Cellular positioning (1)

ABSTRACT

Mobile positioning technology has become an important area of
research, for emergency as well as for commercial services. Mobile
positioning in cellular networks will provide several services such as,
locating stolen mobiles, emergency calls, different billing tariffs
depending on where the call is originated, and methods to predict the
user movement inside a region. The evolution to location-dependent
services and applications in wireless systems continues to require the
development of more accurate and reliable mobile positioning
technologies. The major challenge to accurate location estimation is in
creating techniques that yield acceptable performance when the direct
path from the transmitter to the receiver is intermittently blocked. This is
the Non-Line-Of-Sight (NLOS) problem, and it is known to be a major
source of error since it systematically causes mobile to appear farther
away from the base station (BS) than it actually is, thereby increasing
the positioning error.
In this paper, I present a simple method for mobile telephone tracking
and positioning with high accuracy. Through this we will discuss some
technology used for mobile positioning and tracking

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CONTENTS
page no.

1. Introduction to cellular positioning.
1.1. Introduction to mobile technology.

3
4

2. Positioning Systems.
2.1. Global Positioning System.

6

2.2. Differential Global Positioning System.

7

2.3. Wide Area Augmentation System.

8

3. Need for Geolocation

11

3.1. Architecture of a Geolocation.

12

4. Technology used for Geolocation
4.1 Handset based mobile positioning.

14

4.2 Direction based mobile positioning.

15

4.3 Distance based mobile positioning.

16

5. Location tracking curve method

19

6. Conclusion

24

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1.

Cellular Positioning

In recent years, GPS is gaining more and more momentum for
LBSs. This tendency is certainly fostered by advances achieved in GPS
receiver technology during the recent years, which have made it possible
to build low-cost GPS receivers that need only low power consumption
and to increasingly integrate them into more and more consumer
products like cellular phones and PDAs.

However, positioning methods were also developed for
cellular networks, for which an important driving force was the E-911
mandate in the United States. The introduction of E-911 was scheduled
in two phases, one that obliged operators to provide location data
obtained on the basis of radio cells not later than April 1998, and the
other that prescribed to enable positioning under much stronger accuracy
demands not later than December 2005. This second phase resulted in
the development of advanced positioning methods and their installation
in cellular networks. Examples are E-OTD, U-TDoA, and A-FLT. An
important alternative to these pure cellular methods is the integration of
GPS so that the cellular network provides terminals with assistance and
correction data of the satellites and thus helps to significantly increase
position accuracy and decrease the TTFF. This method is known as AGPS.

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1.1. INTRODUCTION TO MOBILE TECHNOLOGY

Configuration of a typical mobile telecommunication network.
As shown in Figure, the mobile telecommunication network includes a
several base stations (BSs) T 1 to T N for providing mobile
telecommunication service to a mobile subscriber through a mobile
telephone M1, a base station controller (BSC) for controlling the BSs T
1 to T N, and a mobile telephone switching office (MTSO) for
connecting the BSC to another BTS or a PSTN (Public Switched
Telephone Network).

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In a cellular mobile telecommunication network, the whole
service area is divided into a several coverage areas having respective
base stations (BS). Each BS coverage area is called a "cell." Each BS is
provided with a frequency of a range between 450 to900 MHz More
than one cells can use same frequency. Only condition is that no two
adjacent cells must have same frequencies. An MTSO controls these
BSs so that a subscriber can continue his call without interruption while
moving between different cells. The MTSO can reduce the time required
for calling a subscriber by locating the cell of the subscriber. In case of
an emergency like a fire, or a patient needing first aid treatment, the
mobile subscriber should be accurately located. Tracking the location of
a mobile subscriber within the boundary of a cell in a mobile
telecommunication network is known as "location based services
Mobile technology includes mainly two functions. They are call fixing
and hands-off process. All the BSs are sending a signal of power 25 to
30w to the mobile unit. When a user switches ON his mobile, it will
search for the strongest signal and got connected to that BS. Then the
mobile unit sends an identification signal to the BS. When he fixes a
call, the BS accepts the request and sends the request to the BSC and
MTSO. Then the MTSO will searches where the subscriber is and
connects the call.
When a user moves to another cell the MTSO will change the frequency
allotted to it and allots the frequency of the new BS. For both these
processes GEOLOCATION of the mobile unit is essential.

