Repeater Communication Systems
Two-way radio communications give the businesses ability to keep in touch with their employees. As a result businesses become more efficient, competitive and can give better service to their customers. There are several typical business users, for whom communications are pivotal to what they do - trucking, taxi, and public safety, to name a few. All such operations use dispatcher to communicate with people out in the field by receiving and relaying business information upon which they act. To conduct efficient communications they use repeaters.
All repeaters basically work the same way. Most stand-alone high power repeaters are normally one channel, but can have multiple channels. These systems operate uncoded, and the users must check to see if the channel is busy before transmitting. The uncoded transmissions can be listened to by people using scanning receivers. One should not confuse "coded" and "encrypted". "Coded" simply means that a group of radios use the same control codes, so only users within that code group can listen. This code (also known as PL or DPL), being sent with the transmission, unsquelches the receiver.
For the repeater to operate efficiently, it must be placed (or rather, its antenna must be placed) at the highest location possible above the terrain. There it can cover a large area on a single frequency and can be keyed up by remote users from far away.
In the USA there are stand-alone commercial repeaters (a.k.a. community repeaters) that can accomodate more than one group of users sharing the same repeater - this is accomplished by assigning a unique code each user group. In most cases digital codes are used for that purpose. Then there are trunking repeaters which themselves are stand-alone repeaters grouped together into a single system. Any such system may have a central controller (most Motorola systems are), or they can have a decentralized control (LTR-based systems are a good example). The former tend to be more complex than the latter.
With the central controller one repeater channel will be designated as control channel, to which every system user will be listening. Mobile user placing a call interacts with the repeater via control channel. Controller validates userís access authorization and assigns a voice channel, and also switches all users in the same group to that channel. The central controller also handles the interconnect calls. On the other hand, with the decentralized control every repeater channel acts as a control channel. Users are assigned to repeater groups and can communicate when the channel becomes available. In large public communication systems multiple trunking sites to be linked together by means of dedicated telco lines or microwave links and resources are shared among many users over large area.
Repeaters receive mobile user transmissions on one frequency and retransmit them on another. Standard are 5 MHz splits for VHF/UHF bands, 45 MHz - for 800 MHz band and 39 MHz - for 900 MHz band. When the repeater receives the transmission from User A, it processes the received signal and retransmits it on another frequency for User B. In the US, repeater power can vary from 10 W to 150 W, depending on the license.
See also: Radios, Base, Repeaters, Telephone Interconnects and Controllers
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