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INTRODUCTION
Mobile Communications Principles:
Each mobile uses a separate, temporary radio channel to talk to the cell site. The cell site talks
to many mobiles at once, using one channel per mobile. Channels use a pair of frequencies for
communication. One for transmitting from the cell site, the forward link, and one frequency for
the cell site to receive calls from the users, the reverse link. Communication between mobile
units can be either half-duplex or full-duplex. In case of half-duplex, transmit and receive
communications between the mobile units are not at the same time, i.e. talking and listening can
not be done at the same time. In case of full-duplex communication, transmit and receive
communication is at the same time, i.e. one can talk and listen at the same time. When
communications between mobile units are within a cell, and if the same is half-duplex, then it
shall require only one pair of frequency. If the same is full-duplex, then requirement of
frequency pair shall be two.When a mobile unit is communicating with a mobile unit outside
the cell, then the requirement of frequency pair shall be one per cell for both half-duplex and
full-duplex communication. Hence the system resources are utilized more if the mobile units
communicate
Early Mobile Telephone System Architecture
Traditional mobile service was structured similar to television broadcasting. One very
powerful transmitter located at the highest spot in an area would broadcast in a radius of up to
fifty kilometers. The “cellular concepts” structure the mobile telephone network in a different
way. Instead of using one powerful transmitter, many low-power transmitters were placed
throughout a coverage area. For example, by dividing a metropolitan region into one hundred
different areas (cells) with low-power transmitters using twelve conversations (channels) each,
the system capacity theoretically could be increased from twelve conversations - or voice
channels using one powerful transmitter- to twelve hundred conversations (channels) using one
hundred low-power transmitters.
Different Type Of Communication Systems:
The different types of communication systems available today can be broadly classified into the
following categories.
● Land line System
● Cellular System
● Satellite System
The evolution of the above Systems had been broadly as a point to point system.
● Two-Way Radio System
The evolution of the Two-way Systems has been both as a point to point and a point to multi
point system.
MOBILE TRAIN RADIO SYSTEMS
Present Day Scenario
A choice of mobile system for a set up is governed mainly by the following facts.
● Coverage area
● Number of subscriber to be catered
● Frequency spectrum available
● Nature of the terrain
● Type of application i.e. voice of data or both
● Integration with other systems
● Future technological migration capability
● Cost of the system
Railways’ Present Day Requirements.
The Train Mobile System’s present day requirements are not just voice transmission, but
also along with voice the system shall be capable of handling data also. Typical applications for
the Modern Train Mobile System are as under.
● Text and status message transmission.
● Automatic Train operation’s critical alarms.
● Train status and alarm information
● Passenger information system control
● Train passenger emergency system
● Closed circuit TV system
Comparison of Various Open Standard Technologies Available Today
MPT1327 System
MPT1327 is an Open Standard for Analog trunked radio networks. The British Department of
Trade and Industry (DTI) developed it in year 1988. In the course of the next twelve months
of development continued and resulted in MPT1343 standards. A system based on MPT1327
generally comprises of several radio channels. At least one of these channels will have been
defined as the CC (Control Channel) and all the other channels are TCs (Traffic Channel). Data
messages between the mobiles and the networks are exchanged on the Control Channel at 1200
bits/sec using FSK (Frequency Shift Keying). Each subscriber in a trunked radio network has a
unique call number. It consists of a prefix (3
digits), a fleet number (4 digits), and a subscriber number within the fleet (2 or 3 digits). After
it has been entered the call number will be converted in the mobile to a 20-bit address. For the
duration of the call a subscriber is exclusively allocated a traffic channel from the available
trunk. Western Railway has already opted for a MPT1327 system for the Motorman and
Controller communication in its Suburban Section in Mumbai Division. This system is as a part
of the TMS project between Churchgate and Virar Sections. M/s Tait New Zealand has supplied
the Mobile System.
Tetra Systems :
TETRA stands for Terrestrial Trunked Radio, covering PMR (Professional Mobile Radio) as
well as PAMR (Private Access Mobile Radio) applications. As a trunked system, it is designed
to be the true follower of MPT1327. TETRA applies digital speech transmission with TDMA
(burst transmission), very fast call setup times, and may use powerful encryption. TETRA is an
Open Standard defined by ETSI (European Telecommunication Standards Institute). TETRA
applies a modulation format called pi/4 DQPSK, TDMA with 4 channels per carrier, and a
carrier spacing of 25 KHz. TETRA does not have a fixed frequency allocation as GSM. But the
systems currently planned or installed in Europe assumes frequencies in the range of 380..400
MHz for public a safety communication, and 410..430 MHz for commercial systems.
