Taking Public-Safety Communications from Analog to Digital
2019-02-28 | 5 min read
Land-mobile-radio (LMR) systems have been the backbone of most public-safety networks for more than a decade. Unfortunately, catastrophic events have revealed flaws in such critical communications systems—especially a lack of interoperability between various agencies, such as police and fire. The result is a slow but steady migration from analog LMR to digital public safety systems using standards like the Association of Public Safety Communications Officials (APCO) Project 25 (P25), Terrestrial Trunked Radio (TETRA), and Digital Mobile Radio (DMR).
In the U.S., agencies have predominantly selected APCO P25 to move LMR systems from analog to digital. APCO P25 is not one standard but a combination of open standards. Those standards cover the design and manufacturing of two-way public safety communications equipment that is compatible, spectrally efficient, and interoperable between both agencies and vendors. While conventional operation dedicates one radio channel to a group, trunking gives users access to a shared variety of radio channels. This option works well for different agencies in a given region—for example, to enable them to communicate seamlessly together. Using two-slot time division multiple access (TDMA), APCO P25 Phase 2 provides increased capacity. Each base station provides two voice channels, enabling two simultaneous conversations on a single channel.
TETRA is the standard for public mobile radio (PMR) across much of the globe. This open standard provides high data throughput and scalability. With local area and wide area coverage, TETRA serves small through large networks, both private and public. It uses the TDMA channel access method to provide four user channels on a single radio carrier. TETRA continues to evolve with the development of new standards, including TETRA Release 2, which features TETRA Enhanced Data Service (TEDS). TEDS enables wideband, high-speed data communications services by utilizing different RF channel bandwidths and data rates for the flexible use of PMR frequency bands. To show the feasibility of interoperability with military communications (MilCom) applications, this release features the mixed excitation liner predictive enhanced (MELPe) voice codec.
The preferred choice for business-critical versus mission-critical applications, DMR is also an open standard. DMR applications range from consumer and commercial uses to industries such as construction and, of course, public safety. DMR enables two calls on the same channel independently. It provides twice the system channel capacity as a standard two-way analog radio system, thanks to two-slot TDMA technology. DMR achieves reverse-channel signaling via the second TDMA time slot, which provides the system operator features like priority call control. Beyond the voice capabilities, DMR supports data applications such as GPS location, text messaging, telemetry, and radio programming. Users can also configure system requirements, such as priorities, features, and operation, to tailor the system to a specific environment.
No matter which standard they choose, public safety and other agencies agree that the move to digital communications is crucial so that in a crisis, everyone on scene can communicate — regardless of jurisdictions or departments. In addition to standard features, such as network coverage, reliability, and security, these agencies need interoperable solutions across networks, devices, and applications. The performance of digital radio systems must be thoroughly tested to the selected standard to ensure correct and accurate operation and to validate key parameters. In emergency and especially disaster scenarios, critical communications can no longer fail to support and connect responders from all agencies.
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