Transcript Slide 1
Novel Multiple-Antenna Systems
Mati Wax
Topics
Location Fingerprinting
Beamforming and SDMA for outdoors WLAN
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Location Fingerprinting
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What is Location Fingerprinting?
A position location technology for rich multipath
environments
The key idea:
The characteristics of the multipath
from a location to the base station
antenna-array can serve as a
unique identifier –
i.e., as the location “fingerprint”
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What is it Good For?
A single-site location technology
− All other network-based techniques require multiple sites
Excels where all other position location techniques suffer
− Multipath is a major impediment to all position location techniques
GPS, DTOA, TOA, DOA
Outdoors application
− Military systems: where GPS is not an option
− Commercial systems: complement GPS in urban canyons
Indoors applications
−
−
−
−
GPS is not applicable indoors
Indoors environments have rich multipath
WLAN systems are widely deployed indoors
WLAN based location fingerprinting is a promising solution
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How does it works?
Fingerprints
Locations
Fingerprint 1
Fingerprint 2
...
Fingerprint N-1
Fingerprint N
Location 1
Location 2
...
Location N-1
Location N
Antenna Array
Signals
Multi-channel
Receiver
Fingerprints
Database
Pattern
Matching
Algorithm
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Fingerprints Data-Base
Created prior to service launch
Raw data obtained by traversing the coverage
area and recording the data of every location
together with its coordinates
Fingerprints extracted by averaging the data
around each grid point
Grid resolution comparable to the achievable
accuracy
May need updating if the environment changes
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Signal strength as Fingerprint?
Can the Received Signal Strength (RSS) at the base station
serve as a good fingerprint?
Absolute signal strength has a poor fingerprint
− Depends on the orientation of the handset
− Depends on many irrelevant parameters (in/out the car?)
− Varies significantly on a wavelength scale due to constructive
and destructive multipath interference
Relative signal strength has a better fingerprint
− Varies significantly on a wavelength scale due to constructive
and destructive multipath interference
− Requires multiple sites
− Poor accuracy
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The Proposed Fingerprints
Spatial fingerprint
− The directions-of-arrival (DOAs) of the multipath rays
− The relative powers of the multipath rays
− Captured by the array covariance matrix
Time-delay fingerprint
− The time-delays of the multipath rays
− The relative powers of the multipath rays
− Captured by the impulse response / power delay profile
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How to Compute the Spatial Fingerprint
The Problem:
− DOA computation of multipath signals is computationally
intensive
The multipath signals are coherent
The solution:
− Use the “signal subspace” as the basis for the spatial
fingerprint
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The Signal Subspace
x(t)= a(θ₁)s(t-τ₁)+a(θ₂)s(t-τ₂)
When moving, the received
vector x(t) stays in the
2-dimensional signal subspace
spanned by a(θ₁) and a(θ₂)
When stationary, the received
vector x(t) stays in a
1-dimensional subspace
a(θ₁)
x(t₁)
x(t₂)
x(t₃)
a(θ₂)
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The Likelihood Function
LR [i] =Tr[PAiR]
where
1 2
Tr[ ] is the trace operator
N-1 N
R is the received covariance matrix
PAi is the projection on the i-th signal subspace Ai
corresponding to Ri, the covariance of i-th location
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The Pattern Matching Algorithm
Pre-compute a set of likelihood functions {LRi } for each Ri,
i=1…N, and search for the minimum Euclidean distance to the
likelihood function LR obtained from the data
Min |
{i}
LR- LRi |²
The Euclidean distance allows:
− Efficient storage by exploiting norm preserving
transformations
− Fast search of minima by exploiting the triangular inequality
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Further Enhancements
Multiple Sites
− Will reduce significantly the ambiguity
− Will enable good accuracy even in low multipath environments
In pure line-of-sight it degenerates to DOA triangulation
Multiple Frequencies
− Will reduce significantly the ambiguity level and improve the
accuracy
Mobility
− Should be exploited to reduce the ambiguity
The ambiguous locations move randomly, while the true locations
follow a smooth track
Time-delay fingerprint
− Will reduce significantly the ambiguity and improve the accuracy
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Field Test Results
AMPS Phone – Test done by University of Maryland, 2001
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References
[1] O. Hilsenrath and M. Wax: “Radio Transmitter Location Finding for Wireless
Communication Network Service and Management”, US Patent 6,026,304, Feb
2000.
[2] M. Wax, Y. Meng and O. Hilsenrath: “Subspace signature matching for location
ambiguity resolution in wireless communication systems” US Patent 6,064,339,
May 2000
[3] M. Wax, S. Jayaraman and O. Hilsenrath: “Location determination in wireless
communication systems using velocity information”, US Patent 6,084,545, July
2000
[4] S. Jaraman, M. Wax and O. Hilsenrath: “Calibration table generation for
wireless location determination”, US Patent 6,101,390, Aug 2000.
[5] M. Wax, S. Jaraman, V. Radionov, G. Lebedev and O. Hilsenrath: “Efficient
storage and fast matching of wireless spatial signatures”, US Patent 6,104, 344,
Aug 2000.
[6] M. Wax and O. Hilsenrtah: “Signature matching for location determination in
wireless communication systems”, US Patent 6,108,557, Aug 2000.
[7] M. Wax and O. Hilsenrath: “Signature matching for location determination in
wireless communication systems”, US Patent 6,112,095, Aug 2000.
[8] M. Wax, S. Jaraman and O. Hilsenrath: “Antenna array calibration in wireless
communication systems”, US Patent 6,232,918, May 2001.
