rejeesh Positioning GSM Telephones - 123seminarsonly.com
Transcript of rejeesh Positioning GSM Telephones - 123seminarsonly.com
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INTRODUCTION
Location related products are the next major class of value added services
that mobile network operators can offer their customers. Not only will operators be able
to offer entirely new services to customers, but they will also be able to offer
improvements on current services such as location-based prepaid or information services.
The deployment of location based services is being spurred by several
factors:
Competition
The need to find new revenue enhancing and differentiating value added
services has been increasing and will continue to increase over time.
Regulation
The Federal Communications Commission (FCC) of the USA adopted a
ruling in June 1996 (Docket no. 94-102) that requires all mobile network operators to
provide location information on all calls to “911”, the emergency services. The FCC
mandated that by 1st October 2001, all wireless 911 calls must be pinpointed within125
meters, 67% of the time. On December 24 1998, the FCC amended its ruling to allow
terminal based solutions as well as network based ones (CC Docket No. 94-102, Waivers
for Handset-Based Approaches).
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There are a number of regulations that location based services must
comply with, not least of all to protect the privacy of the user. Mobile Streams believes
that it is essential to comply with all such regulations fully. However, such regulations
are only the starting point for such services- there are possibilities for a high degree of
innovation in this new market that should not be overlooked.
Technology
There have been continuous improvements in handset, network and
positioning technologies. For example, in 1999, Benefon, a Finnish GSM and NMT
terminal vendor launched the ESC! GSM/ GPS mapping phone.
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NEEDS OF CELLULAR POSITIONING
There are a number of reasons why it is useful to be able to pinpoint the
position of a mobile telephone, some of which are described below.
Location-Sensitive billing
Different tariff can be provided depending upon the position of the cell
phone. This allows the operator without a copper cable based PSTN to offer competitive
rates for calls from home or office.
Increased subscriber safety
A significant number of emergency calls like US.911 are coming from cell
phones, and in most of the cases the caller can not provide the accurate information about
their position. As a real life example let us take the following incident. In February 1997
a person became stranded along a highway during a winter blizzard (Associated press
1997).She used her cellular phone to call for help but could not provide her location due
to white-out conditions. To identify the callers approximate position authorities asked her
to tell them when she could hear the search plane flying above. From the time of her first
call forty hours elapsed before a ground rescue team reached her. An automatic
positioning system would have allowed rescuers to reach her far sooner.
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Enhanced network performance
At the microscopic level, accurately positioning a moving mobile phone
enable the communication network operator to take better decisions on when to hand
over from one cell to next. Macroscopically, long-term monitoring of mobile telephone
positions provides excellent input to the planning of the cellular network
Intelligent transport systems
Many services envisaged under the ITS initiative will require position
information, often in conjunction with a communications channel, to be effective. The
ability to position a mobile telephone could enable services such as providing information
to travelers, more effective dispatch of vehicles in fleets, and detecting traffic incidents
and congestion
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POSITIONING SYSTEM CLASSIFICATION
The positioning systems are classified according to where the
measurements are made and where the position information is used. Two broad
classifications are made: self positioning and remote positioning. The classification is
useful when evaluating a given positioning system for a particular application.
Self positioning
In self positioning the positioning receiver makes the appropriate signal
measurements. There will be geographically distributed transmitters. The positioning
may be collocated with the receiver. The position information can be transmitted to
distant stations from the positioning receiver if needed. An example of self positioning is
GPS.
Remote positioning
In remote positioning there will be receivers at more than one location, the
object to be positioned being the transmitter. The signal measurement is done at the
receivers and the data collected are transmitted to a central site for processing.
Indirect positioning
It is possible to send the position measurement from a self positioning
receiver to a remote site or vice versa. A self positioning system sending data to a remote
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site is called indirect remote positioning system and the other is called indirect self
positioning system
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POSITIONING TECHNIQUES
There are a variety of ways in which position can be derived from the
measurement of signals and these can be applied to any cellular system including GSM.