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2. Positioning Systems
2.1 Global Positioning System
A GPS tracking unit is a device that uses the Global Positioning
System to determine the precise location of a vehicle, person, or other
asset to which it is attached and to record the position of the asset at
regular intervals. The recorded location data can be stored within the
tracking unit, or it may be transmitted to a central location data base, or
internet-connected computer, using a cellular (GPRS or SMS), radio,
or satellite modem embedded in the unit. This allows the asset's location
to be displayed against a map backdrop either in real time or when
analysing the track later, using GPS tracking software (e.g., Telematics
2.0).

Typical GPS tracker architecture
A GPS tracker essentially contains GPS module to receive the GPS
signal and calculate the coordinates. For data loggers it contains large
memory to store the coordinates, data pushers additionally contains the
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GSM/GPRS modem to transmit this information to a central computer
either via SMS or via GPRS in form of IP packets. The diagram depicts
a hardware architecture of an advanced GPS tracker.
2.2 Differential Global Positioning System

Differential Global Positioning System (DGPS) is an
enhancement to GPS that provides improved location accuracy, from the
15-meter nominal GPS accuracy to about 10 cm in case of the best
implementations.
DGPS uses a network of fixed, ground-based reference stations to
broadcast the difference between the positions indicated by the
satellite systems and the known fixed positions. These stations broadcast
the difference between the measured satellite pseudo ranges and actual
(internally computed) pseudo ranges, and receiver stations may correct
their pseudo ranges by the same amount. The digital correction signal is
typically broadcast locally over ground-based transmitters of shorter
range.
The term can refer both to the generalized technique as well as specific
implementations using it. For instance, the United States Coast Guard
(USCG) and Canadian Coast Guard (CCG) each run such a system in
the U.S. and Canada on the long wave radio frequencies between
285 kHz and 325 kHz near major waterways and harbors. The USCG's
DGPS system is nationwide and comprises 86 broadcast sites located
throughout the inland and coastal portions of the United States including
Alaska, Hawaii and Puerto Rico.
A similar system that transmits corrections from orbiting satellites
instead of ground-based transmitters is called a Wide-Area DGPS
(WADGPS).
Operation

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A reference station calculates differential corrections for its own location
and time. Users may be up to 200 nautical miles (370 km) from the
station, however, and some of the compensated errors vary with space:
specifically, satellite ephemeris errors and those introduced by
ionospheric and tropospheric
distortions. For this reason, the accuracy of DGPS decreases with
distance from the reference station. The problem can be aggravated if
the user and the station lack "inter visibility"—when they are unable to
see the same satellites.
2.3 Wide Area Augmentation System
The Wide Area Augmentation System (WAAS) is an air
navigation aid developed by the Federal Aviation Administration to
augment the Global Positioning System (GPS), with the goal of
improving its accuracy, integrity, and availability. Essentially, WAAS is
intended to enable aircraft to rely on GPS for all phases of flight,
including precision approaches to any airport within its coverage area.[1]
WAAS uses a network of ground-based reference stations, in North
America and Hawaii, to measure small variations in the GPS satellites'
signals in the western hemisphere. Measurements from the reference
stations are routed to master stations, which queue the received
Deviation Correction (DC) and send the correction messages to
geostationary WAAS satellites in a timely manner (every 5 seconds or
better). Those satellites broadcast the correction messages back to Earth,
where WAAS-enabled GPS receivers use the corrections while
computing their positions to improve accuracy.
The International Civil Aviation Organization (ICAO) calls this type of
system a satellite-based augmentation system (SBAS). Europe and Asia
are developing their own SBASs, the Indian GPS Aided Geo Augmented
Navigation (GAGAN), the European Geostationary Navigation Overlay
Service (EGNOS) and the Japanese Multi-functional Satellite
Augmentation System (MSAS), respectively. Commercial systems
include StarFire and OmniSTAR.
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2.3.1.Operation

