Frequency offset compensation for WiFi ranging
Methods, systems, and apparatus are disclosed for determining accurate ranging measurements between communication devices. Various embodiments are described for recording timestamps associated with when transmissions are sent and received between the communication devices. The communication devices are configured to determine a difference in their clock frequencies and to communicate this difference with one another. Furthermore, each of the communication devices is configured to compensate for these differences before or after the timestamps are exchanged and to send a compensation indicator of whether the compensation has been performed. If the compensation has not been performed, either of the communication devices can compensate for the clock frequency differences after receiving the timestamps based on the compensation indicator. By using compensated clock frequencies based on a single clock reference, highly accurate ranging measurements are obtained using round trip propagation time calculations.
1. A method for clock compensation with respect to i) a first communication device utilizing a first clock having a first clock frequency, and ii) a second communication device utilizing a second clock having a second clock frequency, the method comprising:
determining, at the first communication device, a first timestamp corresponding to when a first transmission is transmitted by the second communication device;
determining, at the first communication device, a second timestamp corresponding to when a second transmission is received at the second communication device; and
determining, at the first communication device, whether a compensation indicator has been received from the second communication device; and
compensating, at the first communication device, for a difference between the first clock frequency and the second clock frequency to provide a compensated first timestamp and a compensated second timestamp when it is determined that the compensation indicator has not been received from the second communication device,
wherein a presence of the compensation indicator indicates that the first timestamp and second timestamps are received in terms of the first clock frequency and do not need to be compensated.
2. The method of claim 1 , further comprising:
determining, at the first communication device, a third timestamp corresponding to when the first transmission is received at the first communication device based on the first clock frequency; and
determining, at the first communication device, a fourth timestamp corresponding to when the second transmission is transmitted by the first communication device based on the first clock frequency.
3. The method of claim 2 , further comprising:
calculating, at the first communication device, a distance between the first communication device and the second communication device based on the compensated first timestamp, the compensated second timestamp, the third timestamp, and the fourth timestamp, when it is determined that the compensation indicator has not been received; and
calculating, at the first communication device, the distance between the first communication device and the second communication device based on the first timestamp, the second timestamp, the third timestamp, and the fourth timestamp, when it is determined that the compensation indicator has been received.
4. The method of claim 1 , wherein the act of compensating comprises:
evaluating a first equation:
[
t
1
(
2
)
=
(
t
1
(
1
)
)
(
1
+
ppm
1
-
2
)
]
for
t
1
(
2
)
;
and
evaluating a second equation:
[
t
2
(
2
)
=
(
t
2
(
1
)
)
(
1
+
ppm
1
-
2
)
]
for
t
2
(
2
)
,
wherein:
t 1 (1) and t 2 (1) represent the first timestamp and the second timestamp, respectively;
ppm 1-2 represents a difference between the first clock frequency and the second clock frequency in parts-per-million; and
t 1 (2) and t 2 (2) represent the compensated first timestamp and the compensated second timestamp, respectively.
5. The method of claim 1 , wherein the first transmission includes a fine timing measurement request, and
wherein the second transmission includes an acknowledgement of the fine timing measurement request.
6. The method of claim 1 , wherein each of the first transmission and the second transmission conforms to a wireless networking communication protocol.
7. A method for clock compensation between i) a first communication device utilizing a first clock having a first clock frequency, and ii) a second communication device utilizing a second clock having a second clock frequency, the method comprising:
determining, at the first communication device, a first timestamp corresponding to when a first transmission is transmitted by the first communication device;
determining, at the first communication device, a second timestamp corresponding to when a second transmission is received at the first communication device;
compensating, at the first communication device, for a difference between the first clock frequency and the second clock frequency to provide a compensated first timestamp and a compensated second timestamp;
sending, from the first communication device, the compensated first timestamp and the compensated second timestamp to the second communication device; and
sending, from the first communication device, a compensation indicator to the second communication device, the compensation indicator indicating that the second communication device does not need to compensate for the difference between the first clock frequency and the second clock frequency with respect to the compensated first timestamp and the compensated second timestamp.
8. The method of claim 7 , further comprising:
determining, at the second communication device, a third timestamp corresponding to when the first transmission is received at the second communication device based on the second clock frequency; and
determining, at the second communication device, a fourth timestamp corresponding to when the second transmission is transmitted by the second communication device based on the second clock frequency.
9. The method of claim 8 , further comprising:
calculating, at the second communication device, a distance between the first communication device and the second communication device based on the compensated first timestamp, the compensated second timestamp, the third timestamp, and the fourth timestamp.
10. The method of claim 7 , wherein the act of compensating comprises:
evaluating a first equation:
[
t
1
(
2
)
=
(
t
1
(
1
)
)
(
1
+
ppm
1
-
2
)
]
for
t
1
(
2
)
;
and
evaluating a second equation:
[
t
2
(
2
)
=
(
t
2
(
1
)
)
(
1
+
ppm
1
-
2
)
]
for
t
2
(
2
)
,
wherein:
t 1 (1) and t 2 (1) represent the first timestamp and the second timestamp, respectively;
ppm 1-2 represents a difference between the first clock frequency and the second clock frequency in parts-per-million; and
t 1 (2) and t 2 (2) represent the compensated first timestamp and the compensated second timestamp, respectively.
