DMR

Digital mobile radio

Digital mobile radio (DMR) is a limited open digital mobile radio standard defined in the European Telecommunications Standards Institute (ETSI) Standard TS 102 361 parts 1–4[1] 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 and TETRA uses a four-slot TDMA in a 25 kHz channel.

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, given the current limited scope of the DMR standard, many vendors have introduced proprietary features that make their product offerings non-interoperable with other brands.

DMR Tier I

DMR Tier I products are for licence-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 licence 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 licence is obtained by the operator.

Some DMR radios sold by Chinese manufacturers (most notably Baofeng) have been mis-labelled as DMR Tier I. A DMR Tier I radio would only use the PMR446 licence 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.

ADVANTAGES OF DIGITAL TWO-WAY RADIO

Since the first wireless transceiver was installed in a Bayonne, New Jersey police car in 1933, two-way radio has been a mission-critical technology for police, firefighters, search and rescue workers and others on the front lines of public safety. And increasingly, as new models have reduced the size and cost of two-way radios, the technology has been adopted by business professionals as well.

Industries including transportation, education, construction, manufacturing, energy and utilities, private security, government, hospitality, retail, and many others are finding that two-way radio can improve efficiency, worker productivity and responsiveness by allowing mobile teams to share business and customer information instantly.

Through most of its history, two-way radio has meant analog voice — the representation of sound waves as either amplitude modulated (AM) or frequency modulated (FM) radio waves. In fact, this is one of the last areas of professional communications to be touched by digital technology. But that’s changing, very quickly, for very good reasons.

Modulating the voice into digital signals, rather than analog, provides several advantages. First and foremost, digital technology provides better noise rejection and preserves voice quality over a greater range than analog. especially
at the farthest edges of the transmission range, users can hear what’s being said much more clearly — increasing the effective range of the radio solution and keeping users responsive to changing situations in the field.

Depending on the technologies used, digital systems can also be designed to:

  • Make more efficient use of available, licensed RF spectrum
  • Combine voice and data access in the same device, delivering more information while empowering field workers with systems that are more portable, flexible, and much easier to use than two different and incompatible systems
  • Enable integration and interoperability with back-end data systems and external systems
  • Combine analog and digital voice in the same device, easing the migration to digital while preserving investments in analog technology
  • Provide strong, practical, easy-to-use privacy solutions without the significant loss in voice quality that analog scrambling can cause
  • Enable flexible and reliable call control and signaling capabilities
  • Flexibly adapt to changing business needs and new applications through a modular architecture

The clear advantages of digital radio — along with increasing regulatory pressures to use RF spectrum more efficiently — will drive widespread adoption of professional two-way digital radio solutions in the coming years. If you’re using analog today, you’ll almost certainly be migrating to digital tomorrow. Now is the time to research the available technologies so that, when you’re ready to make the move, you’ll choose systems that provide the greatest benefit over the long term.

TDMA: HOW IT WORKS

TDMA stands for “Time-Division Multiple Access.” Like FDMA, or “Frequency-Division Multiple Access,” TDMA is a technology that allows multiple conversations to share the same radio channel. Although the goal is the same, the two technologies work very differently.

6.25KHZ FDMA

In FDMA, a channel frequency is split into smaller subdivisions — for example, splitting a 25 kHz band into two narrower “sub-channels” that transmit side-by-side to achieve 12.5 kHz equivalent spectral efficiency. The same technique can be used to achieve 6.25 kHz equivalent efficiency in a 12.5 kHz channel — although how well this technique will perform hasn’t yet been established in real- world implementations on a large scale. As the subdivisions of a licensed channel become narrower, there’s a growing likelihood of problems due to congestion and interference in an FDMA-based 6.25 kHz-equivalent system, as shown in the illustration.

