Digital Mobile Radio DMR

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.

DMR was designed with three tiers. DMR tiers I and II (conventional) were first published in 2005, and DMR III (trunked) 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.

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 discover improvements that can be made. 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 analogue FM. Forward error correction can achieve a higher quality of voice when the receive 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 analogue FM would otherwise provide degraded but audible voice service. At a higher quality of voice, DMR outperforms analogue FM by about 11 dB. But at a lower quality of voice, analogue FM outperforms DMR by about 5 dB.

Where digital signal processing has been used to enhance the analogue FM audio quality then analogue 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.

DMR Tiers

DMR Tier I

DMR Tier I products are for licence-free use in the European 446 MHz band. In the US, the 446 MHz range is primary US Government with the amateur radio service a heavy secondary user. Some DMR radios that make it across the ocean have caused interference issues with licensed amateur operations. ETSI DMR specifies two slot TDMA in 12.5 kHz channels for Tier I, II and III.

This part of the standard provides for consumer applications and low-power commercial applications, using a maximum of 0.5 watt RF power.

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.

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.

Amateur Radio DMR

It is Tier II that amateurs are implementing in their Mototrbo™ infrastructure networks. The IPSC protocols used by the different brand repeaters are not compatible; it is doubtful the equipment manufacturers will ever standardize for business reasons. Any brand DMR (Tier II) user radio will work on any Tier II system, although some manufacturers offer proprietary features.

The current implementation of DMR utilizes the DSVI AMBE+2™vocoder by agreement of the manufactures; it is not specified in the ESTI standard. Most of the radio manufacturers have implemented the vocoder in licensed software. The forward error correction in the AMBE+2™ is an improvement of the voice quality of older vocoders such as used by D-Star™.

Amateur Mototrbo™ and Hytera DMR networks, from the end user standpoint, operate the same. Amateur Mototrbo™ networks are much larger, cover many more areas, and most are interconnected. I look forward to the day when the multiple vendor infrastructures can be 2 interconnected by the amateur community. Not all the amateur DMR repeaters are connected to the wide area networks; some are standalone either because they have yet to obtain an ISP connection at their repeater site or because they just want to use the repeater for local communications.

Some standalones are operating in dual-mode (analog/digital). Mototrbo™ repeaters operating in dual-mode do not support interconnection via the Internet using IPSC. Some hams have installed DMR repeaters in a vehicle, using 3G/4G cellular wireless services for Internet access. Others have implemented remote bases to interconnect to other networks or radios; it is important to remember that the wide area networks typically have policies prohibiting interconnected traffic, but what is implemented locally and stays local is acceptable. While some may consider network policies prohibiting interconnection to different types of networks political, these policies are really about keeping large networks functioning. Users sometimes don’t realize the hours put in by network operators or the extent of their efforts that are required to keep a linked system running smoothly. There are sometimes issues of poor quality from interconnected technologies because of the vocoding process that would degrade the quality of the network. DMR-MARC has a sandbox available for persons interested in developing and experimenting that is separate from and off the main DMR-MARC network.

Back during the early era of amateur analog repeaters, most everyone used surplus commercial radios. Over time, equipment designed for and targeted to the radio amateur reached the amateur radio marketplace. Today in the DMR marketplace you can find used commercial gear, but new DMR radios are now available with street prices within the range of a typical ham budget. Some amateur DMR users are just using their commercial radios from work with a few extra channels programmed in.

Currently, no manufacturer is marketing an “Amateur” DMR radio; they are building DMR radios for the broader world market. Because of FCC Rules & Regulations for commercial users, DMR radios do not offer FPP (Front Panel Programming) as is the norm for other amateur radios. This is really not an issue because most DMR radios have enough channels to program all possible channels you may want to operate. Most of the DMR radios require a programming cable to program the radio using manufacturer software, while some radios support programming using BlueTooth and even over-the-air programming.

There are police and fire departments, local/state governments and many businesses using DMR Tier II and Tier III; any Tier III capable radio will also work on Tier II systems but neither will work on Tier I. If you have a DMR radio for work, you may be able to program it to also work on amateur repeaters (make sure you have permission) and you will need to contact DMR-MARC about a usable subscriber ID that will work on both networks.

Digital vs. Analog

If you are use to operating on analog FM repeaters, you will have noticed that the audio quality degrades as a station’s signal into the repeater (uplink) gets weaker; you start hearing an increase in noise bursts intermixed with the audio until the signal gets so weak that the station can no long access the repeater or you can not understand the audio because of noise. As you move further from the repeater you will start hearing the same noise bursts into your receiver as the repeater’s signal gets weaker (downlink) until you can no longer hear the repeater. A combination of a station’s weak signal into a repeater and a repeater’s weak signal to the listener can make the usability degrade faster.

The basic difference with digital repeaters is that the audio quality remains the same on the uplink and downlink until the very end of the coverage range; then the audio starts sounding broken (missing portions of the speech) on DMR systems caused by lost packets. The Internet can also drop the UDP packets used for moving traffic between repeaters and bridges, causing the same broken audio affect. Analog static is a thing of the past using DMR.