WAAS reference station in Barrow, Alaska

As with GPS in general, WAAS is composed of three main segments:
the Ground segment, Space segment, and User segment.
2.3.1A.Ground segment
The ground segment is composed of multiple Wide-area Reference
Stations (WRS). These precisely surveyed ground stations monitor and
collect information on the GPS signals, then send their data to three
Wide-area Master Stations (WMS) using a terrestrial communications
network. The reference stations also monitor signals from WAAS
geostationary satellites, providing integrity information regarding them
as well. As of October 2007 there were 38 WRSs: twenty in
the contiguous United States (CONUS), seven in Alaska, one in Hawaii,
one in Puerto Rico, five in Mexico, and four in Canada.

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2.3.1B.Space segment

Current WAAS satellite signal footprint
The space segment consists of multiple geosynchronous communication
satellites which broadcast the correction messages generated by the
WAAS Master Stations for reception by the user segment. The satellites
also broadcast the same type of range information as normal GPS
satellites, effectively increasing the number of satellites available for a
position fix. The space segment consists of three commercial
satellites: Inmarsat-4 F3, Telesat's Anik F1R, and Intelsat's Galaxy 15.
2.3.1C.User segment
The user segment is the GPS and WAAS receiver, which uses the
information broadcast from each GPS satellite to determine its location
and the current time, and receives the WAAS corrections from the Space
segment. The two types of correction messages received (fast and slow)
are used in different ways.
The GPS receiver can immediately apply the fast type of correction data,
which includes the corrected satellite position and clock data, and
determines its current location using normal GPS calculations. Once an
approximate position fix is obtained the receiver begins to use the slow
corrections to improve its accuracy. Among the slow correction data is
the ionospheric delay. As the GPS signal travels from the satellite to the
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receiver, it passes through the ionosphere. The receiver calculates the
location where the signal pierced the ionosphere and, if it has received

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an ionospheric delay value for that location, corrects for the error the
ionosphere created.
3. NEED FOR GEOLOCATION
One of the most powerful ways to personalize mobile services is based
on location. The location based services, provides the subscribers very
best of the service. Recent demands from new applications require
positioning capabilities of mobile telephones or other devices. The
ability to obtain the geo-location of the Mobile Telephone (MT) in the
cellular system allows the network operators to facilitate new services to
the mobile users. The most immediate motivation for the cellular system
to provide MT position is enhanced in accident emergency services. The
geolocation of the mobile user could provide services like

Emergency service for subscriber safety.

Location sensitive billing.

Cellular Fraud detection.

Intelligent transport system services.

Efficient
management.

and

effective

network

performance

and

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3.1
ARCHITECTURE
OF
A
GEOLOCATION
SYSTEM
An example of geolocation system architecture [KOS00] is shown in
Figure. As we said earlier, in order to fix a call the subscriber we are
calling must be located accurately. A geolocation service provider
provides location information .Upon a request from a subscriber for
location information about an MS, the service provider will contact a
location control center querying it for the coordinates of the MS. This
subscriber could be a commercial subscriber desiring to track a mobile
device or a PSAP trying to answer an E-911 call. The location control
center will gather information required to compute the MS’s location.
This information could be parameters such as received signal strength,
BTS ID, TOA of signals, and so on that we discuss later. Depending on
past information about the MS, a set of BS’s could be used to page the
MS, and directly or indirectly obtain the location parameters. These are
sometimes called Geolocation base stations (GBSs). Once this
information is collected, the location control center can determine the
location of the mobile with certain accuracy and convey this information
to the service provider. The service provider will then use this
information to visually display the MS’s location to the subscriber.
Sometimes the subscriber could be the MS itself, in which case the
messaging and architecture will be and location aware services to
subscribers.

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simplified, especially if the application involves self-positioning.
(Fig)2.1 Architecture of a general Geolocation system.