When you try to squeeze two 6.25 kHz signals into one 12.5 kHz channel, you still have to meet the channel’s regulatory emissions mask. In order to do so, the signal deviation (represented by the height and width of the lobes in the illustration) must necessarily be smaller than what can be achieved with a single 12.5 kHz signal. This smaller deviation means reduced sensitivity, which in turn reduces effective signal range in real world conditions. At the same time, there is very little tolerance for errors introduced by oscillator aging, and the 6.25 kHz signal contains more energy near the edges of the mask — making it more prone to adjacent channel interference and near/far interference problems. This results in reduced quality of service in real world conditions.

TWO-SLOT TDMA

By comparison, TDMA offers a proven method for achieving 6.25 kHz equivalency in 12.5 kHz repeater channels — a major benefit for users of increasingly crowded licensed bands. Instead of dividing the channel into two smaller slices, TDMA uses the full channel width, dividing it into two alternating time slots. As a result, TDMA essentially doubles repeater capacity while preserving the well-known RF performance characteristics of the 12.5 kHz signal.

From the perspective of RF physics — that is, actual transmitted power and radiated emissions — the 12.5 kHz signal of two-slot TDMA occupies the channel, propagates, and performs essentially the same as today’s 12.5 kHz analog signals. with the added advantages of digital technology, TDMA-based radios can work within a single repeater channel to provide roughly twice the capacity of analog while offering RF performance equivalent to, or better than, today’s analog radio.

As we will see, the two time slots can potentially be used for a variety of purposes. Most organizations considering TDMA- based two-way radio will probably

 

As we will see, the two time slots can potentially be used for a variety of purposes. Most organizations considering TDMA- based two-way radio will probably be interested in doubling the voice capacity per licensed repeater channel. By enabling 6.25 kHz equivalency, TDMA supports two simultaneous, independent half-duplex calls in a single 12.5 kHz repeater channel.

If you’re used to thinking about analog radio, this two-for-one capacity in two different time slots might seem problematic. Wouldn’t the two calls cut in and out as the time slots alternate, making both conversations nearly impossible to understand?

But remember, this is the digital world, where voices are encoded in bits. Although analog signals represent the actual duration of spoken words, digital signals can encode that duration in a way that allows for significant compression without compromising voice quality. Each TDMA time slot is quite brief — on the order of 30 milliseconds. The circuitry that translates voice into bits is actually able to pack 60 milliseconds worth of digitized speech into each 30 millisecond time slot. The receiver, in turn, unpacks those bits into speech that has its full 60 millisecond time value.

That’s why, with TDMA, two conversations can happen simultaneously and seamlessly via a single repeater. The alternation of time slots is something that happens in the technology only, not in the user’s experience. In fact, digital technology offers better background noise suppression than analog while preserving the integrity of the signal at the farthest reaches of the transmitter’s range — so both digital conversations are likely to be much clearer than a single analog conversation would be over the same channel. And because both conversations use the channel’s full bandwidth, there’s no degradation in range performance, and no added risk of interference with adjacent channels.

 

LOWER EQUIPMENT COSTS

Compared to 6.25 kHz FDMA, two-slot TDMA allows you
to achieve 6.25 kHz equivalent efficiency while minimizing investments in repeaters and combining equipment. This is one reason why TDMA is so well suited to professional applications, where the budget for two-way digital radio may be limited compared to the mission-critical tier.

FDMA requires a dedicated repeater for each channel, plus expensive combining equipment to enable multiple frequencies to share a single base-station antenna. It can be particularly expensive to make combining equipment work with 6.25 kHz signals, and there’s typically a loss in signal quality and range when it’s used this way.

In contrast, two-slot TDMA achieves two-channel equivalency using single-channel equipment. No extra repeaters or combining equipment is required.

ADVANCED FEATURES AND FLEXIBILITY

In a traditional FDMA two-way radio implementation, each transmission occupies a full 12.5 kHz channel. A single channel can accommodate a single, half-duplex call. Proprietary implementations that use FDMA to achieve two 6.25 kHz equivalent channels enable two conversations to take place within a 12.5 kHz channel — but again, both of these conversations are half-duplex, and there’s no flexibility to put the extra capacity to any other use. TDMA-based digital systems with two time slots aren’t bound by these technical restrictions. The two time slots can be used to carry two half-duplex conversations — as with the two sub-channels in an FDMA-based system — but with no need for extra equipment and no danger of reduced performance. Unlike FDMA, however, it’s also possible to use the second TDMA time slot for other purposes.

