Direction finding and FTM positioning in wireless local area networks
A method of combined direction finding (DF) and fine timing measurement (FTM) positioning in a wireless location area network (WLAN) is proposed. A multiple antenna IEEE 802.11 transmitting device (AP) can transmit signal preamble containing multiple Long Training Field (LTF) symbols in a radio frame from multiple antennas, which allows a receiving device (STA) to resolve multiple DF sounding signals transmitted from the multiple antennas and thereby estimating angle of departure (AoD). On the other hand, the AP can estimate angle of arrival (AoA) from radio signals transmitted from the STA. When the radial resolution error of AoD or AoA positioning increases, DF positioning and fine-timing measurement (FTM) ranging can be jointly applied to reduce the radial resolution error and extends the AoD/AoA service area with positing accuracy.
1. A method comprising:
(a) receiving a plurality of direction finding (DF) sounding signals transmitted from a first wireless device via multiple antenna by a second wireless device in a wireless local area network (WLAN);
(b) estimating an angle of departure (AoD), which is an azimuth angle of the first wireless device from the second wireless device, based on the plurality of DF sounding signals;
(c) determining a location of the second wireless device based on the estimated AoD;
(d) when a radial resolution error of the AoD estimation is larger than a threshold, performing a fine-timing measurement (FTM) procedure with the first wireless device to determine a radial distance between the second wireless device and the first wireless device through transmitting an FTM request to the first wireless device by the second wireless device; and
(e) updating the location of the second wireless device based on both the estimated AoD and the radial distance determined by the FTM procedure.
2. The method of claim 1 , further comprising:
obtaining location information, antenna platform orientation information, and antenna positions and/or spatial signature of the multiple antennas of the first wireless device.
3. The method of claim 1 , further comprising:
receiving a null data packet announcement (NDPA) frame from the first wireless device indicating the immediate subsequent transmission of the DF sounding signals.
4. The method of claim 1 , further comprising:
receiving a fine-timing measurement (FTM) frame from the first wireless device, wherein an indication field of the FTM frame is set to a predefined value.
5. The method of claim 1 , wherein the radial resolution error is based on an AoD service area provided by the first wireless device.
6. The method of claim 1 , wherein the radial resolution error is determined based on an offset angle of the second wireless device relative to an antenna boresight of the antenna platform of the first wireless device.
7. The method of claim 1 , wherein the radial resolution error is determined based on the distance between the second wireless device and the first wireless device.
8. The method of claim 1 , wherein the radial resolution error is determined based on received signal strength indicator (RSSI) information.
9. The method of claim 1 , where the second wireless device first performs the FTM ranging in (d) before performing the AoD estimate in (a) and (b).
10. The method of claim 9 , where FTM frames during the FTM ranging in (d) contain multiple long training field (LTF) symbols for the subsequent AoD estimate, and wherein the subsequent AoD is indicated by an indicator in an FTM frame set to a pre-determined value.
11. A wireless station comprising:
a receiver that receives a plurality of direction finding (DF) sounding signals transmitted from an access point (AP) station via multiple antennas in a wireless local area network (WLAN);
a storage device, arranged to store a program code; and
a processor, arranged to execute the program code;
wherein when loaded and executed by the processor, the program code instructs the processor to execute the following steps:
estimating an AoD, which is an azimuth angle of the station from the AP, based on the plurality of DF sounding signals;
determining a location of the station based on the estimated AoD;
when a radial resolution error is larger than a threshold, performing a fine-timing measurement (FTM) procedure with the AP through transmitting an FTM request to the AP to determine a radial distance between the station and the AP; and
updating the location of the station based on both the estimated AoD and the radial distance determined by the FTM procedure.
12. The wireless station of claim 11 , wherein the receiver obtains AP location information, AP antenna platform orientation information, and antenna position information of the multiple antennas.
13. The wireless station of claim 11 , wherein the receiver also receives a null data packet announcement (NDPA) frame from the AP station indicating the immediate subsequent transmission of the DF sounding signals.
14. The wireless station of claim 11 , wherein the receiver also receives a fine-timing measurement (FTM) request frame from the AP station indicating the transmission of the DF sounding signals, wherein a status indication field of the FTM request frame is set to a predefined value.