11. The method of claim 7 , wherein the first transmission includes a fine timing measurement request, and wherein the second transmission includes an acknowledgement of the fine timing measurement request.
12. The method of claim 11 , wherein each of the first transmission and the second transmission conforms to a wireless networking communication protocol.
13. A first communication device configured to utilize a first clock having a first clock frequency, the first communication device comprising:
a network interface configured to:
determine a first timestamp corresponding to when a first transmission is transmitted by a second communication device that utilizes a second clock having a second clock frequency, and
determine a second timestamp corresponding to when a second transmission is received at the second communication device; and
a clock calculation and compensation unit configured to:
determine whether a compensation indicator was received from the second communication device, and
compensate for differences between the first clock frequency and the second clock frequency to provide a compensated first timestamp and a compensated second timestamp when the compensation indicator is not received from the second communication device,
wherein a presence of the compensation indicator indicates that the first timestamp and the second timestamp are received in terms of the first clock frequency and do not need to be compensated.
14. The first communication device of claim 13 , wherein the clock calculation and compensation unit is further configured to not compensate for differences between the first clock frequency and the second clock frequency to provide the compensated first timestamp and the compensated second timestamp when the compensation indicator is received.
15. The first communication device of claim 13 , wherein the clock calculation and compensation unit is further configured to:
determine a third timestamp corresponding to when the first transmission is received at the first communication device based on the first clock frequency; and
determine a fourth timestamp corresponding to when the second transmission is sent to the second communication device based on the first clock frequency.
16. The first communication device of claim 15 , wherein the clock calculation and compensation unit is further configured to:
calculate a distance between the first communication device and the second communication device based on the compensated first timestamp, the compensated second timestamp, the third timestamp, and the fourth timestamp, when the compensation indicator is not received; and
calculate a distance between the first communication device and the second communication device based on the first timestamp, the second timestamp, the third timestamp, and the fourth timestamp, when the compensation indicator is received.
17. The first communication device of claim 13 , wherein the clock calculation and compensation unit is further configured to compensate for differences between the first clock frequency and the second clock frequency by:
evaluating a first equation:
[
t
1
(
2
)
=
(
t
1
(
1
)
)
(
1
+
ppm
1
-
2
)
]
for
t
1
(
2
)
;
and
evaluating a second equation:
[
t
2
(
2
)
=
(
t
2
(
1
)
)
(
1
+
ppm
1
-
2
)
]
for
t
2
(
2
)
,
wherein:
t 1 (1) and t 2 (1) represent the first timestamp and the second timestamp, respectively;
ppm 1-2 represents a difference between the first clock frequency and the second clock frequency in parts-per-million; and
t 1 (2) and t 2 (2) represent the compensated first timestamp and the compensated second timestamp, respectively.
18. The first communication device of claim 13 , wherein the network interface is configured to send the first transmission including a fine timing measurement request, and to receive the second transmission including an acknowledgement of the fine timing measurement request sent from the second communication device.
19. The first communication device of claim 13 , wherein each of the first transmission and the second transmission conforms to a wireless networking protocol.
20. A first communication device configured to utilize a first clock having a first clock frequency, the first communication device comprising:
a network interface configured to:
determine a first timestamp corresponding to when a first transmission is transmitted by the first communication device, and
determine a second timestamp corresponding to when a second transmission is received at the first communication device, the second transmission having been transmitted by a second communication device configured to utilize a second clock having a second clock frequency; and
a clock calculation and compensation unit configured to compensate for differences between the first clock frequency and the second clock frequency to provide a compensated first timestamp and a compensated second timestamp,
wherein the network interface is further configured to cause the first communication device to send the compensated first timestamp and the compensated second timestamp and a compensation indicator to the second communication device, the compensation indicator indicating that the compensated first timestamp and the compensated second timestamp do not need to be compensated by the second communication device.
21. The first communication device of claim 20 , wherein the network interface is configured to:
determine a third timestamp corresponding to when the first transmission is received at the second communication device; and
determine a fourth timestamp corresponding to when the second transmission is transmitted by the second communication device.
22. The first communication device of claim 21 , wherein the clock calculation and compensation unit is further configured to:
calculate a distance between the first communication device and the second communication device based on the compensated first timestamp, the compensated second timestamp, the third timestamp, and the fourth timestamp.
23. The first communication device of claim 20 , wherein the clock calculation and compensation unit is further configured to compensate the first timestamp and the second timestamp by:
evaluating a first equation:
[
t
1
(
2
)
=
(
t
1
(
1
)
)
(
1
+
ppm
1
-
2
)
]
for
t
1
(
2
)
;
and
evaluating a second equation:
[
t
2
(
2
)
=
(
t
2
(
1
)
)
(
1
+
ppm
1
-
2
)
]
for
t
2
(
2
)
,
wherein:
t 1 (1) and t 2 (1) represent the first timestamp and the second timestamp, respectively;
ppm 1-2 represents a difference between the first clock frequency and the second clock frequency in parts-per-million; and
t 1 (2) and t 2 (2) represent the compensated first timestamp and the compensated second timestamp, respectively.
24. The first communication device of claim 20 , wherein the network interface is configured to send the first transmission including a fine timing measurement request, and to receive the second transmission including an acknowledgement of the fine timing measurement request sent from the second communication device.
25. The first communication device of 20 , wherein each of the first transmission and the second transmission conforms to a wireless networking communication protocol.