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]
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.
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