What is a DMR System?
Digital mobile radio (DMR) is an open digital mobile radio standard defined in the European Telecommunications Standards Institute (ETSI) Standard TS 102 361 parts 1–4 and used in commercial products around the world. DMR, along with P25 phase II and NXDN are the main competitor technologies in achieving 6.25 kHz equivalent bandwidth using the proprietary AMBE+2 vocoder. DMR and P25 II both use two-slot TDMA in a 12.5 kHz channel, while NXDN uses discrete 6.25 kHz channels using frequency division. The standard has become popular within the amateur radio community due to the relatively lower cost and complexity compared to other commercial digital modes.
DMR was designed with three tiers. DMR tiers I and II (conventional) were first published in 2005, and DMR III (Trunked version) was published in 2012, with manufacturers producing products within a few years of each publication.
The primary goal of the standard is to specify a digital system with low complexity, low cost, and interoperability across brands, so radio communications purchasers are not locked into a proprietary solution. In practice, many brands have not adhered to this open standard and have introduced proprietary features that make their product offerings non-interoperable.
Technical Specifications of a DMR System:
The DMR interface is defined by the following ETSI standards:
TS 102 361-1: Air interface protocol
TS 102 361-2: Voice and General services and facilities
TS 102 361-3: Data protocol
TS 102 361-4: Trunking protocol
The DMR standard operates within the existing 12.5 kHz channel spacing used in land mobile frequency bands globally but achieves two voice channels through two-slot TDMA technology built around a 30 ms structure. The modulation is 4-state FSK, which creates four possible symbols over the air at a rate of 4,800 symbols/s, corresponding to 9,600 bit/s. After overhead, forward error correction, and splitting into two channels, there is 2,450 bit/s left for a single voice channel using DMR, compared to 4,400 bit/s using P25 and 64,000 bit/s with traditional telephone circuits.
The standards are still (as of late 2015) under development with revisions being made regularly as more systems are deployed and improvements are discovered.  It is very likely that further refinements will be made to the standard, which will necessitate firmware upgrades to terminals and infrastructure in the future to take advantage of these new improvements, with potential incompatibility issues arising if this is not done.
DMR covers the RF range 30 MHz to 1 GHz.
There are DMR implementations, (as of early 2016), that operate as low as 66 MHz (within the European Union, in 'Lo-Band VHF' 66–88 MHz.)
The DMR Association and manufacturers often claim that DMR has superior coverage performance to analog FM. Forward error correction can achieve a higher quality of voice when the received signal is still relatively high. In practice, however, digital modulation protocols are much more susceptible to multipath interference and fail to provide service in areas where analog FM would otherwise provide degraded but audible voice service. At a higher quality of voice, DMR outperforms analog FM by about 11 dB. But at a lower quality of voice, analog FM outperforms DMR by about 5 dB.
Where digital signal processing has been used to enhance the analog FM audio quality than analog FM generally outperforms DMR in all situations, with a typical 2–3 dB improvement for "high quality" voice and around 5 dB improvement for "lower quality" voice. Where digital signal processing is used to enhance analog FM audio, the overall "delivered audio quality" is also considerably better than DMR. However, DSP processing of analog FM audio does not remove the 12.5 kHz requirement so DMR is still more spectrally efficient.
Different Tiers of DMR:
DMR Tier I
DMR Tier I products are for license-free use in the European PMR446 band. Tier I products are specified for non-infrastructure use only (meaning without the use of repeaters). This part of the standard provides for consumer applications and low-power commercial applications, using a maximum of 0.5 watt RF power.
Note that a license-free allocation is not present at this frequency outside of Europe, which means that PMR446 radios including DMR Tier I radios can only be used legally in other countries once an appropriate radio license is obtained by the operator.
Some DMR radios sold by Chinese manufacturers (most notably Baofeng) have been mislabeled as DMR Tier I. A DMR Tier I radio would only use the PMR446 license free frequencies, and would have a maximum transmitted power of 0.5 W as required by law for all PMR446 radios.
Although the DMR standard allows Tier I DMR radios to use continuous transmission mode, all known Tier I radios currently use TDMA, the same as Tier II. This is probably due to the battery savings that come with transmitting only half the time instead of continuously.
DMR Tier II
DMR Tier II covers licensed conventional radio systems, mobiles and hand portables operating in PMR frequency bands from 66–960 MHz. The ETSI DMR Tier II standard is targeted at those users who need spectral efficiency, advanced voice features and integrated IP data services in licensed bands for high-power communications. A number of manufacturers have DMR Tier II compliant products on the market. ETSI DMR specifies two slot TDMA in 12.5 kHz channels for Tier II and III.
DMR Tier III
A portable radio compatible with the DMR Tier III digital radio standard.
DMR Tier III covers trunking operation in frequency bands 66–960 MHz. Tier III supports voice and short messaging handling similar to TETRA with built-in 128 character status messaging and short messaging with up to 288 bits of data in a variety of formats. It also supports packet data service in a variety of formats, including support for IPv4 and IPv6. Tier III-compliant products were launched in 2012.