Stoke-on-Trent Repeaters
GB7ST
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DMR OVERVIEW
Digital Mobile Radio (DMR) is a digital radio standard specified for professional mobile radio (PMR) users developed by the European Telecommunications Standards Institute (ETSI), and first ratified in 2005.
The standard is designed to operate within the existing 12.5kHz channel spacing used in licenced land mobile frequency bands globally and to meet future regulatory requirements for 6.25kHz channel equivalence. DMR provides affordable digital systems with low complexity and offers voice, data and other supplementary services. Today, products designed to its specifications are sold in all regions of the world.
The DMR protocol covers unlicensed (Tier I), licensed conventional (Tier II) and licensed trunked (Tier III) modes of operation, although in practice commercial application is today focussed on the Tier II and III licensed categories.
One of the major advantages of DMR is that it is an ETSI open standard and the DMR Association administers an interoperability programme which ensures compliance to the standard. Interoperability is key to the success of DMR and provides customers with vendor choice. All of the Simoco Xd range of products fully comply to the ETSI standard and their operation does not rely on any non-standard proprietary features to operate.
The diagram above shows the relative audio performance between analogue and DMR radios and the effect that this has on user experience. A user of an analogue radio at the edge of the coverage area, where the signal is weak, experiences a lot of RF noise. A user of a DMR radio in the same area experiences clear audio which creates not only a better user experience but also creates a perception that the coverage is better than analogue.
Full Duplex Telephony
The ability to offer users full duplex calls is one of the many benefits of TDMA technology. Certain user groups need to be able to connect to external standard telephony systems in a way that is completely seamless to an external user who is used to fully duplex calls.
DMR KEY FEATURES
Frequency Efficiency
One of the principle benefits of DMR is that it enables a single 12.5kHz channel to support two simultaneous and independent calls. This is achieved using Time Division Multiple Access (TDMA) which retains the 12.5kHz channel width and divides it into two alternating timeslots 1 and 2 (illustrated in the figure below) where each timeslot acts as a separate and independent communication path. In the diagram Radios 1 and 3 are talking on time slot 1 and Radios 2 and 4 are talking on timeslot 2.
In this arrangement each communication path is active for half of the time in 12.5 kHz of bandwidth and so each uses an equivalent bandwidth 6.25 kHz. This is known as having an efficiency of one talk path per 6.25kHz of spectrum, but with DMR the channel as a whole maintains the same profile as an analogue 12.5kHz signal.
This enables DMR radios to operate in a licence holders existing 12.5kHz channels meaning there is no need for re-banding or re-licensing in order to realise 6.25kHz effective frequency efficiency. By comparison, 6.25kHz FDMA solutions are not compatible with existing 12.5kHz channels, increasing the complexity of migration from analogue to digital technology.
Digital Voice Quality
DMR digital technology provides better noise rejection and preserves voice quality over a greater range than analogue, especially at the farthest edges of the transmission range. One of the reasons that DMR has an excellent range performance is that a great deal of effort was put into selecting Forward Error Correction (FEC) and Cyclic Redundancy Check (CRC) coders when developing the standard. These coders enable receiving radios to detect and automatically correct transmission errors by analysing bits inserted into messages that enable the receiving radio to tell if there is an error. The DMR standard specifies over 14 different coders to be used, each matched to different types of traffic that is being transmitted. Through the use of coders and other techniques, digital processing is able to screen out noise and re-construct signals from degraded transmissions. Users can hear everything being said much more clearly – increasing the effective range of the radio solution and keeping users responsive to changing situations in the field.