PLMR Specialized Mobile Radio - SMR

Specialized mobile radio (SMR) systems allow communication between mobile radios and one or more radio towers. SMR systems evolved to the allow trunking of radio channels (more efficient radio channel use). These systems use automatic radio channel assignments compared to manual frequency channel selection used in LMR systems.

SMR systems commonly provide advanced services not possible with traditional LMR systems. SMR systems always use radio channel pairs, whether or not the subscriber units operate half-duplex (requiring push-to-talk) or full duplex. Although direct mobile-to-mobile communications is possible (called “talk around”), it is not the usual mode of operation. Talk around (the direct connection between mobile radios) is useful if the subscriber units leave the coverage area of the system, or in case of system failure.

Originally, SMR systems were limited to a single tower site. SMR systems ordinarily cover a range of about 25 miles by using a single tower. Linking between different SMR systems can be difficult and roaming between SMR systems by a given subscriber being very difficult. More recent technological innovations have provided for advanced features such as multi-site networks, roaming capability, etc. Some SMR systems are interconnected but this is not typical. It is possible to connect SMR radio systems to the public telephone network. However, the primary use of SMR systems is dispatch operation.

This figure shows a typical two-way SMR dispatch radio system. In this diagram, a single radio channel is shared by several mobile radios that operate within the radio coverage limits of a high power radio base station. A dispatcher communicates to all the mobile radios by using a high-powered radio transmitter. Because the mobile radios mounted in vehicles transmit at a lower power than the dispatcher’s transmitter, this SMR system has several receiving antennas. When a mobile radio is communicating with the dispatcher, the receiver selects which antenna has the best signal (called a voting receiver).

SMR Dispatch System Diagram

   Private Land Mobile Radio - PLMR Books

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Introduction to Private Land Mobile Radio - PLMR Book Described are the basics of PLMR technologies including simplex, half-duplex, and full duplex operation. The different types of squelch systems are covered including carrier controlled squelch, tone controlled squelch, and digital squelch. The basics of analog and digital trunked radio systems are provided along with how and why analog trunked radio systems are converting to digital trunked radio systems. $14.99 Printed, $11.99 eBook

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