Software Defined Radio: The interoperability solution?

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.