# US 8,787,191 B2 · Method and apparatus for determining distance in a Wi-Fi network > Agents: this data is available as JSON from the patentgrep API and MCP server — see https://patentgrep.com/llms.txt - Number: US 8,787,191 B2 - Document ID: US8787191B2 - Status: Granted - Kind: B2 - Application number: 13/297,202 - Filed: 2011-11-15 - Published: 2013-05-16 - Granted: 2014-07-22 - Earliest claimed: 2011-11-15 - Assignees: QUALCOMM Incorporated - Applicants: Qifan Chen; Kai Shi; Ning Zhang - Inventors: Qifan Chen; Kai Shi; Ning Zhang - Web page: https://patentgrep.com/patent/US8787191B2 ## CPC - H04W 24/08: Testing, {supervising or monitoring} using real traffic - G01S 7/40: Means for monitoring or calibrating - G01S 13/765: {with exchange of information between interrogator and responder} - H04J 3/0667: {Bidirectional timestamps, e.g. NTP or PTP for compensation of clock drift and for compensation of propagation delays (arrangements for monitoring round trip delays in packet switching networks H04L43/0864)} - H04W: WIRELESS COMMUNICATION NETWORKS (broadcast communication H04H; communication systems using wireless links for non-selective communication, e.g. wireless extensions H04M1/72) - G01S: RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES - H04J: MULTIPLEX COMMUNICATION (transmission in general H04B; peculiar to transmission of digital information H04L5/00; systems for the simultaneous or sequential transmission of more than one television signal H04N7/08; in exchanges H04Q11/00; stereophonic systems H04S) ## Abstract A method and apparatus for improving the accuracy of a round trip time (RTT) estimate between a first device and a second device are disclosed. The method involves calculating an acknowledgement correction factor and a unicast correction factor. These correction factors are used to compensate for symbol boundary time errors resulting from multipath effects. ## Claims ### Claim 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. ### Claim 2 (depends on 1) 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. ### Claim 3 (depends on 1) 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. ### Claim 4 (depends on 1) 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. ### Claim 5 (depends on 4) 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. ### Claim 6 (depends on 1) 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. ### Claim 7 (depends on 1) 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. ### Claim 8 (depends on 1) 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. ### Claim 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. ### Claim 10 (depends on 9) 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. ### Claim 11 (depends on 9) 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. ### Claim 12 (depends on 9) 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. ### Claim 13 (depends on 12) 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. ### Claim 14 (depends on 9) 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. ### Claim 15 (depends on 9) 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. ### Claim 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. ### Claim 17 (depends on 16) 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. ### Claim 18 (depends on 16) 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. ### Claim 19 (depends on 18) 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.