The important measurements are the Time of Arrival (TOA), the Time Difference of
Arrival (TDOA), the Angle of Arrival (AOA) and Carrier phase. All these measurement
put the object to be positioned on a particular locus. Multiple measurements give multiple
loci and the point of their intersection gives the position. If the density of the base
stations is such that more measurements can be done than required then a least square
approach can be used. If the measurements are too few in number the loci will intersect at
more than one point result in ambiguous position estimate.
In the following discussion we assume that the mobile station and base
station are lying in the same plane. This is approximately true for most networks unless
the geography include hilly topology or high rise buildings.
Time of Arrival (TOA)
In a remote positioning system this involves the measurement of the
propagation time of a signal from the mobile phone to a base station. Each measurement
fixes the position of the mobile on a circle. With two stations there will be two circle and
they can intersect in a maximum of two points. This gives rise to an ambiguity and it is
resolved by including a priory information of the trajectory of the mobile phone or
making a propagation time measurement to a third base station. The TOA measurement
requires exact time synchronization between the base stations and the receiver should
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have an accurate clock, so that the receiver knows the exact time of transmission and an
exact TOA measurement have made by the receiver.
The measurement of the round-trip time of a signal from a source to
destination and echoed back to the source can be made. This does not require exact
synchronization and is the common method of measurement of TOA.
Figure 1 Circular trilateration
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A system that makes use of TOA for positioning can be called as circular-
circular-circular system.
Time Difference of Arrival (TDOA)
A mobile phone can listen to a series of base station and measure the time
difference between each pair of arrival. Each TDOA measurement fixes the position of
the mobile phone on a hyperbola. With more than two stations there will be more than
two hyperbola and they intersect at a unique point. This is a self positioning system. The
inverse approach yield a remote positioning system i.e. each base station listen to the
mobile station and measure the TOA and sent each TOA to a central site where the
TDOA measurement is made and the position is estimated. A positioning system based
on TDOA measurement is called hyperbolic-hyperbolic system.
Figure 2 Hyperbolic trilateration
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An important issue in TDOA approach is the requirement of time
synchronization irrespective of the positioning method-self or remote. In a self
positioning system the base stations are the transmitters. The transmitted signal should
leave the transmitters at the same time. Otherwise there will be a bias error in the
hyperbolic locus. In remote positioning the base stations are the receivers. There must be
a known relationship between the receiver clocks at the base station, or again bias error
will result.
Angle of Arrival (AOA)
In this method the angle of arrival of a signal transmitted form a base
station to a mobile station (or mobile station to base station) is measured. The locus is a
straight line. With two measurement two straight lines are obtained and the uniquely
intersect at a point. This is also called triangulation method.
Figure 3 Location by triangulation
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Carrier Phase
The phase of the carrier has the potential to provide position information.
The receiver can measure the phase of the carrier but it can not measure the integer
number of cycles between transmitter and receiver. Another problem is the need of
maintaining continuous lock on the carrier signal. Despite of the problems the carrier
phase method is successfully used in the GPS. The application of carrier phase
positioning to GSM will be challenging due to the problems associated reconstructing the
carrier from the Gaussian minimum shift keying modulated signal, with no guarantee that
the carrier will be continuous.
Cell of Origin (COO)
Cell of Origin (COO) requires no modification to the handset or networks
and so is able to be used as the Location Finding System for existing subscribers but is
less accurate than the other methods employed. Some would argue however that the
accuracy of COO in cities is more than adequate for information services owing to the
small cell size. The accuracy of COO is though questionable when the Location Finding
System is required for assisting with emergency services.
COO is the only technology that is widely deployed in wireless networks
today. This scheme is used to meet Phase 1 911 emergency services requirements in the
USA, wireless office location specific billing applications and some location-specific
information request services.
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In this system, the mobile network base station (BTS) cell area is used as
the location of the caller. Positioning accuracy generally depends upon the size of the
cell. Although other schemes offer higher degrees of positioning accuracy than cell of
origin, its main advantages are that speed of response in getting a location fix is fast
(typically around three seconds) and that as no handset or network upgrade is required, it
can be used to provide location specific services to existing customer bases
Heterogeneous systems
Different positioning methods can be mixed. A common example is radar
in which TOA and AOA methods are mixed. Mixing of positioning system has
advantages e.g. circular angle positioning system can measurement using a single base
station.
