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doc.: IEEE 802.11-14/1263r0
Submission
Direction Finding Positioning for 802.11
Date: 2014-09-15
September 2014
James Wang, MediatekSlide 1
Name Company Address Phone email James Wang Mediatek 408-526-1899
88109 [email protected]
Gabor Bajko Mediatek [email protected]
Jianhan Liu Mediatek [email protected]
KC Chou Meidatek [email protected]
Tom Pare Mediatek [email protected]
Alvin Hsu Mediatek [email protected]
James Yee Mediatek [email protected]
Authors:
doc.: IEEE 802.11-14/1263r0
Submission
Baseline Fine-Timing Measurement (FTM)
REVmc supports FTM protocol.
STA1 exchange FTM frames with STA2 to measure time of flight (TOF) or the Round Trip Delay (RTD/2).
STA1 computes its range to STA2.
ToF = [(t4-t1) – (t3-t2)]/2Range = C * ToF
James Wang, Mediatek
September 2014
doc.: IEEE 802.11-14/1263r0
Submission
Baseline FTM Positioning• To calculate its position, STA1 performs range measurements to multiple APs (via FTM frame exchange) and STA1 obtains APs’ positions.• Baseline FTM positioning has the following drawbacks
– possibly involves switching to different channels in which APs operate on– consumes high power due to long session of FTM frame exchange– dense AP deployment is required– low scalability: FTM traffic load increases when more STAs perform positioning
James Wang, MediatekSlide 3
AP1AP2
AP3
AP4
Submission
STA1
September 2014
doc.: IEEE 802.11-14/1263r0
Submission
Direction Finding Positioning• Direction finding positioning is achieved from either
– AOD (Angle of Departure): The transmitter transmits through multiple antennas and the receiver (tracker) resolves the angle of departure (relative to the antenna platform orientation of the transmitter) based on the received signals.
– AOA (Angle of Arrival): Receiver (tracker) employs multiple antennas to receive signal and resolves angle of arrival relative to its antenna platform orientation.
• In both AoD and AoA, only one device with multiple antennas is needed.
• Only one way signal transmission is required for AoD or AoA.
AoAAoD
James Wang, MediatekSlide 4
September 2014
doc.: IEEE 802.11-14/1263r0
Submission
Typical Use Case: AoD Positioning
• AP with multiple antenna platform is deployed in strategic location such as ceiling or wall in a venue.
• AP broadcasts DF signals to STAs at scheduled instances or pseudo-periodically for AoD.
• STAs obtain AP location and antenna platform orientation information either a priori or via downloading from AP
• STAs perform AoD by listening to the broadcast DF signal from AP.
• STAs compute their position based on AoD
September 2014
James Wang, MediatekSlide 5
AP1
STA3
STA2
STA1
doc.: IEEE 802.11-14/1263r0
Submission
Another Use Case – Asset Tracking
• One or multiple APs equipped with multiple antennas are deployed in a venue.
• STA continue to transmit signal to APs.
• One or multiple APs tracks STA’s position via AoA measurements.
September 2014
James Wang, MediatekSlide 6
AP1
AP2
STA1
AoA
AoA
doc.: IEEE 802.11-14/1263r0
Submission
AoD and Position
James Wang, MediatekSlide 7
h θ
ϕ(x3,y3):3
(x2,y2):2
(x4,y4):4
(x1,y1):1
AP
STA
(x,y)Y
X
Z
)()(.)(sin)cos(
2
tntsetr k
Rj
k
kk
• AoD can be estimated from the received signals of the 4 antennas (k=1,2,3,4)
• STA’s location (x, y) can be solved using θ and ϕ if the AP antenna locations and the height (relative to the floor) are known
– h = r*cos(θ)– x = r*sin(θ)*cos(ϕ)– y = r*sin(θ)*sin(ϕ)
4,3,2,1)( 22 kyxR kkk
• ϕ is the angle from X-axis on X-Y plane and θ is the angle to Z axis.
