Electronic Design

Can Public-Safety Radio’s P25 Survive LTE?

Project 25 (P25 or APCO-25) is a suite of North American digital radio communication standards for digital public safety radio communications. It was launched in 1988 as a step beyond the old-fashioned two-way voice contact between first responders and their dispatchers, with the dispatchers serving as the link to other agencies when necessary, generally over a telephone line.

It began when Congress directed the Federal Communications Commission (FCC) to collect recommendations from users and manufacturers. Based on the recommendations of the Association of Public-Safety Communications Officials-International (APCO), Project 25 then came into existence. In scope, this was unprecedented, but it wasn’t just happening in North America. Europe’s Terrestrial Trunked Radio (TETRA) protocol standards are a parallel effort, with much in common, but the two are not compatible.

The Incident Command System

P25 is about radios and interoperability, but hardware is only one aspect of the problem that public-safety professionals were addressing. Interoperability is one part of a possible solution, but it has to fit into broader picture. At nearly the same time that P25 was emerging, there were major efforts to rationalize and standardize the process by which individual public-safety organizations handled incidents and the ways that multiple agencies worked together when a tempest of smaller “incidents” escalated into a calamity.

The part of the larger effort that made more comprehensively interoperable radios necessary is the Incident Command System (ICS), which defines how those radios will be used (Fig. 1).1 ICS is a scalable structure for managing incidents ranging from a traffic crash to a major disaster. It provides a common framework for temporarily managing groups of people from agencies that do not routinely work together.

1. The Incident Command System embraces structure and planning. Even incidents involving police, fire, and emergency personnel only require fairly simple command structures.

Consider the recent Colorado wildfires, which involved multiple federal, state, county, and local police organizations, as well as local, state, and National Forest Service/Bureau of Land Management firefighters on the ground and airborne. Plus, private agencies such as the Red Cross were faced with finding food and shelter for the recently displaced. Trained volunteer amateur radio operators from amateur radio emergency services organizations offered support too. All of those people need a management structure within which to work (Fig. 2).

2. Not all ICS “incidents” are disasters. They may just be events that call for multiple layers of coordination. For example, the Thales Liberty Multiband Land Mobile Radio was used at the 2009 Kentucky Derby. (courtesy of DHS S&T Command, Control and Interoperability Division)

Although ICS is a management philosophy, it’s both a driver for the development of interoperable communications hardware and the tool for managing the problems that require interoperability. Imagine, for example, the difficulties involved in coordinating cops and firefighters, ground crews with chainsaws and bulldozers, borate bombers and helicopters, Red Cross workers, and caterers. (You have to feed these crews.) Plus, you have to keep meddlesome mayors and county supervisors in the loop and feed useful information to the media in a bad situation that threatens to get worse every minute.

ICS emphasizes planning and practice ahead of time and allows for information-sharing, learning, and rapid adaptation. But in application, it needs to evolve continuously. It’s impossible to standardize any aspect of incident control and declare rigidly that’s how things will be done for every future incident.2

P25 Concepts

Understanding ICS puts P25 into a context. After that, it’s all about standards. On a technical level, there is one fundamental rule for P25. Compliant radios may communicate in analog mode with legacy radios and in either digital or analog mode with other P25 radios. Beyond that, P25 standards allow considerable flexibility.

Some organizations use a single frequency. Others have multiple frequencies and use trunking to assign channels. With trunking, free channels are assigned by predefined trunked radio systems (TRS) protocols.

In operation, a control channel transmits data from the site controller that runs the TRS. All of the field radios in the system then continuously monitor the control channel. P25 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.

In a major incident, trunking systems assign priorities and share channels among agencies. New talk groups automatically preempt other routine communications, and lower-priority messages experience a busy signal.

Trunked radio systems aren’t optimal for all situations. For example, in a terrorist attack that also involves ambulances and firefighters, tactical law enforcement units are better served by going off-network and using direct radio-to-radio communications and portable or vehicular repeaters. P25 accommodates this flexibility.

