Software Defined Radio: The interoperability solution?

Nov 1, 2003 12:00 PM, by John Powell

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With local and state public safety first responder agencies spread across 10 discreet RF bands, lack of interoperability is the significant problem most often quoted in after-action reports from major public safety incidents. Most citizens do not realize that their police and fire officers cannot communicate directly with each other by radio, a well-publicized obstacle faced by the New York City fire and police departments during the terrorist attacks at the World Trade Center on September 11, 2001

While there has been much discussion about this problem in council chambers and statehouses across the country, and significant legislation proposed inside the Beltway, money to improve interoperability is only now starting to trickle into public safety budgets. Software Defined Radio promises perhaps the best long-term interoperability solution, providing the field officer with a belt-worn subscriber unit offering the ability to communicate in real-time with whomever he or she needs communication within limits established by agency managers. However, the technology still faces significant technical challenges as it grows from its infancy at the Department of Defense and from within commercial development enterprises.

Local, state and federal public safety agencies operate mission critical voice systems in ten discreet bands, ranging from 30 MHz to 869 MHz, and employing a number of different air-interface protocols. These bands are based upon the historical assignment of higher frequencies as the need for additional public safety spectrum was identified, and as technology provided cost effective equipment for the higher bands. In reality, this is not a bad situation because each of the various bands offers propagation characteristics that, when properly applied, support particular public safety user communities. For example, the lower bands work well for wide area rural systems with low population densities, while the higher frequencies with their improved building penetration characteristics are more appropriate for urban environments.

Three key interop categories

However, operational requirements highlighted by the horrific events of September 11 are driving agencies to demand dramatically improved interoperability between users. Interoperability is divided into three categories, with each having specific characteristics:

Day-to-day interoperability represents perhaps 95 percent or more of all interoperability use. It is the communications that takes place between officers from adjacent agencies, or with overlapping jurisdictions (such as city and county police) who back each other up on incidents. In the fire service it is the concept of automatic aid where the closest unit responds regardless of jurisdiction. In general, the communications links required for day-to-day interoperability are in-place, cover a specific geographic area, and are regularly used.

Task force interoperability supports planned events, though they could range over a wide geographic area. A narcotics surveillance is a one example of such use, and intercommunications between units of a fire strike team enroute to the wildfires in Southern California is another. Special events such as county fairs and political conventions also use task force interoperability. The links for task force interoperability are preplanned ahead of the event, and only exist during the event.

Mutual aid interoperability represents the least used, but most demanding of the three categories. It is the often overwhelming response to major disasters such as earthquakes, fires and hurricanes where there can be no detailed preplanning for the event and communications links are established as the event unfolds and responders arrive at the scene. Fortunately, overall response strategies are usually in place, defining the general nature of the communications that will be needed.

It is within these three categories that technology must be applied. The solutions being implemented today, primarily cross-band links, are not spectrum efficient, nor do they provide needed coverage in many situations. Patch systems require one RF path in each band for each conversation path to be linked, channels that must be taken from the available inventory of one of the participating agencies in each band. Coverage is limited to that area that is covered by the overlapping RF footprint of all of the involved channels. Couple these limitations with the operational requirements to set up and monitor each of the patch links, along with the limitations of the patch technology itself (delays in channel keying, inability to detect “busy” talkgroups on trunking systems, and the lack of support for end-to-end encryption) and the usability of these systems can quickly reduce their benefit to interoperability.

Radio-to-radio needed

The real need for a public safety officer when responding to any of these situations is to carry a radio on the belt that offers the ability to reach whomever she or he needs to communicate with in real time. That need must be supported radio-to-radio without the use of infrastructure for on-scene use at many incidents.

Enter Software Defined Radio. As already demonstrated by companies such as Thales and Vanu, SDR offers the potential of providing a multi-band platform that supports a number of different public safety waveforms. A waveform is the “software package” that defines the air interface and protocols necessary to enable communications using a particular technology. Project 25 Phase I, M/A-COM EDACS and Motorola SmartNet are all examples of waveforms that could be supported on an SDR platform. Equipping a field officer with a radio that supported the waveforms of agencies in that geographic area, along with a common national interoperability waveform, would enable that officer to have direct interoperability without the need for enabling infrastructure.

Additionally, SDR offers important other benefits to public safety. The ability to download software onto an existing platform allows for the addition of new features to existing waveforms and supports forward migration to new technologies with new waveforms.

On-the-street reality?

However, SDR is not yet an on-the-street reality. Through a contract with DoD, Thales has developed its MBITR portable radio that is not yet type-certified for public safety use. Likewise, Vanu Inc has demonstrated a multi-mode multi-band radio operating on a Compaq iPAQ platform. This latter product is a proof-of-concept device, requiring repackaging into a ruggedized public safety housing, with the proper form factor and supporting characteristics. Both require added waveforms to be of significant benefit to the user community, though they have proven the SDR concept is viable. Vanu is also developing a proof-of-concept base station for use in rural areas where a single SDR processor will support waveforms for public safety as well as for commercial cellular voice and for commercial wireless data.

That is not to say that significant technological hurdles have been overcome. The development of a public safety portable subscriber set is hindered by a number of technology requirements: battery capacity, antenna development and physical form factor remain major issues, as does the cost of the overall product. SDR processors and broadband power amplifiers require significant power. Public safety users require a battery that will support a radio for shifts that routinely extend to 12 hours. Today's multiband antennas are physically too large to carry if they have to support a very wide range of frequency bands. And all of the hardware, including an appropriate battery must fit in an easy-to-carry package with an appropriate form factor that supports ease of use. Finally, the product must be affordable; while agencies will be willing to pay a premium for the features provided, all are constrained by today's limited public budgets.

But perhaps the greatest impediment to fielding a useful SDR product will be the licensing of intellectual property (patents). Each of the major public safety waveforms, with the exception of analog FM and Project 25 digital, will require negotiations with those manufacturers that could introduce major delays.

SDR will make its way into the public safety radio services. As with any new technology, introduction will be slow and the initial products costly. But the benefits obtained from the technology will eventually drive demand up and cost down. Will SDR be the ultimate solution to the interoperability problem? Only time will tell, but research to date certainly supports that claim, and significant R&D funds from both private and public sources are being devoted to the development effort.

John S. Powell is an electrical engineer and a director of the Software Defined Radio Forum, representing government agencies. Among his many achievements, Powell served as statewide Police Communications Coordinator for the University of California system. He is a life-member and past-president of the Association of Public Safety Communications Officials-International (APCO). He is a member of the Project 25 Steering Committee and has been a member of the California Legislature's Joint Committee on Fire, Police, Emergency and Disaster Services, APCO Law Enforcement Service Committee. Powell is past-chairman of the Interoperability Subcommittee of the National Coordination Committee, a federally chartered advisory committee to the FCC regarding implementing the 700 MHz public safety band and past-chairman of the FCC-chartered State Interoperability Executive Committee for the State of California, Governor's Office of Emergency Services. Recently, Powell has been consulting for several agencies within the U.S. Justice Department, the Department of Homeland Security, and the Executive Office of the President of the United States on issues related to public-safety communications interoperability, development of next-generation public safety broadband equipment and standards, and Software Defined Radio.

Interoperability:

The ability to talk to who you need to talk to when you need to talk (data and voice, realtime).
NOT the ability to talk with everyone all of the time!