Electronic Design

Radio Interoperability—It's Harder Than It Looks

Emergency management can be difficult. Designing the systems that provide seamless communication between personnel presents some equally tough challenges.

Emergency management can be difficult enough. Designing the systems that provide seamless communication between personnel presents some equally tough challenges.

Fire swept through the hills above the cities of Berkeley and Oakland, Calif., on Oct. 21, 1991. Known as the Tunnel Fire, it destroyed more than 2800 homes and damaged almost 700 more. It also burned some 1500 acres. And while it caused $1.5 billion in damage, its worst toll reverberated in the death of 25 people.

It was the country’s worst fire in terms of loss of life and property since the Great San Francisco Earthquake and Fire of 1906. Since then, experts have studied the Tunnel Fire to reveal strengths and weaknesses in how public safety agencies respond to catastrophes.

Multiple companies of firefighters battle such blazes according to the principle of mutual aid. For the Tunnel Fire, they came from all of the neighboring cities around San Francisco Bay. But during this fire, many companies couldn’t connect to Oakland’s fire hydrants because their cities used 2.5-in. hose couplings, while Oakland fire units used 3-in. couplings. The problem drew scrutiny in the press and in the state legislature because it was easy to grasp, and solutions seemed obvious.

Yet a parallel problem existed in 1991 and persists today. Communications systems—from first-responders’ handhelds to the networks used by dispatchers, firefighters, police, water-bomber pilots, public works personnel, and ambulance crews—are only now emerging from incompatibilities as frustrating as those hoses and hydrants.

A set of communications standards known as APCO 25, Project 25, or simply P25, has been the focus of those inconsistencies in emergency communications systems. The project was conceived by the Associated Public Safety Communications Officers (APCO), a trade association of mostly police and fire service providers, but many are now involved in the standards effort.

A universal standard must address variations in local customs (Fig. 1). For example, some fire departments place all fire-ground communications on a separate tactical channel, and the incident commander handles all communications to dispatch. Other departments want dispatch to monitor and respond directly to fireground comms.

In terms of hardware, some departments use a singlefrequency system for communications. Others have multiple frequencies and use trunking to assign channels (trunking is a term borrowed from the publicswitched telephone network).This addresses the incompatibilities that arise when police, firefighters, public works personnel, and others all rely on their own separate repeaters, which could lead to problems in a crisis situation.

Usually, the police repeater gets more use than that of the road department. But if the police use an extra repeater during an emergency, accessing the road department’s repeater may be very difficult. In a trunked system, though, any given repeater can be switched into a radio circuit as needed. Today, systems in the 700-, 800-, and 900-MHz bands are generally trunked. Below 512 MHz, trunking is allowed if it doesn’t interfere with exiting radio systems in surrounding areas.

The most up-to-date trunking systems assign priorities and share channels among agencies. When a major incident occurs, the additional talk groups automatically preempt other routine communications, making more capacity available for mission-critical messages. The lower-priority messages experience a busy signal.

Traditional non-trunked systems required additional channels to create a hierarchy of networks when there was a large incident or multiple simultaneous incidents. The problem was that some of those additional channels might already have had incumbent users, resulting in confusion and contention.

Whenever mutual-aid operations bring outside resources into a jurisdiction, there must be a method for integrating resources into communications, both when they’re dispatched and when they arrive. Of course, trunked radio systems aren’t optimal for all situations. Sometimes, it’s better to allow interior teams to off-network and use direct radio-to-radio communications and portable or vehicular repeaters.

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Variations in frequency assignments among agencies and the characteristics of different bands introduce their own complications. Federal agencies (FBI, ATF, DEA, Forest Service, etc.) and local governments use VHF frequencies not otherwise allotted between 136 to 174 MHz. Furthermore, the feds employ UHF frequencies between 380 and 400 MHz and between 402 and 420 MHz. (Radiosondes, satellite, and space exploration frequencies fill that 2-MHz gap.) Local government agencies are allotted UHF frequencies from 450 to 512 MHz as well as the 700- and 800-MHz bands.

Frequency also impacts in-building coverage. VHF high-band signals don’t propagate as well from inside buildings as do 700- and 800-MHz UHF. Within those licensed bands, there are layers and layers of equipment, starting with the firefighters’ personal radios. The first issue concerns how those firefighters’ signals reach their intended audiences.