Variations in frequency assignments among agencies, along with the characteristics of different bands, introduce their own complications. Federal agencies such as the Federal Bureau of Investigation (FBI), Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF), the Drug Enforcement Agency (DEA), and the Forest Service and local governments use available VHF frequencies between 136 and 174 MHz. Other federal agencies 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 between 400 and 402 MHz.) 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 UHF 700- and 800-MHz signals. Within those licensed bands, there are layers and layers of equipment, starting with the firefighters’ personal radios. This brings up an interesting illustration of the complexity involved in making decisions about direct communications versus trunking.

A firefighter inside a building might need an immediate burst of water from a truck just outside to deal with a sudden flare-up. That firefighter would have a personal radio. There also would be a radio on the truck. Is it better to communicate the need point-to-point over a path of a hundred feet or to go through a trunked repeater atop a building several miles away? What if elements of the trunking infrastructure fail or are sabotaged? One solution is to make the elements of the infrastructure themselves mobile. People try to answer these kinds of questions after simulations or actual catastrophes.


P25 standards describe eight open interfaces (Fig. 3). The Common Air Interface (CAI) Requires P25-compliant radios to be able to communicate with any other CAI radio, regardless of manufacturer. CAI also provides for interoperability with legacy equipment. Further, it deals with interfacing between repeaters and other subsystems, roaming capacity, spectral efficiency, and the manner in which channels are reassigned and reused.

3. P25 is an ambitious effort to provide transparent interoperability across not just radio, but all communications modes used by public-safety agencies.

The Inter RF Subsystem Interface (ISSI) standard focuses on how RF subsystems work with each other and the ways they can be connected into wide-area networks (WANs). The Fixed Station Interface (FSI) defines what makes up voice and data packets and command and control messages, as well as voice and data encryption and connections between radios and telephone networks.

The Console Subsystem Interface (CSSI) standard describes messaging for interfacing a console subsystem to a P25 RF subsystem. A console is the hardware used by a dispatcher or a supervisor who deals with personnel operating where the incident is taking place. There is also a trunked console interface in ISSI. The Network Management Interface (NMI) provides specs for all networked elements of the RF subsystem.

The Subscriber Data Peripheral Interface (SDPI) describes a port through which mobiles and portables can connect to laptops or data networks. The Data Network Interface (DNI) takes that down a level to the RF subsystem connections to computers, data networks, and external data sources. Finally, the Telephone Interconnect Interface describes how P25 works with the Public Switched Telephone Network (PSTN).


P25-compliant technology was deployed in phases to get something into people’s hands and to provide for feedback from the field.

Phase 1 radio systems operate in 12.5-kHz analog, digital, or mixed mode using frequency-division multiple-access. Data rates are limited, and bandwidths are wide. Phase 1 uses the IMBE voice codec, the original implementation of the Digital Voice Systems Inc. (DVSI) proprietary Multi-Band Excitation (MBE) technology. (IMBE is “Improved MBE.”)

Phase 2 uses DVSI’s AMBE+2 voice codec to reduce the needed bit rate so one voice channel only requires 6000 bits/s (including error correction and signaling). It also advances console interfacing between repeaters and other subsystems.

In lieu of the more familiar analog Tone-Coded Squelch System (CTCSS), P25 employs Digital-Coded Squelch (DCS) codes for access control in the form of a 12-bit network access code (NAC).

Enter LTE

The trouble with standards is that they get outflanked by technology, and that’s what’s happening to 700-MHz P25. Its capacity and bandwidth are being obsoleted by the latest and anticipated next generations of cellular technology. In particular, better analog-to-digital converters (ADCs) and digital signal processors (DSPs) have made software-defined radio a reality, although a reality that must be approached carefully (see “Professional Mobile Radio Goes Digital With DSPs”).

