DMR

The introduction of digital PMR products is bringing great benefits to the users of professional radio. It is however, bringing a degree of complexity in that there are many different types of digital radio protocols now on offer – DMR, dPMR, NXDN, TETRA, P25 – some standardised and some proprietary. None of these protocols are compatible with each other and all bring different attributes to the table. All, however, are more efficient in terms of use of spectrum and also improve voice quality at the edges of a coverage area, when compared to analogue systems due to the efficiencies of digital processing.


It is important that the user understands some basic differences in systems technology in order to make the right choice. Some differences in specific products are the result of the way different vendors have implemented features. Other differences are due to fundamental differences in the underlying technology used by the protocol. These underlying differences impact system scalability, power efficiency, feature possibilities and both access to and use of spectrum.
Broadly speaking there are two underlying technologies to the various protocols; TDMA - used by DMR, TETRA and P25 Phase 2 - and 6.25 kHz FDMA (Frequency Division Multiple Access) - used by NXDN and dPMR. TDMA divides up spectrum using timeslots; user A gets a few milliseconds of access to the bandwidth, then its user B’s turn. FDMA, in contrast divides spectrum into discreet channels; user A has 100% use of a small slice of spectrum and user B has 100% use of another small slice of spectrum. There are a number of consequences of these two approaches.