Novel Multiple-Antenna Systems
Mati Wax
Topics
 Location Fingerprinting
 Beamforming and SDMA for outdoors WLAN
2
Location Fingerprinting
3
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”
4
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
7
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
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[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