Round trip time determination

1. A method of calculating round trip time (RTT) between a first wireless device and a second wireless device, the method comprising:

sending an 802.11 downlink (DL) trigger frame with the first wireless device, wherein the DL trigger frame comprises an indicated location for a synchronization sequence within uplink (UL) Multiple-Input Multiple-Output (MIMO) frames responsive to the DL trigger frame;

determining a first time, wherein the first time corresponds to a time at which the DL trigger frame was sent;

receiving, in response to the DL trigger frame at the first wireless device, a plurality of UL MIMO frames from a plurality of wireless devices, including a UL MIMO frame from the second wireless device, wherein the UL MIMO frame from the second wireless device has the synchronization sequence at the indicated location;

determining a second time corresponding to a time at which the UL MIMO frame from the second wireless device was received; and

determining the RTT between the first wireless device and the second wireless device based on the determined first time and the determined second time.

2. The method of claim 1 , further comprising determining the second time is further based on the synchronization sequence within the UL MIMO frame from the second wireless device.

3. The method of claim 2 , wherein the synchronization sequence directly follows a preamble of the UL MIMO frame from the second wireless device.

4. The method of claim 2 , wherein the synchronization sequence comprises:

a Zadofff-chu sequence,

a Long Training Field (LTF)-like sequence,

an M sequence,

a Gold sequence,

a Costas sequence,

a Bjork sequence set,

a Kasami sequence, or

a Zero-Correlation Zone sequence,

or a combination thereof.

5. The method of claim 1 , further comprising:

storing a value of the RTT;

receiving, at the first wireless device, an RTT request; and

in response to receiving the RTT request, sending the stored value of the RTT.

6. The method of claim 1 , wherein determining the RTT further comprises determining a short inter-frame spacing (SIFS) value.

7. The method of claim 6 , wherein the SIFS value is determined from information provided from the second wireless device to the first wireless device.

8. The method of claim 7 , wherein the provided information is included in the UL MIMO frame from the second wireless device.

9. The method of claim 6 , wherein the SIFS value is obtained from a database.

10. A method of calculating round trip time (RTT) between a first wireless device to a second wireless device, the method comprising:

receiving, with the second wireless device, an 802.11 downlink (DL) trigger frame sent by the first wireless device, wherein the DL trigger frame comprises an indicated location for a synchronization sequence within uplink (UL) Multiple-Input Multiple-Output (MIMO) frames responsive to the DL trigger frame;

determining, at the second wireless device, a UL MIMO frame length;

determining a synchronization sequence length based at least in part on the determined UL MIMO frame length;

and

sending, from the second wireless device to the first wireless device, the UL MIMO frame with the synchronization sequence at the indicated location within the UL MIMO frame and having the determined synchronization sequence length.

11. The method of claim 10 , wherein the indicated location for the synchronization sequence is immediately after a preamble of the UL MIMO frame.

12. The method of claim 10 , wherein the indicated location for the synchronization sequence is at an end of the UL MIMO frame.

13. The method of claim 10 , wherein determining the synchronization sequence length comprises determining a length of the synchronization sequence such the UL MIMO frame, including the synchronization sequence, reaches a certain length.

14. A first wireless device comprising:

a wireless communication interface configured to send an 802.11 downlink (DL) trigger frame, wherein the DL trigger frame comprises an indicated location for a synchronization sequence within uplink (UL) Multiple-Input Multiple-Output (MIMO) frames responsive to the DL trigger frame;

a memory; and

a processing unit communicatively coupled to the wireless communication interface and the memory, the processing unit configured to cause the first wireless device to:

determine a first time corresponding to a time at which the DL trigger frame was sent;

receive, in response to the DL trigger frame via the wireless communication interface, a plurality of UL MIMO frames from a plurality of wireless devices, including a UL MIMO frame from a second wireless device, wherein the UL MIMO frame from the second wireless device has the synchronization sequence at the indicated location;

determine a second time corresponding to a time at which the UL MIMO frame from the second wireless device was received; and

determine a round trip time (RTT) between the first wireless device and the second wireless device based on the determined first time and the determined second time.

15. The first wireless device of claim 14 , wherein the processing unit is further configured to cause the first wireless device to determine the second time further based on the synchronization sequence within the UL MIMO frame from the second wireless device.

16. The first wireless device of claim 14 , wherein the processing unit is further configured to cause the first wireless device to:

store a value of the RTT in the memory,

receive, via the wireless communication interface, an RTT request, and

in response to receiving the RTT request, send the stored value of the RTT via the wireless communication interface.

17. The first wireless device of claim 14 , wherein the processing unit is further configured to cause the first wireless device to determine the RTT by determining a short inter-frame spacing (SIFS) value.

18. The first wireless device of claim 17 , wherein the processing unit is configured to cause the first wireless device to determine the SIFS value from information provided from the second wireless device to the first wireless device.

19. The first wireless device of claim 18 , wherein the processing unit is configured to cause the first wireless device to extract the provided information from the UL MIMO frame from the second wireless device.

20. The first wireless device of claim 17 , wherein the processing unit is configured to cause the first wireless device to obtain the SIFS value from a database.

