dPMR: A Narrowband Option?

A new ETSI standard offers 6.25-kilohertz FDMA for cost-effective business applications. Solving the shortage of radio channels affecting radio users is an ongoing dilemma around the globe. Considerable discussion has revolved around the idea of two channels in a 12.5-kilohertz bandwidth and how and when it could be implemented. Several years ago during a European Telecommunications Standards Institute (ETSI) meeting, a representative of a major radio manufacturer claimed that TDMA in a 12.5-kilohertz channel was technically impossible.

A short while later, one of the ETSI member companies, ICOM, presented some working prototypes of 6.25-kilohertz FDMA radios. ICOM officials also explained about the development of a narrowband 4FSK modulation scheme, which would comply with the U.S Federal Communications Commission (FCC) emission mask mandated in the telecom regulator¡¦s requirement for LMRs to have a 6.25-kilohertz capability originally by 1 January, 2005, and subsequently extended to 2011. The technology fulfilled the equivalent requirement in the European radio standards.

It became clear early on that the 4FSK technique combined with the latest half-rate digital voice coders (vocoders) offered a workable solution. A solution was proposed to ETSI to develop a European standard for digital private mobile radio (dPMR) for 6.25-kilohertz channels. This work was undertaken within the ETSI Digital Mobile Radio (TG-DMR) committee, and the resulting European standard, TS102 490, was published in 2006.

Digital Private Mobile Radio (dPMR) Applications
Tier 2
Semi-secure networks Emergency services/ambulance Business/industry wide-area networks

Licence Free
Business local area communications
Shopping centre security groups
Building site workers
In-store/hotel workers

dPMR and DMR
Both dPMR and digital mobile radio (DMR) digital radio protocols have been published as European standards; DMR is the TDMA two time-slot system with 12.5-kilohertz channels, and dPMR is the FDMA system with 6.25-kilohertz channels. Both systems are equal in that the capacity of the 6.25-kilohertz FDMA system is identical to that of a TDMA time slot in 12.5-kilohertz. Also, both dPMR and DMR currently employ the same vocoder ¡V AMBE+2 from Digital Voice Systems ¡V so the voice quality is effectively identical too.

Each standard has applications to which it is most suited. TDMA is most efficient when operated under a controlling infrastructure of base station repeaters. FDMA, on the other hand, is preferable when operation is mostly peer-to-peer or direct mode, although infrastructure-based systems are possible. Given that a significant percentage of business and industry users operate small radio systems not requiring infrastructure, FDMA provides the most cost-effective answer.

In both cases, FDMA and TDMA come close to the ideal of doubling the available radio channels, but neither is perfect. FDMA filter performance achieves near double capacity, and the TDMA time-slot guard bands also acheive near double capacity. There is only the consideration that TDMA needs to find users for both time slots with the same geographical area before it approaches double capacity in its true meaning, In most instances, only half the channel will be in use.

DPMR MoU Members
CML Microcircuits
CTE International
Flyde Microsystems
ICOM
Kenwood

dPMR in Use
The most noticeable feature of dPMR is the performance at extreme range when compared with analogue FM. The 4FSK scheme results in sensitivity largely superior to analogue, and the digital demodulation and error correction gives excellent audio quality on the fringes of coverage. When this is combined with the possibility of voice data, text, or combined voice and data calls, the dPMR radio becomes a far more than polyvalent tool than its analogue predecessor.

The first dPMR radios compatible with the TS102 490 standard have been available since early 2006 in ICOMs IC-F4029SDR, a handheld radio specifically targeted at the license-free digital PMR bands (446.1-446.2 MHz) in Europe. These bands offer the possibility of a family radio service (FRS) type of service but with digital radios. The dPMR radios are dual mode. Because of the choice of an FDMA architecture, it¡¦s simple to provide both analogue and digital operations, and the same radio can offer 6.25-kilohertz digital operation and 12.5-kilohertz operation. This capability is an essential requirement for users looking for a progressive transition from analogue to digital. TS102 490 is an open ETSI standard, and as more member countries open the 446 MHz band for dPMR use, other manufacturers will release compatible products on the market. Likely within a year to 18 months.

dPMR in the Future
The success of the initial dPMR products combined with the support from a broad range of radio industry players has meant that ETSI has decided to expand on the TS102 490 standard and develop an enhanced version of the protocol with significantly greater functionality. This new standard is expected to be completed later this year.

Support from radio industry players has manifested itself in the creation of a memorandum of understanding (MoU) group. The MoU group was established in 2007 to promote dPMR and 6.25-kilohertz FDMA technology. The MoU also includes a technical working group to develop advances in technology, applications and user requirements, as well as making commercial decisions about the choice of vocoders to be used by dPMR products. Members of the MoU are also resolved to support the use of the recently developed ETSI standards for conformance and interoperability for dPMR equipment, something that is vital for true competition and interoperability among different manufacturers.

Written by Pete Hizzey

18/08/2008