Method and apparatus for determining distance in a Wi-Fi network

1. A method for improving accuracy of a round trip time (RTT) estimate between a first device and a second device, the method comprising:

transmitting a unicast packet from the first device to the second device;

determining a unicast correction factor (CORR_UNC) using a processor in the second device, wherein the unicast correction factor is to compensate for multipath effects upon the unicast packet;

embedding the unicast correction factor in an acknowledgment packet in the second device;

transmitting the acknowledgment packet from the second device to the first device;

determining an acknowledgment correction factor (CORR_ACK) using a processor in the first device, wherein the acknowledgment correction factor is to compensate for multipath effects upon the acknowledgment packet; and

calculating the RTT estimate in the first device using the unicast correction factor and the acknowledgment correction factor.

2. The method of claim 1 , wherein the RTT estimate is calculated as TOA−TOD−CORR_UNC−CORR_ACK−K, wherein TOA is a time of arrival of the acknowledgment packet at the first device, TOD is a time of departure of the unicast packet from the first device, and a K is a constant that embodies processing delays associated with the processor of the second device.

3. The method of claim 1 , further comprising:

selectively substituting the unicast correction factor for the acknowledgment correction factor if the second device does not support symbol boundary timing.

4. The method of claim 1 , wherein determining the acknowledgment correction factor further comprises:

ignoring the acknowledgment correction factor if a percentage of the unicast packets successfully transmitted to the second device is lower than a predetermined percentage.

5. The method of claim 4 , wherein determining the acknowledgment correction factor further comprises:

ignoring the acknowledgment correction factor only if an RSSI value of either the first device or the second device is greater than a predetermined value.

6. The method of claim 1 , wherein determining the acknowledgment correction factor further comprises:

estimating a specific modulation coding schedule (MCS) based upon an RSSI value of the second device.

7. The method of claim 1 , wherein determining the acknowledgment correction factor further comprises:

determining a cyclic shift diversity (CSD) value for the second device; and

subtracting a selected multiple of the determined CSD value from the acknowledgment correction factor.

8. The method of claim 1 , further comprising maintaining channel symmetry between the first and second devices by:

sending a first public action frame from the first device to the second device requesting the second device to commence a distance measurement operation;

entering the second device into a configuration mode in response to the first public action frame;

determining an antenna configuration for the first device;

configuring an antenna of the second device using the antenna configuration determined for the first device;

sending a special unicast packet from the first device to the second device to collect RTT information;

sending a second public action frame from the first device to the second device requesting the second device to complete the distance measurement operation; and

restoring the first device to its original antenna configuration.

9. A system for improving accuracy of a round trip time (RTT) estimate between a first device and a second device, comprising:

means for transmitting a unicast packet from the first device to the second device;

means for determining a unicast correction factor (CORR_UNC) using a processor in the second device, wherein the unicast correction factor is to compensate for multipath effects upon the unicast packet;

means for embedding the unicast correction factor in an acknowledgment packet in the second device;

means for transmitting the acknowledgment packet from the second device to the first device;

means for determining an acknowledgment correction factor (CORR_ACK) using a processor in the first device, wherein the acknowledgment correction factor is to compensate for multipath effects upon the acknowledgment packet; and

means for calculating the RTT estimate in the first device using the unicast correction factor and the acknowledgment correction factor.

10. The system of claim 9 , wherein the RTT estimate is calculated as TOA−TOD−CORR_UNC−CORR_ACK−K, wherein TOA is a time of arrival of the acknowledgment packet at the first device, TOD is a time of departure of the unicast packet from the first device, and a K is a constant that embodies processing delays associated with the processor of the second device.

11. The system of claim 9 , further comprising:

means for selectively substituting the unicast correction factor for the acknowledgment correction factor if the second device does not support symbol boundary timing.

12. The system of claim 9 , wherein the means for determining the acknowledgment correction factor further comprises:

means for ignoring the acknowledgment correction factor if a percentage of the unicast packets successfully transmitted to the second device is lower than a predetermined percentage.

13. The system of claim 12 , wherein the means for determining the acknowledgment correction factor further comprises:

means for ignoring the acknowledgment correction factor only if an RSSI value of either the first device or the second device is greater than a predetermined value.

14. The system of claim 9 , wherein the means for determining the acknowledgment correction factor further comprises:

means for estimating a specific modulation coding schedule (MCS) based upon an RSSI value of the second device.

15. The system of claim 9 , wherein the means for determining the acknowledgment correction factor further comprises:

means for determining a cyclic shift diversity (CSD) value for the second device; and

means for subtracting a selected multiple of the determined CSD value from the acknowledgment correction factor.

16. A method for improving accuracy of a round trip time (RTT) estimate between a first device and a second device, the method comprising:

receiving a unicast packet from the first device;

receiving an acknowledgment packet from the second device by the first device;

determining an acknowledgment correction factor (CORR_ACK) using a processor in the first device, wherein the acknowledgment correction factor is to compensate for multipath effects upon the acknowledgment packet, and wherein determining the acknowledgment correction factor further comprises:

determining a cyclic shift diversity (CSD) value for the second device; and

subtracting a selected multiple of the determined CSD value from the acknowledgment correction factor; and

calculating the RTT estimate in the first device using the acknowledgment correction factor.

17. The method of claim 16 , wherein the RTT estimate is calculated as TOA−TOD−2×CORR_ACK−K, wherein TOA is a time of arrival of the acknowledgment packet at the first device, TOD is a time of departure of the unicast packet from the first device, and a K is a constant that embodies processing delays associated with a processor of the second device.

18. The method of claim 16 , wherein determining the acknowledgment correction factor further comprises:

ignoring the acknowledgment correction factor if a percentage of the unicast packets successfully transmitted to the second device is lower than a predetermined percentage.

19. The method of claim 18 , wherein determining the acknowledgment correction factor further comprises:

ignoring the acknowledgment correction factor only if an RSSI value of either the first device or the second device is greater than a predetermined value.