Location Dependent Addressing Using GSM-R

Cellular Positioning
Liao min, Zhong Zhangdui
Modern Communication Institute,
Northern Jiaotong University, Beijing, 100044, P.R.China
Min-liao@263. net, zhduizhonlrGd263 .net

positioning should have n bright future when used for

Abstract LDA. Certainly, due to railway terrain and special
Location Dependent Addressing (LDA) is an requirements of communication services, there will be
effective and efficient addressing in GSM-R, many issues worthy of further research.
which needs location information of mobile - OVERVIEW of GSM-R and LDA
station to forward the call to the correct called GSM-R has been developed very fast as a European
party. It is worth developing the method that mobile communication system for railway. It uses the
is used to introduce GSM-R positioning GSM standard special for railway, which considers the
techniques into the solution of LDA. The requirement of railway communication and services.
paper analyzes the possibility of using GSM- So the main framework is same as GSM network and
R positioning in LDA. The characteristic of critical communication protocol such as mobility
GSM-R compatible with cellular positioning management, radio resources management is not
is given. At last, a location system model as different from GSM. GSM-R just introduces many
well as positioning techniques suitable for special functions, which have been brought into the
GSM-R system are presented. GSM Phase 2+ standard, such as Voice Gio~ipcall
services (VGCS), Voice Broadcast Services (VBS),
etc. At the same time, EIRENE, responsible for the
INTRODUCTION standard of GSM-R, develops other new functions,
I n recent years, it has become apparent that there is a one of which is LDA. LDA is a special addressing by
large demand for mobile location services. E-91 1 calls, which a call can be forwarded to the correct called
fleet management, ITS, and fraud detection all make party according to the location of calling party, not
good use of mobile positioning techniques. More and through the telephone number of the called party. It
more researchers and companies pay attention to can connect the called and calling party fast. For
mobile positioning applications in above services. example, a call from a mobile train driver to the
Location dependent addressing (LDA) is also mobile dispatcher responsible for him or her can be made like
location service that is used i n GSM-R network as a the following way. After the driver just dialing a short
special efficient and effective addressing. It, however, code or only a digit number such as '1' indicating a
receives little attention as a kind of mobile positioning call to the dispatcher, the system can automatically
technique applications. The location information route the call to the relevant dispatcher according to
rcquired in LDA is provided by the Cell Specific the location of the driver. Such kind of call, playing an
Routing or external location system, not the GSM-R important role in train runiiing operation, needs fast
positioning itself. According to the good use of GSM connection and high reliability. Apparently, LDA
positioning techniques in many services, GSM-R meets the challenge.

