Digital mobile radio (DMR) is a digital radio standard for voice and data transmission in non-public radio networks. It was created by the European Telecommunications Standards Institute (ETSI),^[1] and is designed to be low-cost and easy to use. 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.

Specifications

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.^[2] 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 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. In April 2013, Hytera participated in the completion of the DMR Tier III interoperability (IOP) test.^[7]

DMR Association

In 2005, a memorandum of understanding (MOU) was formed with potential DMR suppliers including Tait Communications, Fylde Micro, Selex, Motorola, Hytera, Sanchar Communication, Vertex Standard, Kenwood and Icom to establish common standards and interoperability. While the DMR standard does not specify the vocoder, MOU members agreed to use the half rate DVSI Advanced Multi-Band Excitation (AMBE) vocoder to ensure interoperability. In 2009, the MOU members set up the DMR Association to work on interoperability between vendors' equipment and to provide information about the DMR standard.^[8] Formal interoperability testing has been taking place since 2010. Results are published on the DMR Association web site. There are approximately 40 members of the DMR Association.

The standard allows DMR manufacturers to implement additional features on top of the standards which has led to practical non-interoperability issues between brands, in contravention to the DMR MOU.

Amateur radio use

DMR is used on the amateur radio VHF and UHF bands, started by DMR-MARC around 2010. The FCC officially approved the use of DMR by amateurs in 2014. In amateur spaces, Coordinated DMR Identification Numbers are assigned and managed by RadioID Inc. Their coordinated database can be uploaded to DMR radios in order to display the name, call sign, and location of other operators.^[9] Internet-linked systems such as the UK`s Largest Digital Radio Network DV Scotland Phoenix Network, BrandMeister network, TGIF, FreeDMR and several others (including several previously closed clusters which now connect to larger networks to wide area accessibility), allow users to communicate with other users around the world via connected repeaters, or DMR "hotspots" often based on the Raspberry Pi single-board computer. There are currently more than 5,500 repeaters and 16,000 "hotspots" linked to the BrandMeister system worldwide.^[10] The low-cost and increasing availability of internet-linked systems have led to a rise in DMR use on the amateur radio bands.^[11] The development of Raspberry Pi-based hotspots, often those using the Pi-Star software, has allowed users to connect their radios to one or more internet-linked systems at the same time.^[12] DMR hotspots are often based on the open source Multimode Digital Voice Modem, or MMDVM, hardware with firmware developed by Jonathan Naylor.^[13][14]

Encryption

Encryption was not defined in the DMR standard initially. Each DMR radio manufacturer later added its own encryption protocol. The encryption protocols are therefore incompatible with each other. For example, Hytera's Basic Encrypt encryption is completely incompatible with Motorola's Basic Encrypt encryption or Tytera's Basic Encrypt encryption.

To address this issue, Motorola has proposed common encryption within the DMRA association. It offers 40-bit ARC4 encryption and 256-bit AES encryption.

The implementation of these two algorithms is not known because Motorola DMRA encryption is only available to manufacturers who sign a non-disclosure agreement in addition to paying usage royalties.

Some DMR encryption algorithms have been released, such as the PC4 released in 2015. It's a block cipher specifically designed for DMR radio communication systems. It uses 253 rounds and the key size can vary from 8 bits to 2112 bits. The block size is 49 bits, the exact size of an AMBE+ DMR voiceframe. ^[15]

A firmware that implements PC4 encryption is available for the Tytera MD-380 and MD-390 radios. ^[16]

Although most DMR encryption protocols are proprietary and not public, there is some information that can be used to define the different encryption modes that exist: AMBE frames are encrypted with a key after digitizing the analog signal. And at the reception the AMBE frames are decrypted with the same key and only then that the digital-analog conversation is made.

In Motorola Basic mode there are actually 255 fixed keys. This is a 1-character key. For example, the No. 1 key in Motorola's Basic Encryption is: 1F001F001F0000 This means that each 49-bit AMBE frame is encrypted by XOR with the key 1F001F001F0000. ^[17]

The Basic mode from other manufacturers offers 10, 32 or 64 character keys to produce a 882-bit fixed string of random characters that will be XOR with AMBE frames. But instead of encrypting each AMBE frame with a fixed key, an entire superframe is encrypted with this fixed string. A superframe contains 18 AMBE frames, i.e. 882 bits, and it is these 882 bits that will be encrypted with this 882-bit fixed string.

PC4 encryption mode encrypts an entire 49-bit frame in ECB mode. A single bit that differs makes the entire encrypted block completely different.

For the Enhanced (ARC4) or Advanced (AES) mode, a complete superframe is also encrypted, but a 32-bit IV (initialization vector) is added. Thus, each superframe will be encrypted differently. And there are 2^32 (2 power 32) possible vectors of initialization. Thanks to the initialization vector, the encryption is no longer fixed for the same key, but changes with each superframe.

In the DMR standard there was no place to store this IV, so the IV (with the addition of an error-correcting code, for a total of 72 bits) is inserted into each 49-bit AMBE frame due to 4 bits per frame (the low-order bits). These 4 bits are therefore lost and it degrades the voice quality, which is not the case with fixed ciphers in Basic mode. 18 AMBE frames of 49 bits with 4 bits in each gives 72 bits (18*4).

See also

• dPMR

References

• "FCC Modifies Amateur Service Testing and Emission Rules". Federal Communications Commission. 2015-12-10. Retrieved 2022-07-05.

External links

• DMR Association
• MNDMR - Minnesota's First In-House DMR Repeater Network System
• Radio ID, Inc., Coordinated DMR Identification Numbers for Amateur Radio