For example, device designs for the first-generation of TDMA-based two-way radio include the ability to use the second time slot for reverse-channel signaling. This capability can be used for priority call control, remote control of the transmitting radio, emergency call pre-emption, and more. The second time-slot could also be used for transmitting application data such as text messaging or location data in parallel with call activity — a useful capability, for example, in dispatch systems that provide both verbal and visual dispatch instructions.

TDMA-based systems also offer the flexibility to adapt as new applications emerge to make additional use of the two time slots — preserving initial investments while providing an open path to future usage models for digital two-way radio. For example, the future roadmap for two-slot TDMA applications includes the ability to temporarily combine slots for increased data rates, or to use both slots together to enable full-duplex private calls.

Additional capabilities will also emerge, as driven by the real-world needs of two-way radio users in the professional marketplace. By choosing TDMA, professionals can immediately gain benefits such as 2:1 voice capacity and reverse-channel signaling within a single channel, with the option to add other capabilities as they become available. FDMA, in contrast, is optimized for a single purpose — half- duplex calling.

LONGER BATTERY LIFE

One of the biggest challenges with mobile devices has always been battery life. In the past, there have only been a couple of options for increasing the talk time on a single battery charge. One way is to increase battery capacity. Battery manufacturers have already done a remarkable job of maximizing capacity, but further gains are only possible by increasing the size of the battery pack — and therefore decreasing portability.

The other option is to decrease transmit power, which is by far the most energy-intensive function of two-way radio. But this means decreasing transmission range and increasing the potential for interference from other devices — an unacceptable tradeoff in professional situations.

Two-slot TDMA provides another, very effective option. Since each call uses only one of the two slots, it requires only half of the transmitter’s capacity. The transmitter is idle half the time — that is, whenever it’s the unused time-slot’s “turn.”

For example, in a typical duty cycle of 5 percent transmit, 5 percent receive, and 90 percent idle, the transmit time accounts for roughly 80 percent of the total current drain on the radio’s battery. By cutting the effective transmit time in half, two-slot TDMA can thus enable an up to 40 percent reduction in current battery drain, or an up to 40 percent improvement in talk time. As a result, overall battery consumption per call is dramatically reduced, enabling much longer usage time in the field between recharges. Modern digital devices also include sleep and power-management technologies that increase battery life even further.

THE RIGHT CHOICE FOR PROFESSIONAL TWO-WAYDIGITAL RADIO: TDMA

For professional users, digital two-way radio in licensed bands is the wave of the future. Whether they’re using analog radio today, or looking to implement their first two- way radio system, business organizations of all kinds will soon be choosing their first digital two-way radio solutions. The advantages and opportunities are simply too great to ignore — in transportation, education, construction, manufacturing, energy and utilities, private security, small municipalities and many other industries.

For most enterprises in these professions, TDMA provides the best method for achieving 6.25 kHz equivalent efficiency in licensed 12.5 kHz channels:

  • TDMA is being leveraged in European and U.S. standards initiatives aimed at providing greater spectral efficiency for the land mobile radio market.
  • Unlike FDMA methods of rebanding existing channels into discrete 6.25 kHz channels, properly designed two-slot TDMA systems fit cleanly into existing channel structures, with no rebanding or relicensing necessary.
  • TDMA improves capacity today, while offering a path to compliance with further channel efficiency requirements that may be mandated in the future.
  • Because it increases capacity without the need for additional repeaters and other infrastructure, TDMA can lower the overall costs of implementing digital two-way radio.
  • TDMA offers the performance and flexibility to support the functional requirements of mobile professionals in virtually any industry.