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 roducing GSM-R n system affeGts the reliability of LDA. Thirdly,
n, it is necessary to get the location QoS of external location system may be not good,
information of calling party in LDA. Just like the including response time, update period of location
definition of EIRENE specification, location information, etc. Long response time and update
information in present LDA systems is provided by period will affect its usefulness directly. Due to
two sources. One is Cell Specific Routing of GSM-R coming from extemal system, QoS of location
network itself and the other I S external system. In estimate is not considered according to the LDA but
GSM-R network, each cell has its ID number like that the external location system itself. So the requirement
i n GSM network. As a result, the network can get of LDA is usually hard to mect. Specially, in GSM-R
location information of each mobile station in form of network, LDA call as a kind of important train
cell ID number. Cell Specific Routing using in LDA running operation call, is hoped to have short call
can find the observed mobile running train with the setup time. Thus, there is a requirement of short
accuracy of cell. Then the network routes the call response time of location estimate. It’s hard for IN-
from the driver to the dispatcher managing the based LDA to overcome the challenge.
specific cell. If the border between dispatchers is same GSM positioning techniques have been developed fast
as that between cells, Cell Specific Routing will work in recent years. As a similar network, GSM-R should
well. I n fact, however, the requirement is hard to meet. have many potential abilities to positioning. Main
It I S not surprising that there are two more dispatchers positioning techniques including signal strength
in the same cell. EIRENE suggests that VGCS is used measurement, Angle of Arrival (AOA), Time of
in this situation. The network calls many dispatchers Arrival (TOA), Time Difference of Arrival (TDOA),
using group call, with the train number in the call hybrid technique and positioning system architectures
information. Then only the relevant dispatcher such as mobile-based system, network-based, hybrid
answers the call and contacts the train driver at last. system used in GSM can be developed similarly i n
Although Cell Specific Routing is not a final solution, GSM network. Furthermore, due to characteristic of
i t is still used when it fails to get location information GSM-R and LDA, there are other advantages of
from external system. LDA in this situation has low introducing GSM-R positioning into LDA. At the
efficiency due to the low accuracy of location same time, more considerations should be made
estimate. before using suitable positioning techniques and
The other IS IN-based (Intelligent Network) LDA. architecture.
External location system provides location
Advantage provided by GSM-R and LDA
information of the mobile train driver, as well as the
There are many factors affecting the positioning
relationship between short code and its corresponding
accuracy and efficiency. Due to GSM-R special
telephone number, which is stored in the database of
network architecture and railway terrain, sometimes it
IN network. When a LDA call starts, MSC
is easier to overcome them than GSM positioning.
interrogates the IN database for the real telephone
Hearability of non-serving BS ’ s
number of the called party. The external location
In general, each mobile station I S only covered by one
system such as GPS, balise, can provides high
BS in cellular network. In most positioning techniques,
accuracy of location, so that the LDA call can be
however, non-serving BSs are involved. The
connected between the calling and called parties.
hearability of signals to or from these BSs may cause
Despite its high accuracy, disadvantage of IN-based
some measurements to be impractical when the
LDA IS not ignored. Firstly, it increases the
mobile station is closed to its serving BS. The signal
complexity of LDA. Furthermore, the interface
must be faint for the non-serving BS. As a result, i t
performance between GSM-R network and external

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 may be impossible to collect the required stations and BSs are in the same line. Furthermore, tlie
measurements from the non-serving BSs in a short busy lines of railway are normally in the area of
enough time. Redundancy configuration of GSM-R plateau. Thus NLOS seldom occurs and higher
wireless coverage gives a better solution about the performance of positioning is expected to obtain than
issue. As a professional network, GSM-R is required GSM network from this aspect.
to have high reliability. Reliability of wireless Advantage from LDA
coveragc makes a very important role in the reliability From tlie definition of LDA, it is concluded that
of GSM-R conimunication. In order to meet the location information request is invoked by GSM-R
challenge, many redundancy techniques have been network, 'while the call is triggered by the calling
taken in GSM-R network. Figure 1 shows these party. At least three benefits are attains. Firstly, the
techniques in details. observed mobile station is always active. Thereby, it is
1 2 3 not essential to consider MS not-reachable, detached
or purged. These three things must be given enough
consideration in other common cellular positioning
applications. For instance, externai location client may
request the location information of a not-reachable
Figure I(a) Cell overlap coverage MS. Secondly, location client is tlie GSM-R network
I/- itself but not the external one. Transmission time of

. c-ecz3-
Fieuie I ( b ) Nctwork ovctldo coveraw
location information gets shorter. In other word, LDA
positioning is convenient. Finally, Gateway mobile
location center (GMLC), an absolutely necessary
Cell overlap coverage is showed in Figure l(a), while entity in GSM LCS network (will explain later)
network overlap coverage is showed in Figure 2(b). becomes useless and will be canceled in tlie LDA
Both of them ensure that each mobile station is positioning architecture. At the same time, response
covered by two BSs, one of which does not transmit time of positioning procedure will decrease due to
until the working one fails. So the mobile station can fewer entities.
get good coverage unless both BSs have failures, Need only two-dimensional location inforniation
which seldom occurs. The redundancy mechanism According to the railway requirement, only two-
gives a good chance to solve the measurement of diniensional location information will be provided by
signal from one more BSs to the observed mobile the LDA positioning system. Thus number of BSs
station. These two BSs can receive strong signal for involved in signal nieasurenient becomes few. For
the MS. I n GSM-R network, only one BS works all instant, at least four BSs are involved in TDOA
the time. A modification should be made that two BSs techniques, when three-diiiiensional location estimate
work together when LDA call takes place. is required. Three BSs, however, are enough on the
Non-Line of Signal (NLOS) request of two-dimensional estimate.
With NLOS propagation, tlie signal arriving at the BS Direction of train running
from the mobile station is reflected or diffracted and There is a definite direction of train when it is running.
takes a long path than the direct path. Five mobile It is evaluable information because the location
positioning narrated above are based on the Line of L,,,,,.
= L,,,d + . (1)
Signal (LOS). In other word, NLOS will degrade the estimate would be gained as followed when present
accuracy of location estimate. NLOS usually occurs in positioning fails:
urban or niontanic terrain, which there are inany high Where L,,, is the present location estimate, with Lo,,
buildings or mountains. Along the railway, mobile

