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Digital Mobile Radios (DMR)

What is DMR?

Digital Mobile Radio (DMR) technology was developed by the European Telecommunications Standards Institute (ETSI). DMR is used worldwide
by professional mobile radio users. DMR digital mobile radio use a 2 channel TDMA, time division multiple access scheme. TDMA radios use a time slot to communicate together, which helps avoid interference.

The great benefit of DMR is how it provides multiple voice channels for every one RF channel. It does this by using two-slot TDMA (Time Division Multiple Access) technology. A 12.5 kHz channel is divided into two independent time slots. DMR achieves the 6.25 kHz channel equivalence (6.25e), specified by many global regulators seeking greater spectral efficiency. DMR is designed to work for consumer and short-range business users. They require low power, and are cost-optimised radios. Even professional users for whom radios are critical to their business or organization, and emergency services for whom radios are a vital mission critical tool, DMR is reliable and efficient. Have a private radio network can cover the required area as needed. With DMR base stations connected to wide area networks, a DMR system can cover a city, state, country, or globe.

DMR has three tiers.

Tier 1 (Unlicensed) is a single channel specification originally for the European unlicensed dPMR446 service. It is a single channel FDMA 6.25 kHz bandwidth; the standard supports peer-to-peer (mode 1), repeater (mode 2) and linked repeater (mode 3) configurations. Tier I standard has been expanded into radios for use in other than the unlicensed dPMR446 service.

Tier 2 (Conventional) is 2-slot TDMA 12.5 kHz IF peer-to-peer and repeater mode
specification. Each time slot can be either voice and/or data depending upon system needs. Most amateur radio implementations of DMR are using voice on both time slots. Any brand DMR (Tier 2) radio will work on a Tier II system.

Tier 3 (Trunking)was built upon Tier 2, adding trunking operation involving multiple
repeaters at a single site. Vender specific protocols expanded the
trunking to multiple site operations.

The current implementation of DMR utilizes the DSVI AMBE+2™
vocoder; it is not specified in the ESTI standard. Most of the radio manufacturers have implemented the vocoder in licensed software.

  Talk Groups (TG)

Talk Groups (TG) are for groups of users to share time slots without distracting and disrupting other users of the time slot. Note that only one Talk Group can be using a time slot at a time. If a radio is not programmed to listen to a Talk Group, you will not hear that Talk Group’s communication.

Base Stations

The DMR Base Station has a CACH data channel transmitted between the individual TDMA bursts. This enables the base station to transmit announcement information to DMR mobile and portable devices.

DMR systems support both voice and data services. On a trunked network, channels can be dynamically allocated as required. With DMR, priority levels ensure traffic with the highest priority will get passed through.

The maximum coverage of a TDMA system is indeed influenced by distance. Most DMR vendors, provide 0.5ppm frequency accuracy in the terminals. The theoretical limit is around 150km. There are many other factors that impact radio coverage. There are many DMR systems deployed with coverage areas of up to 100km. 

DMR radios satisify the FCC’s narrow-band mandate by being 6.25 kHz equivalent. DMR is more spectrum efficient than other 6.25 kHz modes since no guard band is needed for the two channels. DMR as 1/3 the channel RF bandwidth of an analog 25 kHz radio signal. It also has extended battery life and great voice quality, often better than other digital voice radios.

Filed under: DMR | Posted on June 9th, 2016 by John Sonnenberg


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