The case for open digital radio standards

When vendors work to agreed standards, those managing radio systems have a choice of supplier which can bring down prices, and improve quality. Customers are not at risk of being unable to source products for replacement or expansion through the demise of a sole supplier.

To properly qualify, a standard needs to be non-proprietary, so not under the control of one vendor. It should be defined and controlled by an independent body with a proper process for resolving conflict between interested parties. Some standards only define the air interface, while others cover line interfaces as well.

Equipment designed and produced to a standard by one vendor should be interoperable with equipment from another vendor. However, there is often room for differences of interpretation of a standard, and vendors can add proprietary features not covered by the standard itself. It is wise to look for standardized interoperability test procedures or results from certified interoperability laboratories.

The advent of digital radio has increased the importance of standards. With conventional analog FM, interoperability for basic features is inherent. However digital equipment cannot interoperate unless the same protocols are used. For example, if different vocoders are used, speech cannot be understood; if different control signaling is used, users cannot communicate call setup information.

Digital radio standards developments

In 2000, there were only two non-proprietary open digital PMR or LMR standards; TETRA and APCO P25. TETRA (Terrestrial Trunked Radio) was developed by the European Telecommunications Standards Institute for large, national networks run by government agencies for public safety organizations and others. APCO P25 was designed primarily for public safety users. While TETRA originated in Europe and APCO P25 in the United States, both standards have been widely adopted outside their area of origin.

In 2005, European Telecommunications Standards Institute (ETSI) published the DMR (Digital Mobile Radio) standard with three tiers for business and professional systems with low complexity and low cost. ETSI has since defined dPMR (digital Private Mobile Radio), an FDMA variant for DMR Tier 1, which now provides equivalent FDMA standards for Tier 2 and Tier 3.

Digital Mobile Radio (DMR)

DMR now provides a full set of air interface standards covering voice and data services, and conformance tests. Current developments will add interfaces, encryption and application protocols to the standard.

DMR provides a low-complexity digital standard to replace analog radio. DMR is promoted as a data and voice standard that can operate in 6.25kHz channel equivalence mode.

• Tier 1 is aimed at applications such as sport, family vacations and commercial enterprises such as retail. It is license-free and permits up to 500mW transmit power output. Operation is peer-to-peer, so it requires no repeaters.
• Tier 2 is digital conventional. It achieves 6.25kHz channel equivalence through 2-slot TDMA on an existing 12.5kHz narrowband FM channel. Designed for easy migration of analog to digital, the output spectrum must fit in to the existing 12.5kHz narrowband FM channels used by legacy analog systems. The choice of modulation scheme and associated symbol rate are critical. 4FSK modulation is used with an associated symbol rate of 4800symbols/sec. Each symbol carries 2 bits of data, so the equivalent data rate is 9600bits/sec. Both the downlink (base station to terminal) and uplink (terminal to base station) use this modulation.
• Tier 3 DMR Trunked is a digital replacement for MPT 1327, aimed at applications that will benefit from trunking efficiency. These include organizations responsible for critical infrastructure, such as utilities, transportation, oil, and gas.

dPMR

Digital Private Mobile Radio (dPMR) is an ETSI standard developed after DMR, and using FDMA to divide the 12.5kHz channel into two 6.25kHz sub-channels. The implications of this will be covered next week when we compare the standards. Just as there are three tiers of DMR, there are three modes of dPMR. Mode 1 is peer-to-peer, Mode 2 is conventional (repeaters and infrastructure), Mode 3 has managed sites, each with a beacon channel (dPMR terminology for trunking with control channels).