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 location information, v and t meaning intersection of a hyperbola provided by TDOA
running time from the last location to between two BSs and a straight line from AOA
the new destination. In general location application, decides the location of mobile station.
this equation is meaningless because the mobile A logical architecture of GSM LCS is defined in
station runs in non-fixed direction GSM03.71 by ETSI, as showed in Figure 3.
Now it is easy to draw a conclusion that introducing .
mobile positioning into LDA is feasible and L

convenient.

LDA logical model MLC Extemal

It is necessary to select the positioning technique and chen t
v
architecture before a LDA logical model is given.
Figure 3 Generic LCS Logical Architecture
As we known, there are many positioning techniques
LCS is logically implemented on the GSM structure
used in cellular system. In typical GSM-R network,
through the addition of one network node, the Mobile
directional antennas are installed along the rails in
Location Center (MLC). A number of new interfaces
order to get the elliptical coverage of cells. In the large
is named. A LCS procedure is made out briefly below.
stations, sector cells are made while normal cells are
External LCS client accesses in a GSM network via
installed in other areas. Directional antennas of BSs
the Gateway Mobile Location Center (GMLC) with
benefit AOA techniques. Another question is
the location request of a mobile station. Then GMLC
Geometric Dilution if Precision (GDOP) caused by
may request routing information of observed mobile
railway layout. In certain scenarios, the GDOP will
station from HLR. After performing registration
have a significant effect on the accuracy of the system.
authorization, it sends positioning request including
Specifically, railway cells with the BS arrangeiiient
QoS information to the Serving Mobile Location
may be in a straight line. But TOA and TDOA are
Center (SMLC) via VMSC. SMLC manages the
triangular solutions. F r o m the two points, TOA or
overall coordination and scheduling of resources
TDOA combining AOA is likely to increase
required to perform positioning of a mobile station.
performance of mobile positioning along the railway.
It's up to the Location Measuring Unit (LMUs)
The former needs only one BS and the latter needs
controlled by the SMLC to perform positioning
two BSs, as showed in Figure 2.
measurement with the techniques decided by the
I n figure 2(a), TOA determines a circle with AOA
SMLC. After receiving the measurement result sent
making a straight line. The intersection of them is the
by LMU via VMSC, SMLC calculates the final
location of the mobile station. Similarly, the
location estimate and accuracy. ,At last, location
S , estimate is sent to the external LCS client via GMLC.
That's a successful LCS procedure.
LDA, as a special LCS application, should have a
more suitable architecture than above. Figure 4 shows
the procedure of LDA using GSM-R positioning. The
driver on a running train calls the dispatcher
responsible for him or her with dialing a short code
'1 12'. BS forwards the LDA call to MSC, which sends
Figure 2 Hybrid technique for LDA its IMSI to the SMLC for the location estimate. Then
SMLC begins the location positioning same as GSM
network performing. After receiving the location

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estimate from SMLC, MSC look up two databases: Conclusion
DBl for meaning of short code and looking up DB2 This paper has provided the overview of GSM-R and
LDA. It is characteristic of them
suitable for positioning that is
emphasized. Finally, a conclusion
is made that LDA using GSM-R
positioning is feasible and
convenient. Although GSM-R
has bright future, many issues
will be arisen when LDA
combining GSM-R positioning.
Thus it is evident that many
research challenges still exist in
providing high-reliability and
Figure 4 LDA using GSM-R positioning high QoS LDA call in GSM-R
with the driver location infoiination provided by network.
SMLC for dispatcher number. Similarly, if LDA
called party is shunning personnel, DBl and DB3 are Reference
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