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Novel Multiple-Antenna Systems. Mati Wax. Topics. Location Fingerprinting Beamforming and SDMA for outdoors WLAN. Location Fingerprinting. What is Location Fingerprinting?. A position location technology for rich multipath environments. The key idea:

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Novel Multiple-Antenna Systems

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  • Location Fingerprinting
  • Beamforming and SDMA for outdoors WLAN
what is location fingerprinting
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”
what is it good for
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
how does it works
How does it works?


Fingerprint 1

Fingerprint 2


Fingerprint N-1

Fingerprint N


Location 1

Location 2


Location N-1

Location N






Antenna Array




fingerprints data base
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
signal strength as fingerprint
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
the proposed fingerprints
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
how to compute the spatial fingerprint
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
the signal subspace
The Signal Subspace






x(t)= a(θ₁)s(t-τ₁)+a(θ₂)s(t-τ₂)

When moving, the received vectorx(t) stays in the

2-dimensional signal subspace spanned by a(θ₁) and a(θ₂)

When stationary, the received vectorx(t) stays in a

1-dimensional subspace

the likelihood function
The Likelihood Function

LR [i]=Tr[PAiR]






Tr[ ] is the trace operator

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

the pattern matching algorithm
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

  • The Euclidean distance allows:
    • Efficient storage by exploiting norm preserving transformations
    • Fast search of minima by exploiting the triangular inequality

Min| LR- LRi |²


further enhancements
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
field test results
Field Test Results
  • AMPS Phone – Test done by University of Maryland, 2001
  • [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.
summary location fingerprinting
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
who needs outdoors wlans
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
what are the main challenges
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
the solution
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
the base station block diagram
The Base Station Block Diagram

Beam Forming


Standard, Unchanged

802.11 clients



Wavion ASIC



Wavion ASIC




Standard Off-the-shelf RF & Antennas

the asic system on a chip
The ASIC – System on a Chip







Wavion ASIC


Digital Front-end


Channel Estimation





Weight Calc.




Multi Antenna





  • Fully functional WiFi baseband chip
  • Mixed signal
how is beamforming done
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
how is sdma done
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
beamforming gain
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
fcc adaptive antenna rule
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

self backhaul links
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
cumberland us comparative tests
Cumberland, US – Comparative Tests
  • Customer conducted field comparison with conventional AP
    • Clear advantage both in coverage and in capacity


Conventional AP

Area Gain Ratio= 0.13 sq mi/0.035 sq mi = 3.7

summary bf sdma for wlan
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)