DMR has Forward Error Correction (FEC) which can correct small bit errors, slightly extending the usable range and improving communication quality.

Better quality receivers can operate at a lower noise floor, higher power transmitters, and higher gain antenna systems will also extend coverage of both analog and digital systems.

Two-Slot TDMA

DMR Tier II/Tier III occupies a 12.5 kHz bandwidth that two channels share using Time-Division Multiple Access (TDMA). This results in spectrum efficiency of 6.25 kHz per channel. Comparing the spectrum efficiency of DMR to a wideband analog FM, DMR only uses 25% of the bandwidth per talk channel. Each channel can carry either voice and/or data depending on system design. The two time slots are called Time Slot 1 (TS1) and Time Slot 2 (TS2).

For the amateur, this means one repeater allows two separate channels at the same time. Currently most amateur DMR repeater system implementations utilize both channels for voice and some limited text messaging. Typically one channel (time slot) is used for wide-area and the second is local and regional Talk Groups.

For repeater operators, a single two-slot TDMA repeater offers a significant savings over two standalone repeaters to obtain two separate communication channels as only one repeater, one duplexer, and one antenna system is required.

The utilization of TDMA offers about a 40% battery savings on transmit, extending talking time over non-TDMA and analog transmissions for portable users.

The two-slot TDMA implemented in DMR uplinks (portable/mobile to repeater) uses a 30-ms window for each time slot, the 30-ms is further divided into a 27.5-ms frame and a 2.5-ms gap. This means when transmitting, your transmitter is only turned on for 27.5ms every 60ms, resulting in extended battery life for portables. The DMR repeater (downlink) transmits a continuous data stream even if only one timeslot is being used; the 2.5-ms uplink gap is replaced with a CACH burst (Common Announcement Channel) that is used for channel management and low speed signaling.

The 27.5-ms frame consists of a total of 264-bits; 108-bit payload, 48-bit SYNC or embedded signaling, and a second 108-bit payload for a total of 216-bits of payload per frame. The vocoder must compress 60-ms of audio with FEC (forward error correction) into 216-bits of data for transmission. The 2.5ms-gap is used for guard time to allow PA ramping and propagation delay.

Talk Groups

Talk Groups (TG) are a way for groups of users to share a time slot (one-to-many) without distracting and disrupting other users of the time slot. It should be noted that only one Talk Group can be using a time slot at a time. If your radio is not programmed to listen to a Talk Group, you will not hear that Talk Group’s traffic.

The DMR-MARC Mototrbo™ network supports a number of Talk Groups on TS1 including World Wide (TG1, PTT), North America (TG3), and World Wide English (TG13). TS2 is for local, state, and regional Talk Groups. The DCI/TRBO network uses TG3163 for North America and TG3161 for World Wide, and TG3 for World Wide English on TS2.

The DMR standard also supports private calls (one-to-one), encryption, and data. Private calls are not allowed by most of the amateur networks and many consider private calls not amateur friendly; private calls tie up a large number of repeater time slots across the network. Encryption is not legal on amateur radio in the USA but is allowed in Canada! Data and text messaging is supported on some networks.

For simplex traffic, the accepted standard in the amateur community is to use TG99 on TS1 with CC1.

Programming Talk Groups

When programming your DMR radio, you may find it easier to program multiple Talk Groups for receive. I have two RX Group lists programmed in my radios, one for TS1 and one for TS2; this allows my radio to listen to all the possible Talk Groups used on a time slot when I have my radio set to any channel.

There are Talk Groups implemented for individual states and regional on many networks. Some Talk Groups are available all the time, while others only at preprogrammed times. Some Talk Groups require a local user to PTT on the Talk Group to activate it for a period of time. Since only one Talk Group can be active at a time on a time slot, many systems will disable other Talk Groups when a local user is active on a different Talk Group on the time slot. Be ham friendly and try to use Talk Groups that tie up the fewest number of repeaters if you are going to have a long QSO.

Color Codes

DMR repeaters use Color Codes (CC) much like analog repeaters use CTCSS (PL) or DCS. To access a repeater you must program your radio to use the same CC as the repeater. There are 16 different CCs (CC0-CC15). The use of Color Codes is not optional on DMR systems. If your Color Code is not set correctly, you will not be able to access the repeater. The only real purpose of using different Color Codes is when multiple repeaters operating on the same frequency have overlapping coverage areas.

Central Ontario Area Amateur DMR Repeaters/Networks

Uniden and DMR

On April 12, 2017, Uniden announced the latest upgrades for their flagship BCD436HP and BCD536HP scanners. Now, owners of these scanners can add the ability to monitor and scan Digital Mobile Radio (DMR) systems including MotoTRBO, Connect Plus and MotoTRBO Capacity Plus systems with a single paid upgrade, making these scanners the indisputable best-in-class solution for monitoring radio systems. 

Radio Reference

Radio Reference has now incorporated DMR systems into their database and the new Uniden scanners download the database each week.