ON INDOOR POSITION LOCATION WITH WIRELESS LANS

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ON INDOOR POSITION LOCATION WITH WIRELESS LANS

• Present by Y.R Lee

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Outlines

• Introduction
• Related work
• Issues in RSS-based Location fingerprinting
• EXPERIMENTAL TESTBED
• ANALYSIS AND EXPERIMENTAL RESULTS
• FUTURE WORK

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Introduction

• Location aware services are becoming attractive
  with the deployment of next generation wireless
  networks.
• Knowledge of the positions of users combined with
  user profiles could significantly help
• in network planning
• in load balancing
• caching of information closer to the user
• radio resource management designing other
  performance enhancement methods

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Related work

• It is far easier to obtain RSS information than
  the multipath characteristic, the time or angle
  of arrival that require additional signal
  processing.
• RSS-based Techniques
• map the path loss of the received signal to the
  distance traveled by the signal from the
  transmitter to the receiver.
• fingerprinting scheme.
• RADAR system

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RADAR system

• set up only in the hallway where the RSS is
  stable and strong.
• The measurements are all in a single floor with
  three APs.
• A problem with the multifloor environment is that
  two or more very different locations could
  potentially have the same RSS location-fingerprint
  .

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Issues in RSS-based Location Fingerprinting

• Self-positioning
• Instead of three AP's measuring the RSS from a
  MS, the MS could measure the RSS values from
  multiple APs
• Granularity
• The spacing between grid points influences the
  granularity of the position estimate.
• We consider two specific grid spacings 5 ft and
  10 ft to evaluate the tradeoffs between
  performance and delay.

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• Algorithms
• Choose the minimum distance to the measured
  fingerprint. The Euclidean L2 distance (p 2)
  are considered with wi 1 for all entries
• Choose M closest database entries and estimate
  the location based on the average of the
  coordinates of these M points.
• Fault-tolerance
• Sometimes some access points may be disabled but
  user still want to obtain location service

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• Performance
• accuracy
• Delay time
• capacity
• coverage

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EXPERIMENTAL TESTBED
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• located in 4th floor of our eight-story building.
• The floor has dimensions of 45 feet by 105 feet
  and includes more than 10 rooms.
• The WLAN consists of ten access points located
  opportunistically on multiple floors.
• database contains fingerprints for 60 locations.

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• At each location, we calculated the average of 40
  samples. The 40 samples are composed of 5 samples
  in each of the MSs orientation at two different
  times of the day.
• We use a simple a linear search algorithm since
  our database size is rather small.
• Measurements were not taken in a few room.This
  is not because the RSS values were unavailable,
  but because access to these rooms is restricted.

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ANALYSIS AND EXPERIMENTAL RESULTS

• Algorithms and accuracy
• Algorithm 1 compute distance
• Increasing the value of p in the Lp distance
  fails to increase
• So used two weighting schemes,SD-weight and
  NSS-weight
• Algorithm 2 return M estimated locations
• After searching database corresponding to the M
  smallest Euclidean distances (p and wi 1 for
  all i).
• From the M locations, the location is estimated
  as the average
• When M 2 , we called 2-best

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• It is quite clear that there is no improvement in
  weighting the distance with the SD or NSS values.
• Using the 3-best has a better 95 performance
  compared to all other schemes.
• This scheme performs at the same level as the one
  with the three closest locations (3-best).

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Performance-Accuracy Tradeoffs

• Increasing the granularity of the grid only
  slightly improves the positioning accuracy
• But the size of database entries is increased by
  about three times and search time is the same.

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Fault tolerance
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• the average distance error when one AP is down is
  slightly increased compared to the case with no
  AP failure.
• As before,the algorithm that uses the 3-best
  matches tends to perform better than the others.
• This indicates that the position location is
  quite robust to failures.

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Deployment Issues

• How many access points are required for a given
  level of accuracy
• How close the grid spacing should be
• Whether there should be a mixture of algorithms
• Whether it is feasible to provide the required
  accuracy by performing some tradeoffs.

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Performance Vs Architecture

• We need to correlate the performance measures to
  the physical architecture of the building.

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• As the number of obstructing walls increases, the
  accuracy of the positioning technique is
  decreased.
• for every additional wall, there is approximately
  1.8 ft increase in the error
• In our case, two APs can cover one floor of the
  building for communications, but four APs are
  required to keep the average distance error to
  less than 20 feet.

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Hybrid schemes
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• It is possible to slightly improve the accuracy
  of the position error by increasing the
  granularity of the grid in rooms
• there is no improvement by increasing the
  granularity in hallways where the number of
  obstructions between APs and MSs is smaller.
• The conclusion here is that hybrid schemes do
  provide better accuracy and also good performance
  by reducing the search time.

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FUTURE WORK

• This paper has provided a framework for
  systematically analyzing indoor position location
  using WLANs.
• How many access points are required to provide a
  given accuracy with a given granularity of the
  grid in the database
• How the database should be organized for better
  searching speed.