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DMR is a TDMA mode (Time Division Multiple Access) which means that it allows several users to share the same frequency channel by dividing that signal into different time slots. Each frequency has a slot 1 or slot 2 and each slot can be used by different users. Each user transmits in rapid succession using their own time slot. Tier II is 2-slot TDMA 12.5 kHz peer-to- peer and repeater mode specification, resulting in a spectrum efficiency of 6.25 kHz per channel. Each time slot can be either voice and/or data depending upon system needs.
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Time Division Multiple Access (TDMA) is a digital cellular telephone communication technology. It facilitates many users to share the same frequency without interference. Its technology divides a signal into different timeslots, and increases the data carrying capacity.
TDMA Overview
Time Division Multiple Access (TDMA) is a complex technology, because it requires an accurate synchronization between the transmitter and the receiver. TDMA is used in digital mobile radio systems. The individual mobile stations cyclically assign a frequency for the exclusive use of a time interval.
In most of the cases, the entire system bandwidth for an interval of time is not assigned to a station. However, the frequency of the system is divided into sub-bands, and TDMA is used for the multiple access in each sub-band. Sub-bands are known as carrier frequencies. The mobile system that uses this technique is referred as the multi-carrier systems.
In the following example, the frequency band has been shared by three users. Each user is assigned definite timeslots to send and receive data. In this example, user ‘B’ sends after user ‘A,’ and user ‘C’ sends thereafter. In this way, the peak power becomes a problem and larger by the burst communication.
FDMA and TDMA
This is a multi-carrier TDMA system. A 25 MHz frequency range holds 124 single chains (carrier frequencies 200) bandwidth of each kHz; each of these frequency channels contains 8 TDMA conversation channels. Thus, the sequence of timeslots and frequencies assigned to a mobile station is the physical channels of a TDMA system. In each timeslot, the mobile station transmits a data packet.
The period of time assigned to a timeslot for a mobile station also determines the number of TDMA channels on a carrier frequency. The period of timeslots are combined in a so-called TDMA frame. TDMA signal transmitted on a carrier frequency usually requires more bandwidth than FDMA signal. Due to the use of multiple times, the gross data rate should be even higher.
Advantages of TDMA
Here is a list of few notable advantages of TDMA −
Permits flexible rates (i.e. several slots can be assigned to a user, for example, each time interval translates 32Kbps, a user is assigned two 64 Kbps slots per frame).
Can withstand gusty or variable bit rate traffic. Number of slots allocated to a user can be changed frame by frame (for example, two slots in the frame 1, three slots in the frame 2, one slot in the frame 3, frame 0 of the notches 4, etc.).
No guard band required for the wideband system.
No narrowband filter required for the wideband system.
Disadvantages of TDMA
The disadvantages of TDMA are as follow −
High data rates of broadband systems require complex equalization.
Due to the burst mode, a large number of additional bits are required for synchronization and supervision.
Call time is needed in each slot to accommodate time to inaccuracies (due to clock instability).
Electronics operating at high bit rates increase energy consumption.
Complex signal processing is required to synchronize within short slots.
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.
Specifications[edit]
P25 Trunking Channel
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.)[citation needed]
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The DMR Association and manufacturers often claim that DMR has superior coverage performance to analogue FM.[citation needed] 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.[citation needed]
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.[citation needed] Where digital signal processing is used to enhance analog FM audio, the overall 'delivered audio quality' is also considerably better than DMR.[citation needed] However DSP processing of analog FM audio does not remove the 12.5 kHz requirement so DMR is still more spectrally efficient.
DMR Tiers[edit]
DMR Tier I[edit]
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.[3]
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.[4]
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.[5]
DMR Tier II[edit]
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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.[6]
DMR Tier III[edit]
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.
DMR Association[edit]
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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.[7] 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.
See also[edit]
References[edit]
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- ^ETSI. 'DMR Standard Overview'(PDF). ETSI. Retrieved 22 March 2012.
- ^DMR association press release Oct 27 2015 stating revision to standardArchived 2015-11-08 at the Wayback Machine
- ^'Archived copy'. Archived from the original on 2015-02-04. Retrieved 2015-01-12.CS1 maint: archived copy as title (link)
- ^'A guide to the new PMR446 license-free radio frequencies following ECC Decision (15)05'(PDF). Kenwood. Retrieved 7 February 2018.
- ^'Two-way radios and battery life'. Hytera UK. Retrieved 19 October 2020.
- ^'ETSI TS 102 361-1, DMR Air Interface PRotocol'(PDF).
- ^'DMR Association'.