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4. TECHNOLOGIES USED FOR GEOLOCATION
4.1 HANDSET BASED MOBILE POSITIONING
AND TRACKING
4.1.1. GLOBAL POSITIONING SYSTEM (GPS)
A mobile telephone can be located by a mobile telephone itself or
through a mobile telecommunication network. To locate the mobile
telephone by itself, the mobile telephone is provided with a GPS
receiver to calculate its location in latitude and longitude coordinates
based on the location information received from a satellite through the
GPS receiver.

Increases the price and the size of the mobile telephone.

The load on the mobile telephone is increased.

Power consumption is high.

4.1Global positioning system
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4.2 DIRECTION BASED GEOLOCATION
4.2.1 ANGLE OF ARRIVAL METHOD As said earlier there
will be transmission and reception of signals between the mobile unit
and BSs. This method calculates the angle of arrival of signal receiving
at the BS. When a mobile user switches the system ON it receives the
signal from different base stations, may be 3 or 4 or more. The angle of
arrival method two or more base station for the determination. It
measures the direction of signal falling on the base station and measures
the angle of incidence with respect to a normal and determines the
position of the system. Angle of arrival method is not an accurate
method used for the mobile positioning because of its some
disadvantages such as:
• The determination of the system will be in error if the angle of
incidence is changed due to any obstacle like atmospheric particles or
due to scattering etc.
• The accurate location cannot be determined if the mobile user
is in between the BSs, that is in a straight line.
• It cannot be used for the indoor environments.

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The accuracy of the method can be increased by increasing the number
of the base stations used for determination. The direction based mobile
positioning is not used commonly now a day. It is replaced by the
distance based mobile positioning technologies.
4.3 DISTANCE BASED POSITIONING
4.3.1 TIME OF ARRIVAL (TOA)
The TOA method calculates the distance of a mobile telephone and a BS
based on the TOA of a signal transmitted from the mobile telephone at
the BS. It is assumed that the mobile telephone is located at the
intersection point of three circles having the radius of the distances
between the BSs and the mobile telephone. The distance is calculated by
the following equation,
2
2
Ri = C τi = sqrt ( (xi – X ) + (yi – Y) ) where,
C – Propagation speed of electromagnetic wave,
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th

τi – propagation of time from the mobile telephone to i base station,
th

Xi, yi -- location of i base station,
X, Y – mobile position.
4.3.2. TIME DIFFERENCE OF ARRIVAL (TDOA)
The TDOA method assumes that the TDOAs of a signal transmitted
from the mobile telephone at the three BSs define a set of points on a
hyperbola, and the mobile telephone is located at the intersection point
of at least three hyperbolas.

Figure4.3 illustrates a typical TOA method for locating a mobile
telephone.
As shown in Figure 1, three circles C1, C2, and C3, whose radii are the
distance between the mobile telephone M1 and at least three BSs T1, T2,
and T3, are overlapped across an area. The mobile telephone M1 is
located in the overlap area. One approach to locating the mobile
telephone M1 in the overlap area 1 is to use a common chord, as shown
in Figure. 2. When at least three circles C1, C2, and C3 are overlapped
over an area without meeting at one point, the mobile telephone M1 is
considered to exist at the intersection point of three common chords L1,
L2, and L3.
The above method using the common chord is not very accurate in
locating the mobile telephone except in the case where the mobile
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telephone is at an approximate equal distance from the selected BSs and
in a similar propagation environment to each respective BS.

Figure4.4, illustrates the TDOA method of locating a mobile
telephone.
In the case that a first mobile telephone M1 is nearer to the first BS T1,
as shown in Figure 2, the procedure will be described by a way of
example. In Figure 2, two circles C11 and C21 are drawn based on the
TOAs of a signal transmitted from the first mobile telephone M1 at the
first and the second BSs T1 and T2. A first common chord L1 is defined
by the intersection between the circles C11 and C21. But if the path
between the first mobile telephone M1 and the second BS T2 is in an
NLOS condition and the path between the first mobile telephone M1 and
the first BS T1 is in a line-of-sight (LOS) condition, the common chord
L1 is positioned far left from the actual location of the mobile telephone
M1. The effect is the same in the opposite case. If the path between the
first mobile telephone M1 and the second BS T2 is in the LOS condition
and the path between the first mobile telephone M1 and the first BS T1
is in the NLOS condition, the common chord L1 is also far right from
the actual location of the mobile telephone M1..