Cellular technology provides economies of scale. Companies that make P25 communications gear, including Motorola and Thales, also make cellular telephony products. They’re working with P25 public-safety organizations to adapt P25 to the newer technology, and the newer technology to P25, and the government is helping them.

The newer technology is Long-Term Evolution (LTE). The name itself is a positive sign that suggests it will adapt, rather than allow itself to be rapidly obsoleted. The Third Generation Partnership Project (3GPP) defined third-generation (3G) phones, and the International Telecommunications Union-Telecommunications (ITU-T) later standardized them.3

The 3G system is based on wideband CDMA with a 5-MHz bandwidth. It can download data at 384 kbits/s under normal conditions and up to 2 Mbits/s in some instances. High-speed packet access (HSPA) uses higher-level quadrature amplitude modulation (QAM) to get speeds up to 21 or 42 Mbits/s downlink (cell site to phone) and up to 7 and/or 14 Mbits/s uplink (phone to cell site). Then, cdma2000 phones added 1xRTT and Rev A and Rev B modifications that boost speed as well.

While people tend to hype LTE as “4G,” it’s really an advanced 3G standard. It uses orthogonal frequency division multiplexing (OFDM), which divides each channel into smaller 15-kHz subchannels or subcarriers, each of which is modulated with part of the data. In other words, the incoming fast data is divided into slower streams that modulate the subcarriers with either quadrature phase-shift keying (QPSK) or 16-phase QAM (16QAM).

LTE also uses multiple-input multiple-output (MIMO) antenna agility. The data stream is divided between the antennas to boost speed and to make the link more reliable. Combining OFDM and MIMO lets LTE deliver data as fast as 100 Mbits/s downstream and 50 Mbits/s upstream.

Keep in mind that that’s just data. Neither 3G/LTE nor 4G when it truly arrives will use these techniques for voice communications, which still relies upon 2G GSM or cdma2000. This is helpful in maintaining interoperability between LTE devices and P25.

Recent LTE/P25 Announcements

About a year ago, Harris Corp. released its BeOn. According to the company, it’s the first solution that lets subscribers on a cellular or public-safety LTE network talk to each other, exchange text messages, and pass real-time location information to connected team members and the dispatcher’s computer-assisted dispatch system. Harris also says that BeOn provides the integrated P25 feature set, including voice, text messaging, and location services. BeOn had previously been offered without P25 capabilities.

Voice communication services are delivered to first responders as Voice over Internet Protocol (VoIP) data packets using wireless broadband IP data services, via the Harris VIDA IP-based network. The VIDA network platform is a unified voice and data communication system based on P25 standards.

According to Motorola, LTE is enabled by its use of an OFDM air interface, advanced antenna techniques including MIMO and beam forming, flat all-IP architectures, and a common IP core.4 LTE technology, Motorola says, is available in two technologies: paired frequency-division duplex (FDD) and unpaired time-division duplex (TDD).

FDD is standard for the cellular industry, and public-safety narrowband technologies are available. TDD-based systems, commonly called TD-LTE, share the same spectrum for both the downlink and uplink. Also, these systems can be configured to allocate channel capacity for each.

The United States has allocated 10 MHz of paired spectrum in the 700-MHz band for public safety, allowing a 5-MHz channel in each direction. The U.S. government authorized a new 700-MHz LTE network for broadband services for the public-safety community in the tax relief bill that President Obama signed on February 12, says Andy Seybold, a wireless industry analyst.5 The authorization reallocates the cellular 700-MHz D Block to public safety and funds the network with proceeds from future auctions. Initial funding for the network will be $7 billion.

“This legislation also encourages public/private partnerships to help reduce the network costs,” Seybold says. “Some of these partnerships will be with commercial network operators and will include sharing of cell sites, high-speed backhaul, and, in some cases, the day-to-day operation and maintenance of all or a portion of the network.”