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PERFORMANCE CONSIDERATIONS
There are many error sources that degrade the performance of the
positioning system. The measure of the RMS deviation from the measured position (x^,
y^) about the position (x, y).this is called RMS accuracy in two dimension and is given
by the formula
S= (E [(x^-x) 2 +(y^-y) 2]) 1/2
where E is the statistical expectation operator.
The RMS accuracy is a function of the geometry of the base stations and
accuracy of raw locus measurement. The locus errors impose a limit on the accuracy that
can be achieved, but the relative geometry of the base station to the mobile station will
further degrade the accuracy. The amount by which errors are degraded by geometry is
called dilution of precision (DOP). When developing a positioning system the designer
should aim to minimize DOP.
Coverage is the proportion of an area of interest that is provided with an
acceptable level of service by the positioning system. An acceptable level of service
could be the ability to make positioning measurement to a predefined level of accuracy.
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REVIEW OF GSM
GSM networks are very complex communication systems. In the
following paragraphs those aspects of GSM signaling pertinent to positioning
considerations are discussed.
GSM 900 uses two 25 MHz blocks of the radio frequency spectrum, called
uplink and downlink. Each block is divided into125 frequency channels of 200 kHz.
Other systems derived from GSM specifications have similar frequency channel
structure.
GSM employs Time Division Multiple Access (TDMA) schemes on each
frequency channel, dividing it in to time slots of 577µ s duration. Blocks of eight time
slots are grouped in to form a frame. Frames in turn, are grouped in to multi-frames and
super-frames.
GSM defines a number of logical channels, each of which has a specific
role such as carrying user payload (traffic channels), coordinating base station and mobile
(associated and dedicated control channels), establishing links (common control
channels), and transmitting system parameters (broadcast channels). These logic channels
are mapped in to predetermined time slots of particular frames within the multi-frame
sequence.
The message contained in various logical channels is fitted in to the time
slots using a burst structure. GSM defines a number of burst types with differing formats.
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The format for a normal burst, the most commonly used burst structure, is indicated in
fig.4. Of particular note in the context of positioning is the 26-bit training sequence which
is located in the middle of the burst. This is a pseudo-random sequence chosen for its
correlation properties. By cross-correlating a local copy of the training sequence with the
sequence in the received burst, a GSM receiver is able to estimate the impulse response
of the radio channel as an aid in demodulating the bits in the burst. This training sequence
is also useful for time based positioning measurements. A positioning receiver is able to
use the correlation peak as a time reference for the burst.
Another useful training sequence is the 64 bit training sequence used in
the GSM synchronization burst transmitted by all the base stations on the synchronization
channel (SCH).
Figure 4 the GSM TDMA format
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The area covered by GSM network is divided in to a number of cells, each
served by its own base station, called a base transceiver station (BTS). Each cell is
distinguished by a unique cell identifier and is allocated one or more uplink/downlink
frequency pairs. More than one BTS may be grouped together under the control of a base
station controller (BSC). In turn, several BSCs are controlled by a mobile service center
(MSC), which handles tasks such as call routing and serves as the interface between the
mobile network and the fixed telephone network. In GSM specification a mobile
telephone is referred to as mobile station (MS).
GSM uses a number of techniques to increase capacity, some which have
implications for positioning. One of these is the use of sectored cells. In this case BTSs of
more than one cell may be located at a particular site, each BTS serving only a sector of
the area around that site. A common configuration is to have three collocated BTSs, each
providing a 120 degree coverage pattern. Where such a configuration is in use, the
constraint on the BTS coverage pattern offers extra information for use by a positioning
system.
The training sequences in different burst structures of GSM lend
themselves to measurement of time and indeed there are also higher-level features of
GSM signaling which measure time and could be used for positioning. GSM is a TDMA
system, the successful operation of which requires that all signals arrive at the BTSs at
the appropriate time. Since the signals arriving at the base stations originate from
different distances, the time at which the signals are sent must be varied. GSM achieves
this by having each BTS send each MS connected to it a timing advance (TA), which is
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the amount by which the MS must advance the timing of its transmission to ensure that it
arrives in the correct time slot.
