for a duration of a reception window for each of the plurality of ranging signal transmissions;
the processing circuitry being further configured to:
calculate a rolling maximum value, M, of the correlation values for the duration of the reception window for each of the plurality of ranging signal transmissions;
detect a completion of a burst of ranging signals;
calculate a sum of the rolling maximums;
determine a peak value of the sum of the rolling maximums;
record a time of the peak value referenced to an end of the ranging signal transmissions;
determine a geo-location of the second WD based at least in part on a time of transmission of a ranging signal and the time of the peak value of the sum of the rolling maximums.
14. An airborne station, comprising:
a transmitter configured to transmit a plurality of ranging signals in a burst of N transmissions;
a receiver configured to:
receive, for each ranging signal in the burst, a response signal from a ground station;
extract from each response signal a received sequence of bits; and
correlate bits of the received sequence with bits of an expected sequence to determine a correlation value, the correlation value being indicative of an extent to which the received sequence matches the expected sequence; and
processing circuitry in communication with the transmitter and receiver, the processing circuitry configured to:
calculate rolling maximum values M of correlation values determined for a reception window during which spread spectrum signals are received;
calculate a sum of the calculated rolling maximum values M;
determine a peak value of the calculated sum;
determine a geo-location of the ground station based at least in part on a time of transmission of a ranging signal and a time of occurrence of the peak value referenced to an end of the plurality of ranging signal transmissions.
15. The airborne station of claim 14 , wherein the response signal is a direct sequence spread spectrum signal (DSSS) and the extracting includes despreading.
16. The airborne station of claim 14 , wherein the rolling maximum value M is defined as:
For
k
=
0
to
(
W-s
)
,
M
k
+
s
2
=
max
{
C
}
k
k
+
s
where W=number of samples in the reception window, C are correlator values, s=number of samples in the assumed maximum jitter of the response packet, and
max
{
C
}
k
k
+
s
is a maximum correlation value between k and k+s.
17. The airborne station of claim 14 , wherein the processing circuitry is further configured to set a duration of the reception window based on an assumed short interframe spacing (SIFS) jitter.
18. The airborne station of claim 14 , wherein the rolling maximum values M are calculated over a duration equal to an expected short interframe spacing (SIFS) jitter.
19. The airborne station of claim 14 , wherein the expected sequence of bits is determined by steps that include:
constructing medium access control (MAC) header bits based on using a transmitter address (TA) of the ranging signal as a receiver address (RA) of the response signal;
adding preamble and header bits to the MAC header bits to produce a packet; and
scrambling the packet using a predetermined seed to produce the bits of the expected sequence.
20. The airborne station of claim 14 , wherein the processing circuitry is further configured to detect a completion of a burst of ranging signals.