18-01-2013, 02:45 PM
Indoor Geolocation Research at TCS
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Geolocation: Few General Remarks Contd..
Networked applications are often implemented in a layered architecture or
protocol stack, and several layers of the protocol stack can benefit from local
-ization. Knowing the objects location not only promote context awareness at the
application level, but also bumps up low level functionalities such as routing,
service discovery and resource management.
Few Performance and Related Measures:
The key metric for evaluating a localization technique is the accuracy defined as,
How much is the estimated position deviated from the true position. The
accuracy is denoted by an accuracy value and precision value(e.g.15 cm accuracy
over 95% of the time). The precision indicates how often we expect to get at least
the given accuracy. The accuracy of a location sensing system is often used to
determine whether the chosen system is applicable for certain applications.
Calibration also plays a very important role. The uncalibrated ranging readings
are always greater than the true distance and are highly erroneous due to
transmit and receive delays. Device calibration is the process of forcing a
device to conform to a given input/output mapping.
TOA/TDOA Based Local Positioning
Time of Arrival (TOA) or Time Difference of Arrival (TDOA) is chosen as the basis
in our work as they avoid having an elaborate database of empirical
measurements or simulated signal strengths. Furthermore, the task of imitating
or manipulating TOA/TDOA on the scale of a few tens or hundreds of
nanoseconds is substantially harder and thus brings an inherent security .
As is well known, the time measurements between the unknown position (of
mobile terminal (MT)) and the fixed positions (access points (APs) or base
stations (BSs) are converted to distance measurements and these are used in the
process of multilateration to evaluate the position coordinates. In TOA, we
require the knowledge of transmitting time of MT (through time stamping, say)
and synchronization of the clocks of MT and the BSs. It is possible to omit the
start time measurement by introducing an additional BS at a known position and
measuring the TDOA at the receivers of a signal transmitted by the MT. In TDOA
we still need synchronization between the BSs, which can be further eliminated by
using differential time difference of arrival (DTDOA)
Main Issues in TOA/TDOA Based Local Positioning
Note: After the determination of the distances, there are different methods for
estimating the location. Triangulation uses the geometric properties of triangles
to compute object locations. It is a positioning procedure that relies on angle
measurements with respect to the known landmarks. Triangulation is
subcategorized into lateration—using distance measurements— and angulation—
using angle or bearing measurements . Trilateration uses ranges to three known
node positions to find the coordinates of unknown nodes. Multilateration uses
more than three ranges.
Three main issues:
(1) Base-station geometry and synchronization issues
(2) Time of arrival estimation
(3) Algorithm for multilateration.
All the three are addressed in our research.
Base-Station Geometry and Synchronization: General
Issues
Any positioning technique requires MT to be in contact with a specified minimum
number of base stations at all times. The task of remaining in contact with several
base stations becomes more difficult to achieve if the maximum measurement
distance of the system, at which the required observable (TOA here) can be
measured with the required accuracy is short. The only solution then is to install
base stations more closely, yielding a denser and more expensive infrastructure
for the entire necessary coverage. Synchronization refers to resolving different
senses of time in MT and BSs. Even for systems which achieve the same
maximum measurement distance, there can still be significant differences in
coverage area owing to the choice of time synchronization techniques. These
issues should be kept in mind in arriving at the required number of base stations.
Further, these BSs should be installed at carefully selected positions (basestation
geometry); for example in one of the DTDOA-based system they can be
placed in corners or edges of the room to have good line-of-sight (LOS)
conditions for almost all positions.