A further mechanism in GSM which may optionally be employed to
improve the efficiency of handovers is pseudo synchronization. Each MS monitors the
time differences between the epochs of the different BTSs in its vicinity. These
measurements are called observed time differences (OTDs) and are used to facilitate
handover by estimating the amount the timing of the mobile would have to be advanced
/retarded if it were handed over to another BTS.
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LOCUS MEASUREMENT WITHIN GSM
SPECIFICATION
There are three ways that position loci can be derived using the signaling
aspects inherent in GSM specification. Propagation time using TAs, TDOA using
observed time difference of arrival (OTD) and angle of arrival from the sector
information.
It is possible to implement a circular-circular-circular system using TA.
For this the mobile station should be artificially forced to listen to more than two Base
Transceiver Stations (BTS). There are two difficulties with this scheme. First, artificially
forced handover to sub optimal BTS will degrade call quality and reduce system capacity.
Second, under GSM specification TA is reported in units of bit period, which equates a
locus accuracy of 554m. This is an optimistic value for accuracy, since multi-path will
degrade the accuracy further. The rms accuracy of such a system is likely to be even
worse due to DOP considerations.
Since OTD measurements are made by mobile without forcing a
handover, they are potentially greater utility for positioning than TAs. Under the current
GSM specification, the resolution of an OTD measurement is 554m. Assuming that
network is synchronized, it is possible to use the OTDs to implement a TDOA
positioning system. The GSM specification does not require that the network to be
synchronized. The level of uncertainty in the degree of synchronization will even further
degrade the accuracy of a GSM positioning system which simply operates with the
current specifications.
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Some networks use sectored cells. A mobile connected to a sectored base
station can be crudely located using knowledge of the base station’s reduced geographic
coverage. While this information is insufficient to provide an accurate position estimate,
on occasion it might be sufficient to resolve a twofold ambiguity resulting from an
insufficient number of measurements, thus alleviating the need for additional propagation
time (TA) or TDOA (OTD) measurements.
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GSM POSITIONING ARCHITECTURES
In this section we examine three different position architectures that could
be used to position GSM mobile phones: mobile-based, network-based, hybrid
positioning. There could be significant differences in system architectures affecting
infrastructure costs, coverage, the total number of users that can be supported and number
of users that can be simultaneously positioned. The needs of a given positioning
application will determine where the position information is required, the position update
rate for each object being tracked, the number of objects to be tracked, and the net value
of the position information. The various architectures need to be evaluated in light of
these requirements to select the most appropriate one.
Mobile-based Positioning
This is a form of self positioning. The Mobile Station (MS) uses the
signals from the BTSs to determine the position. There are a number of techniques that
could be used to calculate position, but the basis is likely to be TDOA. For an MS based
TDOA systems to work two fundamental changes need to be made to GSM equipment.
The first is to modify the MS so that it is able to make accurate TDOA measurements,
much more accurate than the current one bit resolution of OTDs. Such measurements
include algorithms to reject multi-path. Accordingly within the mobile there will be a
locus function which will accurately determine the TDOA. This can be done by
processing the burst information to locate the epoch of the training sequence. The
simplest logical channel on which to carry out this processing is the broadcast control
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channel (BCCH) because its bursts are not subject to frequency hopping or power control
and is repeated more frequently than the SCH.
The second modification arises from the need of network synchronization.
There are two options. The first is to tightly synchronize the network. This can be
achieved by placing GPS time transfer receivers at each base station. The alternative is to
provide information to the MS about the synchronicity of the network. This could be
provided by special purpose monitoring receivers which measure the timing offset
between different BTSs and sent this timing data to MS via a traditional data link such as
the GSM short message service.
To accurately provide a robust cellular positioning system with high
accuracy and good coverage, it will be necessary to integrate many other sources of
information. A key part of such implementation will be sophisticated software called
fusion function which will fuse the information from a variety of sources. A key source
of information is the location of the base stations. Other possible sources of information,
which can be used to increase accuracy and to resolve ambiguities, include TAs, signal
strength indications, and sector information. On many occasions, the position information
calculated in the mobile will be needed at another location. This position information
could be sent from the mobile to another location using SMS. This would constitute
indirect positioning system.