Imagine a firefighter cornered by a flareup inside a burning building. An engine outside the building can deliver a stream immediately to where the firefighter needs it. But would the firefighter talk to the truck through the building, via a repeater on a hill 10 miles away, or talk to the truck point-to-point?

Huge blazes like the Tunnel Fire are less common than fires in single buildings, but a fire in a skyscraper presents parallel problems. Primarily, the system must ensure that teams can communicate adequately at various levels inside those buildings and with their support equipment outside.

One solution is to move the repeaters closer. Some large buildings even have their own active systems with amplifiers that boost outside signals, but such active systems are rare. A more versatile approach involves local portable repeaters carried inside the structure in a suitcase or backpack.

These repeaters can work floor-to-adjacent-floor for the team inside the building, while at the same time pushing more RF power from outside. But even then, the incident commander on the outside may not clearly hear signals coming from the interior teams.

A recent evolution is multiple antennas, with one for in-building communications and one for external communications. Alternatively, vehicular repeaters parked nearby can provide both internal and external coverage.

Moving up from immediate tactical communications, there are multiple layers of command networks. SAFECOM, the U. S. Department of Homeland Security’s public safety communications and interoperability program, has names for all of them.

Firefighters may have their own personal-area network (PAN), which could include their Personal Alert Safety System (PASS) device, self-contained breathing apparatus (SCBA) monitor, and body-vitals reader. Otherwise, they could just have their handheld radio in its pouch.

The PAN ties in to the jurisdictional-area network ( JAN) and the incident-area network (IAN). The JAN is the day-today radio system that dispatches calls through the radio network. The IAN would represent the equivalent for on-scene communications.

Most departments’ JAN would be the trunking or conventional radio systems they use to dispatch equipment and personnel. Their IAN would be the radio-to-radio tactical channel used when there’s a fire and on-scene units need to communicate with the incident commander.

Radio dispatchers and chief officers use an extended-area network (EAN) to communicate with other cities, counties, or state agencies during a large incident that requires mutual aid, regional hazmat teams, or other specialized resources.

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When there’s a brush or forest fire, firefighters and vehicles are usually dispersed in remote areas where a JAN hasn’t been established or over hilly or even mountainous terrain. Getting coverage between the incident commander or the sector commanders and their dispersed teams is tougher than in a city. Not only that, these incidents can last days or weeks, and repeaters must be remotely powered.

Despite the location of the emergency, there are limits to how many channels a communications system can support. There also are interoperability and interference issues when multiple services respond to the same event, leading to challenges in determining communications modes.

Looking forward, digital modes are favored for bandwidth control, but there’s always the issue of degradation. When the bit error rate (BER) is too high, digital modes simply go from working to not working. There’s no picking up a faint cry for help through the static.

This is significant because the Federal Communications Commission is requiring all agencies that use VHF and UHF radio communications to reduce their channel bandwidth by half to provide more capacity by 2013. Agencies on the 700-MHz band must do so by 2017.

In the plus column, digital modes enable new software applications. An incident commander could manage incidents on a handheld computer and then send that information to other commanders and dispatch via the JAN. Also, it should be possible to use compact video cameras mounted on telescoping masts on the command vehicle to get a bird’s-eye view of the scene. Then it would transmit the information to other locations using Internet protocol standards.

P25 succeeds Project 16, a 1970s-era effort that anticipated the first 800-MHz radio licenses for public-safety and other uses. P16 created basic performance standards and feature sets, but failed to produce a signaling standard. P16-compliant systems from different manufacturers were incompatible with one another. Workarounds were developed, but the results were unsatisfactory.

The P25 set of standards was produced through the joint efforts of APCO, the National Association of State Telecommunications Directors (NASTD), selected federal agencies, and the National Communications System (NCS), according to the Project 25 Technology Interest Group (www.project25.org).

Standardized under the Telecommunications Industry Association (TIA), the P25 suite involves digital land mobile radio (LMR) services for local, state/provincial, and national (federal) public safety organizations and agencies. The standards specify eight open interfaces that more or less follow the JAN/IAN/EAN model, but with finer granularity (see the table and Fig. 2).

The first to be implemented, the Common Air Interface (CAI) standard, addresses the shortcomings of P16. Compliant radios must be able to communicate with any other CAI radio, regardless of manufacturer. CAI also provides for interoperability with legacy equipment, interfacing between repeaters and other subsystems, roaming capacity, and spectral efficiency/channel reuse.