21. A second wireless device comprising:

a wireless communication interface configured to receive an 802.11 downlink (DL) trigger frame from a first wireless device, wherein the DL trigger frame comprises an indicated location for a synchronization sequence within uplink (UL) Multiple-Input Multiple-Output (MIMO) frames responsive to the DL trigger frame;

a memory; and

a processing unit communicatively coupled to the wireless communication interface and the memory, the processing unit configured to cause the second wireless device to:

determine a UL MIMO frame length;

determine a synchronization sequence length based at least in part on the determined UL MIMO frame length;

and

send, to the first wireless device via the wireless communication interface, the UL MIMO frame with the synchronization sequence at the indicated location within the UL MIMO frame and having the determined synchronization sequence length.

22. The second wireless device of claim 21 , wherein the processing unit is configured to cause the second wireless device to determine the synchronization sequence length comprises at least in part by determining a length of the synchronization sequence such that the UL MIMO frame, including the synchronization sequence, reaches a certain length.

23. A first wireless device comprising:

means for sending an 802.11 downlink (DL) trigger frame, wherein the DL trigger frame comprises an indicated location for a synchronization sequence within uplink (UL) Multiple-Input Multiple-Output (MIMO) frames responsive to the DL trigger frame;

means for determining a first time corresponding to a time at which the DL trigger frame was sent;

means for receiving, in response to the DL trigger frame, a plurality of UL MIMO frames from a plurality of wireless devices, including a UL MIMO frame from a second wireless device, wherein the UL MIMO frame from the second wireless device has the synchronization sequence at the indicated location;

means for determining a second time corresponding to a time at which the UL MIMO from the second wireless device frame was received; and

means for determining a round trip time (RTT) between the first wireless device and the second wireless device based on the determined first time and the determined second time.

24. The first wireless device of claim 23 , further comprising means for basing the determined second time on the synchronization sequence within the UL MIMO frame from the second wireless device.

25. The first wireless device of claim 23 , further comprising:

means for storing a value of the RTT,

means for receiving, at the first wireless device, an RTT request, and

means for, in response to receiving the RTT request, sending the stored value of the RTT.

26. The first wireless device of claim 23 , further comprising means for determining a short inter-frame spacing (SIFS) value.

27. The first wireless device of claim 26 , further comprising means for determining the SIFS value from information provided from the second wireless device to the first wireless device.

28. The first wireless device of claim 27 , further comprising means for extracting the provided information from the UL MIMO frame from the second wireless device.

29. The first wireless device of claim 26 , further comprising means for obtaining the SIFS value from a database.

30. A second wireless device, the comprising:

means for receiving an 802.11 downlink (DL) trigger frame sent by a first wireless device, wherein the DL trigger frame comprises an indicated location for a synchronization sequence within uplink (UL) Multiple-Input Multiple-Output (MIMO) frames responsive to the DL trigger frame;

means for determining an UL MIMO frame length;

means for determining a synchronization sequence length based at least in part on the determined UL MIMO frame length;

and

means for sending, to the first wireless device, the UL MIMO frame with the synchronization sequence at the indicated location within the UL MIMO frame and having the synchronization sequence length.

31. The second wireless device of claim 30 , further comprising means for locating the synchronization sequence immediately after a preamble of the UL MIMO frame.

32. The second wireless device of claim 30 , further comprising means for locating the synchronization sequence at an end of the UL MIMO frame.

33. The second wireless device of claim 30 , wherein the means for determining the synchronization sequence length comprises means for determining a length of the synchronization sequence such that the UL MIMO frame, including the synchronization sequence, reaches a certain length.

34. A non-transitory computer-readable medium having instructions embedded thereon for calculating round trip time (RTT) between a first wireless device to a second wireless device, the instructions including computer code for:

sending an 802.11 downlink (DL) trigger frame with the first wireless, wherein the DL trigger frame comprises an indicated location for a synchronization sequence within uplink (UL) Multiple-Input Multiple-Output (MIMO) frames responsive to the DL trigger frame;

determining a first time corresponding to a time at which the DL trigger frame was sent;

receiving, in response to the DL trigger frame at the first wireless device, a plurality of UL MIMO frames from a plurality of wireless devices, including a UL MIMO frame from the second wireless device, wherein the UL MIMO frame from the second wireless device has the synchronization sequence at the indicated location;

determining a second time corresponding to a time at which the UL MIMO frame from the second wireless device was received; and

calculating the RTT between the first wireless device and the second wireless device based on the determined first time and the determined second time.

35. The computer-readable medium of claim 34 , wherein the computer code for determining the second time includes computer code for basing the determination of the second time on the synchronization sequence within the UL MIMO frame from the second wireless device.

36. The computer-readable medium of claim 34 , further comprising computer code for:

storing a value of the RTT,

receiving, at the first wireless device, an RTT request, and

in response to receiving the RTT request, sending the stored value of the RTT.

37. A non-transitory computer-readable medium having instructions embedded thereon for calculating round trip time (RTT) between a first wireless device to a second wireless device, the instructions including computer code for:

receiving, with the second wireless device, an 802.11 downlink (DL) trigger frame sent by the first wireless device, wherein the DL trigger frame comprises an indicated location for a synchronization sequence within uplink (UL) Multiple-Input Multiple-Output (MIMO) frames responsive to the DL trigger frame;

determining, at the second wireless device, an UL MIMO frame length;

determining a synchronization sequence length based at least in part on the determined UL MIMO frame length;

and

sending, from the second wireless device to the first wireless device, the UL MIMO frame with the synchronization sequence at the indicated location within the UL MIMO frame and having the determined synchronization sequence length.

38. The computer-readable medium of claim 37 , further comprising computer code for determining a length of the synchronization sequence such that the UL MIMO frame, including the synchronization sequence, reaches a certain length.