Seybold notes that device vendors will also gain from this new network. “New devices will be needed to serve the public safety network only (Band 14) or to also provide services on the AT&T and Verizon Wireless 3G and 4G networks when a public safety unit is out of its network’s coverage, which will certainly be the case during network construction over the next three to five years,” he says.

Harris Corp. recently concluded a demonstration, begun in March, of a dedicated LTE for public-safety network with the cops on the street in cities around the U.S. and the network core at the company’s headquarters in Chelmsford, Mass.

In Massachusetts, Harris provided a dispatcher and an LTE packet core from Nokia Siemens Networks. The public-safety officers were at LTE pilot locations in Miami, Las Vegas, and Monroe County, N.Y. The demonstration showcased the system’s ability to allow distant access to the core’s high capacity.

During the demonstration, the dispatcher could view the location of the police vehicle, know whether it was available for communications (communication may not be appropriate during certain surveillance situations), and engage in a push-to-talk call through the Harris BeOn application, which provides a P25 feature set over a broadband connection.


  1. The best overview of ICS can be found at the Federal Emergency Management Agency (FEMA) Web site
  2. For an interesting discussion of how communications problems develop even after everybody has interoperable radio, see “Interoperability: Stop Blaming the Radio
  3. Louis E. Frenzel, “What’s the Difference Between 3G and 4G Cellular Systems?
  4. The Beginning of the Future: 4G Public Safety Communications Systems"
  5. Andrew M. Seybold, “Seybold’s Take: Public safety’s 700-MHz LTE network an opportunity for vendors,” Fierce Wireless, March 14, 2012.


Professional Mobile Radio Goes Digital With DSPs

Joy Ji, Texas Instruments

Professional mobile radio (PMR) systems—the private wireless communications systems deployed by first responders (police and fire departments), military, and, increasingly, many businesses—received a wakeup call in the early 1990s. Interoperability was a problem.

Emergency response personnel from various agencies were sometimes challenged to work together because they couldn’t communicate effectively. As the number of users of PMR has grown steadily, doubling over the last 10 years, its wireless spectrum has become very crowded and interoperability is becoming a requirement.

Digital To The Rescue

Just as the wireless cellular network learned over the last 20 years, PMR system designers are realizing that digital hardware and software technologies offer a degree of flexibility, adaptability, and programmability not found in analog components. Most experts in the industry expect the PMR market to become completely digital by 2020 because analog has seemed to reach its limit.

The shift to digital was ushered in with the development of several digital radio standards. First, the major suppliers of PMR equipment developed their own proprietary digital radio components. Unfortunately, these vendor-specific standards could not interoperate with each other. Subsequently, several standards groups such as the European Telecommunications Standards Institute (ETSI) and several organizations in the United States and other countries around the globe began the development of open specifications for digital PMR radio.

Some of these specifications were regional solutions, while others addressed a niche market segment. For instance, Tetra (Terrestrial Trunked Radio) is widely deployed in Europe, P25 in North America, and PDT (Police Digital Trunking) in China. Other standards such as DMR (Digital Mobile Radio) and dPMR (digital Private Mobile Radio) have also been implemented in various regions. Developed by JVC Kenwood, the vendor-specific NXDN standard has been widely implemented as well. Many of the PMR manufacturers are evaluating or designing digital PMRs that can provide interoperability and support for as many standards as possible by reprogramming the system before the user goes to the field.

The cost of migrating from analog to digital PMR systems is not minor. First responders, the military, and other government-funded agencies have typically blazed the trail to digital PMR because it is easier to justify the costs in the name of public safety or national security. Now, newer, broader commercial applications of digital PMR are emerging in construction, hotels, transportation, large manufacturing facilities, delivery companies, commercial security, and other applications. Deploying higher volumes of digital PMR components in these areas will drive down the cost of PMR systems, opening additional applications to the technology.

The Challenges Of Success

Adding more applications means fitting more users into an already crowded wireless spectrum. A significant advantage of digital PMR technology is its ability to squeeze more efficiency from the spectrum. For instance, many digital PMR standards are reducing channel widths to 6.25 kHz so more users can occupy a certain frequency band. This can be done while maintaining backward compatibility and interoperability with previous generations of the PMR standards, which featured wider channels.