GSM System:
● The GSM (Global System for Mobile Communication) MoU Association, a Swiss
registered Corporation, is the principle body responsible for promoting and evolving the
GSM wireless platform worldwide. Today GSM is the most successful implementation
of a global wireless standard using digital technology for point to point operations. There
are over 293 members representing 120 countries/areas. The overall objective of the
GSM MoU Association is “ The promotion and evolution of the GSM900, GSM1800,
GSM1900 systems, and the GSM platform for international roaming, for the provision of
standardized services.
Global System for Mobile Communications - Railway or GSM-Railway is an international
wireless communications standard for railway communication and applications. A sub-system
of European Rail Traffic Management System (ERTMS), it is used for communication between
train and railway regulation control centers. The system is based on GSM and EIRENE -
MORANE specifications which guarantee performance at speeds up to 500 km/h (310 mph),
without any communication loss.
GSM-R is a secure platform for voice and data communication between railway operational
staff, including drivers, dispatchers, shunting team members, train engineers, and station
controllers. It delivers features such as group calls (VGCS), voice broadcast (VBS), locationbased
connections, and call pre-emption in case of an emergency. This will support applications
such as cargo tracking, video surveillance in trains and at stations, and passenger information
services.
GSM-R is typically implemented using dedicated base station towers close to the railway.
The distance between the base stations are 7–15 km. This creates a high degree of redundancy
and higher availability and reliability. The train maintains a circuit switched digital modem
connection to the train control centre at all times. This modem operates with higher priority than
normal users (eMLPP). If the modem connection is lost, the train will automatically stop. In
Germany, Italy and France the GSM-R network has between 3000 and 4000 base stations.
In the UK, over 14,000 km of GSM-R enabled railway is planned, with 280 km currently in
operation.GSM-R cab radio for use in UK trains is currently being developed by Network Rail
and is undergoing trials in the Strathclyde and Glasgow areas of Scotland. It is currently in use
on London Overground's East London Line, between Dalston Junction and New Cross Gate.
Upper system
GSM-R is one part of ERTMS (European Rail Traffic Management System) which is composed
of:
● ETCS (European Train Control System).
● GSM-R.
Frequency band
In Europe, GSM-R uses a specific frequency band:
● 876 MHz — 880 MHz: used for data transmission (uplink)
● 921 MHz — 925 MHz: used for data reception (downlink)
However GSM-R can operate on a number of frequencies that are beeing used around the world
In China GSM-R occupies a 4 MHz wide range of the E-GSM band (900 MHz-GSM). In
Australia GSM-R is being implemented using frequencies in the 1800MHz band.
The used modulation is GMSK modulation (Gaussian Minimum Shift Keying). GSM-R is a
TDMA (“Time Division Multiple Access”) system. Data transmission is made of periodical
TDMA frames (with a period of 4.615 ms), for each carrier frequency (physical channel). Each
TDMA frame is divided in 8 time-slots, named logical channels (577 µs long, each time-slot),
carrying 148 bits of information.
There are worries that LTE mobile communication will disturb GSM-R, since it has been given a
frequency band rather close to GSM-R. This could cause ETCS disturbances, random emergency
braking because of lost communications etc.
GSM-R uses
GSM-R permit new services and applications for mobile communications in several domains :
● transmission of Long Line Public Address (LLPA) announcements to remote stations
down the line
● control and protection (Automatic Train Control/ETCS) and ERTMS)
● communication between train driver and regulation center,
● communication of on-board working people
● information sending for ETCS
● communication between train stations, classification yard and rail tracks
Main use
It is used to transmit data between trains and railway regulation centres with level 2 and 3 of
ETCS. When the train passes over a Eurobalise, it transmits its new position and its speed, then
it receives back agreement (or disagreement) to enter the next track and its new maximum speed.
In addition, trackside signals become redundant.
A GSM-R mobile phone used by the National Railway Company of Belgium
A modern GSM-R cab radio
Like other GSM devices, GSM-R equipment can transmit data and voice. New GSM-R features
for mobile communication are based on GSM, and are specified by EIRENE project. Call