Private Land Mobile Radio - PLMR

Land mobile radio (LMR) systems are traditionally private systems that allow communication between a base and several mobile radios. LMR systems can share a single frequency or use multiple frequencies. The LMR industry in the United States is regulated by the FCC in part 90, Private Land Mobile Radio Services, includes various types of private radio services including police, taxi, fire and other types of two-way and dispatch services.

Land mobile radio (LMR) is different from Cellular and Personal Communications Services (PCS) in that it has its roots in "push-to-talk" radio systems. Push-to-talk can also be referred to as half-duplex (or simplex), whereas the half means only half of a conversation can happen at one time. This is sometimes experienced when using a household speakerphone. Full duplex, on the other hand, allows two simultaneous conversations, similar to a classic argument between a coach and a sports official. Push-to-talk communications range from a simple pair of hand-held citizen band (CB) walkie-talkies to a new breed of enhanced, full-duplex cellular-like consumer products. Licensed by the Federal Communication Communications (FCC) as specialized mobile radio (SMR), this radio service is primarily used by the public-safety sector, the industrial & construction sector and the dispatch sector like taxi-dispatch services. Though classified by the FCC as a commercial mobile radio service (CMRS), SMR is not currently classified like cellular and PCS as a common carrier telephone system.

Connection to the public switched telephone network (PSTN) by a two-way radio is usually done by incoming calls routed to a radio interface adapter. During this process, a simplex SMR radio channel is connected to a two-way regular telephone transmission line. The wired portion of the call extends the effective distance of the radio communication link. Even though a two-way wired connection is made, the communication itself usually remains a half-duplex (push-to-talk).

LMR systems can share a single frequency or use dual frequencies. Where LMR systems use a single frequency when mobile radios must wait to talk, this is called a simplex system. To simplify the mobile radio design and increase system efficiency, some LMR systems use two frequencies; one frequency is used for transmission and the frequency for reception. If the radio cannot transmit and receive at the same time, the system is called half duplex. When LMR systems use two frequencies and can transmit and receive at the same time, this is called full duplex. When a company operates a LMR system to provide service to multiple users on a subscription basis (typically to companies), it is called a public land mobile radio system (PLMR).

This figure shows a two-way radio system. In this example, a high-power base station (called a “base”) is used to communicate with portable two-way radios. The two-way portable radios can communicate with the base or they can communicate directly with each other. To transmit, the mobile radio user must push-to-talk.

   Related Private Land Mobile Radio - PLMR Definitions

Specialized Mobile Radio (SMR)
Dispatch
Trunked Radio
TETRA
MPT 1327 

   Private Land Mobile Radio - PLMR Books

more details

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

Emerging Public Safety Wireless Communication Systems	Land-Mobile Radio System Engineering	Voice and Audio Compression for Wireless Communications
Emerging Public Safety Wireless Communication Systems Robert I. Desourdis Jr., David R. Smith, William D. Speights,Hardcover,Series: Artech House Mobile Communications Series, English-language edition,Pub by Artech House, Incorporated	Land-Mobile Radio System Engineering Garry C. Hess,Hardcover, Edition: 1,Series: Artech House Mobile Communications Series, English-language edition,Pub by Artech House, Incorporated	Voice and Audio Compression for Wireless Communications Lajos Hanzo, F. Clare Somerville, Jason Woodard,Hardcover, Edition: 2,Series: IEEE Series on Digital & Mobile Communication Series, English-language edition,Pub by Wiley, John & Sons, Incorporated

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