Mobile Train Radio Communication

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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 with each other in full-duplex mode. ??????

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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.
? Landline 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 radio or mobiles refer to wireless communications systems and devices which are based on radio frequencies, and where the path of communications is movable on either end. There are a variety of views about what constitutes mobile equipment. For US licensing purposes, mobiles may include hand-carried, (sometimes called portable), equipment. An obsolete term is radiophone.
Some mobile radios are mounted in aircraft, (aeronautical mobile), shipboard, (maritime mobile), on motorcycles, or railroad locomotives. Power may vary with each platform. For example, a mobile radio installed in a locomotive would run off of 72- or 30-volt DC power. A large ship with 117V AC power might have a base station mounted on the ship's bridge.

2.Disambiguation: Two-way versus telephone
The distinction between radiotelephones and two-way radio is becoming blurred as the two technologies merge. The backbone or infrastructure supporting the system defines which category or taxonomy applies. A parallel to this concept is the convergence of computing and telephones.
Radiotelephones are full-duplex (simultaneous talk and listen), circuit switched, and primarily communicate with telephones connected to the public switched telephone network. The connection sets up based on the user dialing. The connection is taken down when the end button is pressed. They run on telephony-based infrastructure such as AMPS or GSM.
Two-way radio is primarily a dispatch tool intended to communicate in simplex or half-duplex modes using push-to-talk, and primarily intended to communicate with other radios rather than telephones. These systems run on push-to-talk-based infrastructure such as Nextel's iDEN, Specialized Mobile Radio (SMR), MPT-1327, Enhanced Specialized Mobile Radio (ESMR) or conventional two-way systems. Certain modern two-way radio systems may have full-duplex telephone capability.

2.1 Mobile Radio

The mobile radio is a two-way communication gadget that operates through radio frequencies. As such, the channel of information and messages in a mobile radio is variable. Used to be known as radiophone, the earlier versions of the mobile radio, were one-way communication systems used for broadcast.
Contemporary mobile radio systems can have as much as a hundred channels and may be controlled by microprocessors. These types require the use of software to encode channels and operate their integrated functions. The mobile radio, also known as a two-way radio system, allows the exchange of messages only with other mobile radios through push-to-talk (PTT) functions. A mobile radio also features wireless transceivers, making mobile radios portable. Mobile radio systems may be used for communications in aircraft, ships, automobiles, and other vehicles. The power supply on which mobile radios run depend on the type of vehicle these are mounted on.
A mobile radio system is composed of a transceiver and microphone with a push-to-talk key. It has an antenna that links to the transceiver. Since most types of mobile radio are used in moving vehicles, where the surrounding noise can be loud, some mobile radio types come with an external speaker. Other models have headsets and microphones with noise-reduction capabilities.

2.2. How does a mobile radio work?

Most mobile radios operate on a single band of frequency. The radio transceiver contains transmit and receive frequencies. Very high frequency (VHF) and ultra high frequency (UHF) allow a mobile radio to operate on maximum coverage. This means that its average operating range is from 150 to 470 MHz.
To transmit a message, the PTT key must be pressed during talk-time to allow the voice message to be dispatched by the sending party. During this period, the sending party cannot hear or receive any incoming messages from the mobile radio. Once the PTT button is released, the sender may hear the response of the receiving party.

2.3. Why do we need a mobile radio

The use of mobile radio in transportation, security, and general operations makes communication fast, efficient and safe. It allows control centers to monitor location of vehicles and dispatch announcements to several receivers simultaneously. Additionally, the range of its area coverage is very high and is not dependent on a cellular network, which may fluctuate during emergency situations.
Different types of mobile radio are portable and capable of withstanding shock and severe weather conditions. Most countries impose certain requirements on the manufacture, sale and use of two-radio systems. This helps ensure that the communication gadget functions according to standards and that its use does not interfere with other communication systems.

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