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5. LOCATION TRACKING CURVE METHOD
The method proposed by us for tracking the location of a mobile
telephone using curves connecting the points where circles intersect one
another, the circles radii being the distances between BSs and the mobile
telephone. The steps involved are:

Flowchart showing the steps involved in locating a mobile telephone
Description
When a location service is requested about a specific mobile telephone
by a user or a network, the location data processor draws two circles C1
and C2 with their respective centers set at BSs T1 and T2 based on the
TOAs of a signal transmitted from the corresponding mobile telephone
M1 or M2 to the two BSs T1 and T2 located near the mobile telephone
M1 or M2. The two circles C1 and C2 define a common chord L1.

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Figure3. 5, illustrates a proposed method for mobile telephone location.
However, if each mobile telephone M1 or M2 is placed in a different
propagation environment with respect to the BSs T1 and T2, the location
of the mobile telephone M1 or M2 can not be determined by the
common chord L1. Therefore, we use location tracking curves TR1 and
TR2 connecting the same two intersection points P1 and P2 of the two
circles C1 and C2, instead of the common chord L1.
The two curves TR1 and TR2 have their middle points intersecting
the line ST, which connects the positions of the two BSs T1 and T2 and
the parts of two circles C1 and C2 drawn to connect the two intersection
points P1 and P2. Instead of the common chord L1, the location data
processor uses the curve TR1 for the mobile telephone M1 and the curve
TR2 for the mobile telephone M2. It prevents the location error caused
by the multi-path fading or the NLOS path characteristics.
Determination of the location tracking curve
The BS with smaller variances should be selected to draw reference
circles based on the variances

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Figure 3.6, illustrates the determination of location tracking
curve.
From Figure 6, assuming that the first and the second BSs T1
and T2 selected for use in the location tracking are present at positions
(x1, y1) and (x2, y2), respectively, in the second-dimensional
coordinates, the location data
processor draws the two circles C1 and C2 with the coordinates (x1, y1)
and (x2, y2) of the two BSs T1 and T2 at their centers The curve
connects the two points P1 and P2 at which the two circles C1 and C2
intersect each other. The coordinates of the intersection points P1 and P2
are (xA, yA) and (xB, yB), respectively.
Since the mobile telephone is near the first BS T1 with respect to the
common chord L1, the variances of the TOAs of a signal transmitted
from the mobile telephone at the first BS T1 will be larger than those of
the signal at the second BS. Therefore, reference circles TR1 to TR4 are
drawn with respect to the second BS T2 with smaller variances, as
shown in Figure 6.