A Network Management Interface standard specifies a common network-management scheme to manage all network elements of the RF subsystem. P25 also includes interface standards for porting between radios and different kinds of fixed-station equipment.

A Data Network Interface standard connects the RF subsystem to computers, data networks, or external data sources. An Inter RF Subsystem Interface (ISSI) standard enables radios to establish wide-area networks. Yet other standards deal with interconnection with ordinary telephone systems.

The CAI standard has multiple implementation phases. Phase 1 systems operate in analog, digital, or mixed-mode channels with 12.5-kHz bandwidth, using FM at 9600 bits/s/channel, or quadrature phase-shift keying (QPSK) at the same data rate with half the bandwidth. These systems are currently shipping.

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Phase 2 systems will be able to use either two-slot time-division multiple-access (TDMA) and frequency-division multiple-access (FDMA) modulation. Phase 2 work also involves console interfacing between repeaters and other subsystems, as well as manmachine interfaces for console operators that would facilitate centralized training, equipment transitions, and personnel movement.

During this year’s International Wireless Communications Expo in Las Vegas, Thales Communications demonstrated its Liberty P25-compliant multiband radio, which supports multiple public-safety frequency bands. The show demo was to introduce a $6 million-plus contract between Thales and the U.S. Department of Homeland Security’s Science and Technology (S&T) Directorate.

Thales is conducting a huge ongoing field-demonstration project involving public safety organizations across the country. It was intended to demonstrate the Liberty’s compatibility with any and every P25 radio currently in the field.

The software-defined Liberty interacts with P25 radios that operate in the 136- to 174-MHz VHF band and the 360- to 400-MHz, 402- to 420-MHz, 450- to 512-MHz, 700-MHz, and 800-MHz UHF bands (Fig. 3). It also is backward-compatible with earlier analog FM systems. Actual availability is scheduled for 2009. Cost will be approximately $4000 to $6000 per unit.

Like earlier P25-compliant radios with more limited bandwidth, Liberty evolved from military radios—in Liberty’s case, the AN/PRC-148. Like Motorola, M/ACOM, and EF Johnson, and other companies, Thales has a long history in defense electronics. Its parent company, the Thales Group, is a French amalgamation of the British defense company Racal and the former Thomson CSF. Alcatel is also an investment partner.

For domestic security reasons, U.S.- based Thales Communications is firewalled from the parent. It operates as a proxy-regulated company, free of foreign ownership, control, and influence. As such, it’s considered a 100% American company by the U.S. government, approved to work on the full spectrum of U.S. government projects and positioned to support strategic partnerships in developing key technologies for the defense market.

The AN/PRC-148 covers 30 to 512 MHz. It has a maximum output of 5 W and incorporates a number of special features for use on the battlefield. Liberty, on the other hand, doesn’t need to operate below the VHF public-safety band, so it has its own custom RF section. Thales would not discuss details of intermediate frequencies and analog-to-digital conversion, but it did say that Liberty’s single rubber-ducky antenna covered all subbands, from VHF-Low to 800 MHz.

Moving beyond Liberty, the use of common vocoder hardware ensures P25 interoperability. Digital Voice Systems makes all of the P25 vocoder products used today. Its latest chips, the DSP-based AMBE-3000 (Advanced Multi-Band Excitation, a proprietary algorithm), operate from 2.0 to 9.6 kbits/s (Fig. 4). To accommodate different radio platforms, the AMBE-3000 has multiple interface ports as well as operating modes that support parallel use of two ports or packetized data through a single port.

Beyond its vocoder functionality, the AMBE-3000 offers automatic voice/ silence detection (VAD), noise suppression, adaptive comfort noise insertion (CNI), dual-tone multi-frequency (DTMF), and call-progress tone detection/ regeneration, echo cancellation, low-power modes, and frame-by-frame “on-the-fly” rate switching.

In addition, the chip has variable-rate forward error correction (FEC). The FEC combines block and convolution codes with up to four bits of Viterbi soft-decision decoding. It enhances speech intelligibility in the presence of background noise and other degraded channel conditions, even with BERs of up to 20%.

There’s still much to be done standards-wise. For a quick overview on the road ahead, see “Still Far To Go,” www.electronicdesign.com, ED Online 18655.

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