• Algorithms such as Bartlett beamformer or MUSIC can be used to solve AoD
(Note that the STA height cannot be determined by AoD of a single AP.)
September 2014
doc.: IEEE 802.11-14/1263r0
Submission
Switched Antenna Implementation• In a switched antenna implementation, AP transmits through different
antennas at different time slots.• Receiver estimates the relative signal phases of antennas at different time
slots to compute the AoD.
Transmitter
Time Slot i
Time Slot i+1
Receiver estimate phase difference
James Wang, MediatekSlide 8
September 2014
doc.: IEEE 802.11-14/1263r0
Submission
Proposed 802.11 Signal for AoD
• In contrast to the switched antenna implementation, 802.11 devices have been using multiple antennas simultaneously for transmit or receive (e.g., MIMO, STBC)
• A multiple antenna 802.11 device can transmit signal preamble containing multiple LTF symbols, this allows the receiving device to resolve signals from different antennas.
• AoD of the transmit device can be solved by using the resolved signal from each antenna.
• Note no special signal waveform format is required for AoA.
September 2014
Slide 9 James Wang, Mediatek
doc.: IEEE 802.11-14/1263r0
Submission
Recover Signals by LTF Processing
• P matrix (NVHTLTF x NVHTLTF ) consists of a number of orthogonal row vectors, where NVHTLTF is number of LTF symbols.
• On the transmit device, non-pilot tones of each LTF symbol (n=0, 1, .. NVHTLTF -1) in a transmit antenna chain are multiplied by an entry from the row vector of the P matrix.
• On the receive device, the signal from each transmit antenna is recovered by the inner product of the corresponding row vector of the P matrix and received LTF symbols.
September 2014
James Wang, MediatekSlide 10
doc.: IEEE 802.11-14/1263r0
Submission
Radial Resolution Error
• Assume that an AP on the ceiling (7 meter height) has four antennas positioned at 4 corners of a 10 inchx10 inch platform
• Note that radial resolution error increases rapidly (reduced dilution of precision) at higher θ angles
• This limits the position accuracy when STA is away from AP (large r)
James Wang, MediatekSlide 11
Plot of every 2 degree in Φ, θ
h=7m
θ
Poor radial resolution
Increased radial
resolutionerror
September 2014
r
Higher θ
doc.: IEEE 802.11-14/1263r0
Submission
Combined DF Positioning and FTM Ranging
• DF positioning and FTM ranging can be jointly used to reduce radial resolution error (when r > r1) and extends the AoD service area.
• Accurate region can be significantly increased with combined DF and FTM ranging (Note that STA’s height can be determined)
September 2014
James Wang, MediatekSlide 12
Direction Finding
Only
Combined Direction Finding and FTM Ranging
STA2 (AP)
r1
rSTA1
• Inside the inside inner circle (green area), STA1 uses AoD to determine its position
• In the yellow annulus, STA1 uses both FTM ranging and AoD estimates to determine its position
• AoD determines the azimuth direction of STA1.
• Radial distance (r) of STA1 is determined by FTM ranging
doc.: IEEE 802.11-14/1263r0
Submission
Privacy Issues
• STA Privacy: AoD is a receive-only operation by STA. STA does not transmit signal to AP. AP does not know that STA is doing DF positioning and does not know its position.
• AP can preclude certain STAs from performing DF positioning by not releasing its position and antenna platform information to these STAs.
• However, an AP with AoA capability can track the STA position from STA’s transmit signal.– The only way to protect STA’s privacy is for a STA to conceal its
identity in its transmit signal.
September 2014
James Wang, MediatekSlide 13
doc.: IEEE 802.11-14/1263r0
Submission
Conclusions
• 802.11 direction finding positioning is a promising technique for determining the location in the proximity of an AP– A scalable solution if large number of devices in an area
– Low power consumption
– Privacy for the tracking devices
– Existing 802.11 signal format (e.g., LTF symbols) can be used.
Slide 14
September 2014
James Wang, Mediatek