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The coordinates of the reference circle can be obtained (using minimum
variance) which has its center on the line ST passing through (x1, y1)
and (x2, y2) and passes through (xA, yA) and (xB, yB). Selecting the
center of the reference circle is significant as the mobile telephone is
located on the reference circle. The location data processor selects the
desired curves (reference circles) with respect to the several BSs
selected for location tracking. In Figure 6, as the real location of the
mobile telephone deviates farther from the circle C2 with the second BS
T2 at its center, the center of a reference circle is farther from the
location of the second BS T2. That is, the center of a desired reference
circle is farther from the second BS T2 in the case of a third mobile
telephone M3 (curve C3) than in the case of a fourth mobile telephone
M4. Reference circle selection
The variances of the TOAs of a signal which arrives at the two
BSs T1 and T2 from different paths are used to find the curve on which
the actual location of the mobile telephone is determined. If the TOAs of
the signal at the first BS T1 from N propagation paths are t1, t2, . . . , tN,
the first BS T1 calculates the variances σ of t1, t2, . . . , tN. The location
data processor compares the variances calculated by the first BS T1 with
the variances calculated by the second BS T2 and considers that the
mobile telephone is near to that BS with the larger variances (the first
BS T2 in Figure 6). Hence, the reference circle has its center near to the
BS with the smaller variances (the second BS T2 in Figure 6) on the line
ST.With the larger variances, the center of a reference circle gets farther
to the right from the center of the second BS T2. In order to select the
desired curve, the location data processor initializes the reference circles
with predetermined radii and the variances of TOAs of a signal
transmitted from the mobile telephone located on the reference circles,
and compare the preset variances with real variance measurements.
The location data processor sets a several reference circles based on the
distances between the mobile telephone and the BS with the smaller
variances(the second BS T2) In Figure 6, as an example, the first to the
fourth reference circles TR1 to TR4 have radii twice, three times, four
times, and five times, respectively, of that of BS T2, where all these
points of reference circles TR1 and TR4 are located along the line ST
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The variances of the second BS T2 smaller than those of the first BS T1
are used as a criterion for selecting an optimal reference circle.
Therefore, the location data processor predetermines the reference
variances for the first to the fourth reference circles TR1 to TR4 to be
compared with respect to the second BS T1. It is assumed in the
following description that σ 1, σ 2, and σ 3 are reference variances and σ
1< σ 2< σ 3
The location data processor compares the variances calculated
by the two BSs T1 and T2 and selects the base station with smaller
variances as a reference point to draw the reference circle. If the selected
variances (those of the second BS T2) are σ, the location data processor
compares the selected variances σ, with the preset reference variances σ
1, σ 2, and σ 3.

If σ <= σ 1, the curve of the first reference circles TR1 is
selected.

If σ 1 < σ <= σ 2, the curve of the TR2 is selected.

If σ 2 < σ <= σ 3, the curve of the TR3 is selected.

If σ 3 < σ, the curve of the fourth reference circles TR4 is
selected.
As we have seen, the location data processor selects the optimal curve
(reference circle) for the two BSs among the several BSs, and selects
another optimal circle for another BS pair, and so on.
When curves are selected for all selected BS pairs, the location data
processor obtains the intersection points among the selected curves as
shown in Figure 7. However, as the selected curves do not intersect at
one point due to the multi-path fading or the NLOS effects, the midpoint
of these intersection points is determined as the location of the mobile
telephone.

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Figure 3.7, illustrates the positioning of mobile by the proposed method.
As the three intersection points M1 (xA, yA), M2 (xB, yB), and M3 (xC,
yC) are defined by the three curves TR1 to TR3, the location data
processor considers the mobile telephone to be located at (x, y). While
the three BSs are selected for the location service using the TOAs of a
signal arrived at each BS from a mobile telephone has been described in
the embodiment of the present invention, more BSs can be used to
increase the accuracy in locating the exact position of the mobile station.
th
If N intersection points are defined by location tracking curves obtained
th
according to the present invention and an i intersection point is at (xi,
yi), coordinates (x, y) indicate the location of the mobile telephone.
After the location of the mobile telephone, that is, the intersection
points among the curves are obtained, the location data processor
represents the intersection points in the latitude and the longitude
coordinates and transmits the position coordinates to the network
(BS/BSC/MSC) and the mobile telephone.

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6. CONCLUSION
Our proposal is advantageous in that the location of a mobile
telephone can be accurately tracked even in the multi-path fading and
the NLOS environment, by using more accurate tracking curves
connecting the intersection points among circles with the radii being the
distances between corresponding BSs and the mobile telephone in a
cellular mobile communication system. We have described about
accurate positioning of mobile telephones, which can be used for several
applications. The important considerations to be undertaken while
selecting a location based technology are location accuracy,
implementation cost, reliability, increasing functionality.

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REFERENCES
-J. Caffery, and G. Stuber Jr, ―Vehicle location and tracking for
IVHS in CDMA micro-cells‖, Proc. IEEE PIMRC, 1994
G. Morley, and W. Grover, ―Improved location estimation
with pulse-ranging in presence of shadowing and multi-path excessdelay effects‖, Electronics Letters, vol.31, No.18, 1995
www.ieeexplore.ieee.org
gp.espacenet.com
- www.Google.com

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