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Figure 5 mobile based positioning architecture
Network-based Positioning
Using transmissions of a mobile to work out its position is referred to as
network-based positioning. The simplest implementation is based on TDOA. A number
of geographically distributed positioning receivers are required to monitor transmissions
from mobiles in the area and to be able to make accurate TOA measurements of the
signals from the MSs. A locus function is to be needed to reside in each positioning
receiver. The locus function will process the bursts emanating from the uplink from
mobiles engaged in a call.
A location service center (LSC) will generate TDOA measurement from
the different TOAs and produce position estimate. The fusion function will reside in
LSC. The LSC will also contain the BTS database. It will receive requests for position
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measurement from various application computers; schedule the appropriate positioning
receivers to make the required locus measurement of the nominated MS, collect the locus
measurement from the positioning receivers, fuse all information into a position
measurement, and then return the position measurement to the requested application. The
LSC is likely to sit at the same level of the GSM hierarchy as the MSC.
Positioning receivers could be collocated with BTSs, with many resultant
advantages. Many of the functional elements of a positioning receiver have equivalent
elements in a BTS. For certain BTS configurations, the positioning receiver could tap
into the down converted video stream generated within the BTS and thus not have to
duplicate subsystems such as antenna system, power supply, amplifiers, down converter,
and digitizer. If the LSC is collocated with the MSC, signaling between LSC and the
positioning receiver could be achieved using the existing communication link between
the MSC and BTSs.
As with mobile based positioning, synchronization is an issue. If the
network is synchronized, there is likely to be a timing source available at each BTS to
unambiguously mark the TOA of a given TCH burst on a given frequency channel. In an
unsynchronized network a mechanism is required to synchronize the clocks used at each
positioning receiver to mark the epoch at which a nominated burst arrives. Alternatively
the positioning receiver collocated with a BTS could measure the TOA relative to their
own transmission cycle, and separate monitoring sites would provide the LSC with
timing offset of each BTS, thus allowing TDOAs and subsequently position to be
calculated.
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Figure 6 network based positioning architecture
Network based solutions are important because it allows positioning
without modification of the mobile phones. The other advantage is the leverage that can
be gained from a single positioning measurement. The network operator could use the
information for position based tariffs and provide the mobile user with a variety of
position-based services (e.g., route guidance to the nearest service station).
Hybrid Positioning
Hybrid-positioning combines different aspects of remote-positioning and
self-positioning. Possible hybrid architecture has the locus function residing in the mobile
but the fusion function situated at the LSC. When requested by the LSC, a given mobile
will measure TOA of bursts from various BTSs. These are then sent to the LSC, which
generates TDOA measurements and compute position estimate for that mobile.
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As with mobile based positioning, some form of synchronization is
required. Either of synchronization methods identified for mobile-based positioning is
suitable for use in hybrid architecture. A variant of hybrid architecture is Digital Cursor
system. Instead of transmitting the locus information, this system transmits a replica of
the signal, with the TDOA calculated using cross-correlation of the different replicas.
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SUMMARY
Many people will agree that mobile location services are an important new
category of value-added service for the 21st century. The applications and services
explained in the introduction are of interest to customers and will help to differentiate
network operators initially before becoming an expected part of service within 10 years.
Although the opportunity for operators generated by these new offerings is
both broad and deep, the deployment of location-based services will not begin with the
most complex, technically demanding and feature-rich offerings. Instead, network
operators will use today's technology to differentiate their services, as well as gain market
leadership and critical technical skills. Becoming involved in the development of
location specific products prepares them to be able to efficiently augment their offerings
for when more sophisticated positioning technology and wireless devices enter the
market.
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REFERENCES
1. http://www.mobilepositioning.com
2. Christopher Drane, Malcolm Macnaughtan, and Craig Scott, “Positioning GSM
telephones” IEEE Communication Magazine April 1998.
3. Jeffery H. Reed, Kevin J. Krizman, Brain D.Woerner, and Theodore S.
Rappaport, “An overview of challenges and progress in meeting the E-911
requirement for location service” IEEE Communication Magazine April 1998.
4. Richard Walter Klukas, “A Super resolution Based Cellular Positioning System
Using GPS Time Synchronization” UCGE Reports Number 20114