While these are all known players in the land mobile radio industry, most radio amateurs may recognize only a few of them.
Like other digital radio standards, DMR converts the analog signal (usually from the microphone on the radio) to a stream of digital bits. These bits are used to modulate the radio-frequency carrier, which is decoded at the receiver and converted back into analog form.
This two-for-one approach provides an effective spectral efficiency of 6.25 kHz per channel. The FCC is requiring most commercial users of the VHF and UHF spectrum to move to 12.5-kHz channel spacing by the start of 2013. This requirement does not apply to the Amateur Radio Service. The FCC is also anticipating a further shift to 6.25 kHz per channel sometime in the future, so the DMR standard is set up to achieve that efficiency. The DMR standard supports the mix and match of voice and data. That is, the digital bit stream can be used for data or to carry an analog voice signal. Both TDMA slots can be used for voice communication, or one can be used for voice and the other used for data, or both can be used for data.
Clearly, synchronizing the time interleaving of two radio transmitters is not trivial. However, once you handle the complexity of on/off timing of the mobile radios, two-slot TDMA has a lot of advantages. While the two mobiles on the repeater input are switching on and off, the repeater transmitter stays on continuously while either mobile is active.
For simplex operation, there is no repeater available to coordinate the timing of the two TDMA slots, so simplex operation basically uses the entire 12.5-kHz channel. The DRM standard does include the concept of a Reverse Channel, which allows the open timeslot to be used as a channel,”allowing other transmitters to insert upstream information onto the channel. (This is not implemented on MOTOTRBO today.)
The voice modulation is digitized and compressed using a proprietary AMBE+2 vocoder from DVSI. This is one generation newer than the AMBE+ technology used in D-STAR. Actually, the DMR specification does not specify the vocoder technology, but AMBE+2 is the vocoder that the manufacturers have adopted.
The modulation technique for DMR is states a symbol, rather than a bit, since a symbol can represent more than one bit. The DMR spec calls for 4800 symbols/ second. Since there are two bits per symbol, this gives us a data rate of 9600 bits/second.
The DMR protocol includes Forward Error Correction (FEC) to mitigate the presence of errors in the bit stream. This means that the audio quality stays intact as the signal level drops off, creating errors in the bits. FEC can correct for a relatively small number of bit errors, but when the error rate increases, the audio signal falls apart.
It is very apparent that the DMR Standard was created by a standards body that is used to completely specifying a complex radio standard. The documentation available on the web is very complete and detailed. Compare this level of documentation to what is available on D-STAR and yousee that the D-STAR spec is relatively thin.
No surprise, most of the repeaters showing up on the ham bands are MOTOTRBO, Motorolaimplementation of DMR. MOTOTRBO offers some features over and above what is defined in the standard, while maintaining compatibility with the standard. The DMR industry is motivated to maintain compatibility across manufacturers, else the whole ”concept falls apart.
For backward compatibility, MOTOTRBO repeaters and user radios can operate in analog FM mode, both narrowband (12.5 kHz) and wideband (25 kHz). The wideband FM mode is important to radio hams for operating on existing FM repeaters.
MOTOTRBO radios include the concept of Radio ID, where each radio on a system is assigned a number between 1 and 16776415. Each radio stores a list of aliases that translate these numbers to name, location, or assignment. For ham use, the alias feature will often be used for name and amateur callsign. Alternatively, it could be used to indicate a fixed location such as an Emergency Operations Center (EOC). MOTOTRBO has a robust text-messaging system built into it. Most, but not all, radios have a GPS receiver built in to enable location-based features. Motorola is releasing new software for both the repeaters and the user radios, so the specific features will improve over time.
In photo A is a popular MOTOTRBO UHF handheld radio (Motorola XPR 6550) that covers 403 to 470 MHz. I received several quotes for this model of radio new from dealers in the range of $700-750. On the used market they can be picked up for less than $500. This is significantly higher than other ham radio equipment but actually not that expensive for commercial-quality digital gear.
DMR equipment is available from several land mobile radio companies. Also, earlier this year Yaesu published a paper that argued for the use of DMR and APCO 25 technology in the amateur radio market. The rumors persist that they will have something to show at the Dayton Hamvention® this year. This means an amateur radio manufacturer is adopting DMR or APCO 25 specifically for amateur use. (Recall that Yaesu is now a separate company from Vertex/ Motorola.) This could get interesting.
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