An Overview of Digital Mobile Radio
What Is Digital Mobile Radio?
Digital mobile radio or DMR uses digital signal processing instead of traditional analog signals. A few digital mobile radios are capable of both digital and analog broadcasts and reception. A number of digital mobile radios have built in GPS but that is not a requirement. Many European Union DMR products achieve a 6.25 kHz bandwidth, though there are competing technologies for doing so.
Is DMR the Same Thing as SDR?
Digital mobile radios may be software defined radios, but they are not necessarily the same thing. You can receive DMR broadcasts via a stationary software defined radio and vice versa.
Software Defined Radios are usually designed to use the Software Communications Architecture (SCA), something that doesn’t apply to DMR. Military SDR were originally designed based on standards set by the Joint Tactical Radio System or JTRS.
The Three Tiers of DMR
Digital mobile radio was designed with three tiers. The conventional DMR tiers 1 and 2 were published in 2005. Tier 1 DMR are available for license free use in the EU on the 446 MHz band, though this frequency range is open to amateur radio operators in the US. Level 1 DMR use a maximum of half a watt of RF power. Tier 1 digital mobile radios are not suitable for business applications or emergency use, since there is a limited number of channels and usage is first come, first served. These digital mobile radios are not supposed to use repeaters, nor should the work with telephone interconnects.
DMR tier 2 is for conventional radio systems, typically between 99 MHz and 960 MHz. Tier 2 DMR is for higher power communications and those who need spectral efficiency; DMR tier 2 uses a two slot time division multiple access technology. Tier 2 DMR radios are able to use channels of 12.5 kHz versus the prior minimum of 25 kHz, at a minimum doubling the number of channels available to users. Base stations and direct modes are both allowed with tier 2 digital mobile radios.
DMR tier 3 was published in 2012, but few digital mobile radios meet this standard. Digital mobile radios using Tier 3 build on the TDMA technology of Tier 2. DMR Tier 3 supports both short messaging, voice and data packets, including IPV4 and IPV6.
The Pros of Digital Mobile Radio
Digital mobile radios are PC programmable and firmware upgradable. Analog radios are rarely either.
Tier 2 DMR “narrowband” transmissions carry more voice and data over the same frequency range as was used before. In short, they can send information over much narrower signal frequency ranges than analog radios, allowing you to have more people working with the same frequency band without talking over each other.
Because DMR radios use narrower frequency bands for each channel, other channels can now be used as additional talk channels or data channels.
The Cons of Digital Mobile Radio
Digital mobile radios are more prone to multipath interference than analog radios. The wider range of frequencies they receive leave them vulnerable to this interference.
The quality of a DMR’s signal depends on the quality of the voice service.
What are DMR Antennas?
DMR antennas are simply antennas that work with a digital mobile radio. DMR antennas may be UHF, VHF or HF. Many DMR cover frequencies from 30 Mhz to 1 GHz. There are European digital mobile radios that receive frequencies as low as 66 MHz, the low VHF band.
Since many digital mobile radios are software defined radios receiving a broad signal range, ultrawideband antennas are often used with digital mobile radios.
Digital Mobile Radio Standards
European Union digital mobile radios must meet European Telecommunications Standards Institute Standard TS 102 361. Part 1 of this standard is for the air interface protocol. Part 2 defines the voice and general services and facilities. Part 3 applies to the data protocols. Part 4 is the standard for the trunking protocol.
Digital mobile radios in the EU need to meet standards EN 300 113 and EN 300 390. Those designed for use in the United States need to comply with FCC Part 90 regulations.
Standards Body for DMR
The international standards body for DMR is the Digital Mobile Radio Association. It was established in 2005 and supports the ETSI DMR standardization process and sets standards for interoperability testing. Membership in the DMR association is open to anyone building, using or contributing to DMR standards. Interoperability standards are maintained by the DMR Association and several major DMR manufacturers.