How does DMR Mobile Radio Work

Looking at how DMR mobile radio works and how it uses time division techniques and digital technology to improve its performance.

TETRA Radio Includes:
DMR basics DMR operation - how it works

Other LMR / PMR systems: LMR / PMR basics MPT1327 TETRA P25 DMR dPMR NXDN

Digital Mobile Radio uses digital technology along with a time division approach to enable greater levels of efficiency and performance along with improved spectrum usage.

Looking at how DMR works it can be seen that it utilises many standard features employed in digital communications and provides many facilities which are not possible, or harder to implement when using analogue techniques.

How does DMR work: basics

One of the first steps in understanding how DMR works is to look at the basic specification highlights for the technology.

Parameter	Details
Channel spacing	12.5 kHz
Multiple use	Time division, two slots providing two effective channels
RF modulation	4FSK
Symbol rate	4800 symbols per second
FSK frequencies	+1944, +648, -648, -1944 Hz

DMR mobile radio offers a number of capabilities including voice and data communications, interfacing to external networks, call alert, emergency call, remote monitoring, silent worker, push to talk ID, radio check, all call, etc . .

DMR air interface

DMR digital mobile radio uses a 2 channel TDMA, time division multiple access scheme.

The transmission is split into various blocks:

Burst: Each transmission burst is nominally 30 ms long and this consists of one TDMA element. Each burst can either be for one TDMA channel or the other.
Frame: A frame consists of two TDMA bursts and it is 60ms long.
Super-frame: A DMR super-frame consists of six frames and this is used for voice transmission.

The outbound transmission from the base station, BS, is typically labelled BS TX. It is transmitted continually when the base station is active. Between the individual bursts the BS TX includes a Common Announcement CHannel, CACH and this is used for traffic management and signalling.

DMR air interface burst & slot timing — DMR air interface burst & frame timing

The mobile station, MS, only transmits when it has traffic (either data or voice) to send. Instead of the CACH it incorporates a guard period. This is required to accommodate any propagation delays between different mobile stations.

DMR frame structures

In order to organise the data that is being sent in both directions, the burst and frame structure is defined.

The outward channel is transmitted from the base station and this is active all the time the Base Station is active. The inward transmission from the Mobile Station is only active when it has traffic to carry. After this it stops. This helps to conserve battery power which is more important for mobiles that may only have limited battery capacity.

DMR air interface frame structure — DMR frame structure

The Mobile Station, MS, burst and frame structure is shown below It can be seen that there is a space between the individual time slot bursts and this is to enable the Base Station, BS to accommodate different DMR Mobile Stations that may be located at different distances from the base station. The clear transmission time enables the propagation delays arising from the different distances to be accommodated.

DMR air interface mobile station burst structure — DMR MS burst structure

The DMR Base Station has a CACH data channel transmitted between the individual bursts. This enables the BS to transmit announcement information.

DMR air interface base station burst & frame structure — DMR BS burst & frame structure

DMR also supports a Reverse Channel which is used for signalling. It is used in both directions. In the outward direction the Reverse Channel replaces the Sync field, but in the inward direction a special short burst is used.

For voice transmission a super frame consisting of seven bursts. The individual bursts are labelled A to F.

DMR protocol stack

In line with other digital radio systems, the DMR scheme uses a protocol stack which is based around the ISO scheme. Accordingly different layers address different parts of the functionality and control.

Layer 1 - Air interface / physical layer: This incorporates: modulation & demodulation, transmit / receive switching, radio characteristics, bits & symbol definition, frequency & symbol synchronisation, burst building.
Layer 2 - Data link layer: This incorporates functions including: channel coding, media access control, link addressing, interfacing of voice data, data bearer services, acknowledgement mechanisms, interleaving.
Layer 3 - Call control layer: This includes functionality to provide: base station activation & deactivation, call set-up, maintenance and teardown, destination addressing, built in services, data call control, announcement signalling.