[9] M. Wax, O. Hilsenrath and A. Bar: “Radio transmitter location finding in CDMA
wireless communication systems”, US Patent 6,249,680, June 2001.
[10] M. Wax, A. Bar and O. Hilsenrath: “Measurement of spatial signature
information in CDMA wireless communication systems”, US Patent 6,466,565, Oct
2002.
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Summary – Location Fingerprinting
A Multiple-antenna single-site position location
technology
Excels in rich multipath environments
− Outdoors
− Indoors
Easily integrated with all next-generation
multiple-antenna standards
− Cellular, WLAN, WIMAX, LTE
A lot of interesting open problems for research
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Beamforming and SDMA for outdoor WLAN
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Who needs Outdoors WLANs?
Municipalities / Governments
− Education (Digital Inclusion)
Internet to schools and their neighborhood
− Municipal applications
Meter reading
− Public safety
Video surveillance
− Economic development
Business connectivity, tourism
Cellular Carriers / WISPs
− The lowest cost broadband wireless alternative
− Especially attractive in developing countries
Driven by availability of low-cost embedded clients
− Laptops, handsets, PDAs
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What Are the Main Challenges?
Cope with interference
− Unlicensed band is prone to interference
− Level and nature of interference is unpredictable
Provide extended range
− Mounting sites are expensive to acquire and maintain
Provide uniform coverage
− Minimize dead-spot in coverage
Enable indoors penetration
− Penetration lowers deployment costs
Provide high capacity
− For bandwidth-hungry applications such as video
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The Solution
A 6-antenna base station using
− Beamforming
− SDMA
Based on custom-designed ASIC
Per packet processing on Rx and Tx
Applicable to all off-the-shelf clients
− 802.11b/g
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The Base Station Block Diagram
Beam Forming
& SDMA
RF
RF
Wavion
ASIC
RF
CPU
RF
Wavion
ASIC
Standard,
Unchanged
802.11 clients
RF
RF
Standard Off-the-shelf RF & Antennas
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The ASIC – System on a Chip
Wavion ASIC
Analog
Front-end
Digital
Front-end
Analog
Front-end
Digital
Front-end
Analog
Front-end
Digital
Front-end
Multi-Antenna
Channel Estimation
Beamforming
Modem
MAC
Modem
PCI
Weight Calc.
DSP
Multi Antenna
Sync
SDMA
Fully functional WiFi baseband chip
Mixed signal
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How is Beamforming Done?
Per-packet weights computation based on channel
estimation
− Done in the frequency domain per bin
− Maximum-ratio combining algorithm
Channel estimation based on packet preamble
− Involves a short packet exchange prior to transmission
Continuous on-line calibration of RF-mismatch
− Compensating the transmitter/receiver RF-chain mismatch
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How is SDMA Done?
Done only in the down link
− The random access protocol prevents simultaneous uplink
Requires prior channel estimation to each client
− Involves a short packet exchange to each client
The simultaneously transmitted packets are set to
the same length by zero padding
− Required to prevent uplink transmission during downlink
The corresponding ACKs are transmitted
simultaneously after the packet ends
− In accordance with the 802.11 protocol
The AP resolves the simultaneously received ACKs
− Using the pre-computed weights
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Beamforming Gain
Beamforming gain is composed of two parts:
− Array gain
− Diversity gain
Array gain
− On receive: 10*log6 = 6.5 -7.5 dB
− On transmit: 20*log6 = 13 -15 dB
Diversity gain (over selection diversity)
− 0-6 dB depending on the modulation and multipath
severity
Total beamforming gain
− On receive: 6.5 – 13.5 dB
− On transmit: 13 – 21 dB
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FCC “Adaptive Antenna” Rule
For every 3dB antenna gain above 6dBi, the total
power output shall be reduced by 1dB below 1W
Implication to Wavion AP:
− Antenna gain = 10*log6 + 7.5 = 15 dBi
− Total transmitted power = 30 - (15-6)/3 = 27 dBm
42 dBm directed power to user:
(Directed power) = 27 + 10*log6 + 7.5 = 42 dBm
6 dB greater than the 36dBm conventional limit
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Self-Backhaul Links
Provide cost effective wireless
backhaul
Done in 2.4 GHz using
Beamforming at both ends
Provide 20 dB link gain over
Selection Diversity (SD)
− High throughput
− Robust and reliable link
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Beamforming Gain - SS
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BF vs. Selection Diversity, Outdoor Channel 400ns
BF, 2Mbps
BF, 11Mbps
SD, 2Mbps
SD, 11Mbps
6.5 dB
13.5 dB
-1
PER
10
0
10
10
-2
-3
-5
0
5
10
SNR[dB]
15
20
25
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Beamforming Gain - OFDM
10
0
BF vs. Selection Diversity, Outdoor Channel 400ns
8 dB
12 dB
-1
PER
10
BF, 6Mbps
BF, 54Mbps
SD, 6Mbps
SD, 54Mbps
10
-2
-3
10
-10
0
10
20
SNR[dB]
30
40
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Cumberland, US – Comparative Tests
Customer conducted field comparison with conventional AP
− Clear advantage both in coverage and in capacity
WBS-2400
Conventional AP
Area Gain Ratio= 0.13 sq mi/0.035 sq mi = 3.7
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India, Jaipur – City Deployment
400m
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OLPC, Uruguay – Village Deployment
550m
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Summary – BF & SDMA for WLAN
Enabling technologies for wide-area WLAN
deployments
A cost effective solution based on custom-design
ASIC
Leverages tight integration with PHY and MAC for
optimal performance
Further enhancements:
− Spatial “Nulling” of interference
− Extending to multiple antenna clients (802.11n)
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Thank You