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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 with each other in full-duplex mode.
MOBILE TRAIN RADIO SYSTEMS
Present Day Scenario
A choice of mobile system for a set up is governed mainly by the following facts.
Ø Coverage area
Ø Number of subscriber to be catered
Ø Frequency spectrum available
Ø Nature of the terrain
Ø Type of application i.e. voice of data or both
Ø Integration with other systems
Ø Future technological migration capability
Ø 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.
Ø Text and status message transmission.
Ø Automatic Train operation's critical alarms.
Ø Train status and alarm information
Ø Passenger information system control
Ø Train passenger emergency system
Ø Closed circuit TV system
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