New features and capabilities made possible by the flexibility and programmability of digital technology are also drawing the attention of users. For example, short messaging services (SMS) and other features requiring packet data communications are being added to PMR systems. Programmability is imperative to suppliers of PMR systems since it lets them develop these distinguishing features for their products and differentiate them in the marketplace. When all products in a category must conform to a certain base level standard to interoperate, differentiating capabilities can become critical in the minds of potential users.

Programming PMR’s Future

PMR system designers are finding that the programmability of digital signal processors (DSPs) in high-level languages like C is enabling myriad advantages not possible with other processor types, such as reconfigurable FPGAs or the hardwired ASICs. On a very practical level, many of the digital PMR standards are still evolving.

A programmable DSP-based platform enables designers to quickly react to changes in the standards and rapidly upgrade systems already in the field. Given the major investment that many PMR systems represent, easy upgrades without acquiring new hardware are critical to future-proofing these systems without sacrificing leading-edge capabilities and interoperability. The low power consumption of DSPs is also an important requirement in applications like PMR that involve mobile, battery-operated handsets.

DSPs not only provide a basis for a particular PMR system, perhaps more importantly, they also can form the basis for a PMR platform that can extend across various market segments and use cases well into the future. In fact, the ability to implement one or more PMR radios in software running on a DSP maximizes a system’s interoperability with previous-generation analog radios as well newly emerging digital standards. While users transition from analog to digital, this backward compatibility with analog is extremely effective, allowing organizations a phased migration into the digital world.

This sort of flexibility on the part of DSPs has already enabled PMR radios to support multiple frequency bands, configuration-at-runtime, and various radio modes such as point-to-point and trunking communications. As digital signal processing power continues to increase, cost-effective PMR systems will be able to support multiple digital radio standards concurrently.

From the manufacturer’s point of view, the software reuse made possible by a programmable platform based on DSPs is essential to optimizing business opportunities. With DSPs, control and communications software (baseband and protocol stack) becomes portable among various PMR systems, which can be targeted at differing market segments with various price points and use cases.

Moreover, software in the signal chain becomes portable as well. For example, many of the low-bit-rate voice encoders/decoders (vocoders) that are essential to reducing channel widths were developed for execution on DSPs. These vocoders include advanced features such as noise reduction and forward error correction that ensure a natural-sounding digitized voice.

As more manufacturers deploy digital signal processing technology in PMR systems, the higher production volumes will continue to drive down the cost of systems. In addition, greater integration is possible with digital components, creating another opportunity to reduce costs further.

Indeed, some PMR systems are already taking advantage of DSPs integrated with general-purpose processing (GPP) cores. In particular, these types of PMR systems capitalize on the GPP’s processing capabilities to include applications processing, network protocol, a high-level operating system, graphical user interfaces, or even extensive peripheral connectivity interfaces such as Wi-Fi, Bluetooth, and GPS. The OMAP-L138 and OMAP-L132 DSP + ARM9 processors from Texas Instruments (TI) are well suited to PMR systems.

In addition, systems can combine DSPs with high-performance, low-power analog-to-digital converters (ADCs), digital-to-analog converters (DACs), and RF components to reduce board size, weight, power, and cost. TI’s 12-bit ADC12D1800RF direct RF-sampling ADC can greatly increase flexibility and significantly reduce the RF component count on a wide range of systems, while TI’s DAC34H84 and TRF3705 provide a small, low-power, and low-cost direct RF transmit solution.

As PMR systems continue to evolve, DSPs will certainly be at the heart of this technology’s shift from analog to digital components and will help these radio systems better serve the needs of the marketplace.

Joy Ji is a product marketing manager at Texas Instruments. She has degrees from Texas A&M University and Tarim University.

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