Extending the Software-Defined Radio Concept

November 2004
By Robert Walter and George Duchack

The Adaptive Joint Command, Control, Communications, Computers, Intelligence, Surveillance and Reconnaissance Node (AJCN) is a multimission radio frequency system that would provide seamless interoperable communications with signals intelligence, electronic warfare and psychological operations simultaneously.
Technology demonstration examines feasibility of a multimission system.

The U.S. military is developing a modular, scalable, multifunctional radio frequency system that would provide unprecedented interoperability through its communications and data gateway while performing signals intelligence collection, electronic warfare and psychological operations broadcast. The technology incorporates common radio frequency hardware components networked with pools of processors that are programmed through software to instantiate a variety of radio frequency capabilities and perform multiple radio frequency functions simultaneously.

The Adaptive Joint Command, Control, Communications, Computers, Intelligence, Surveillance and Reconnaissance Node (AJCN) advanced concept technology demonstration (ACTD) is sponsored by the Defense Advanced Research Projects Agency (DARPA); the U.S. Army Communications Electronics Research, Development and Engineering Center; the U.S. Air Force Command, Control, Intelligence, Surveillance and Reconnaissance (C2ISR) Center; the Office of the Deputy Undersecretary of Defense for Advanced Systems and Concepts; and the U.S. Joint Forces Command (JFCOM), Norfolk, Virginia, which also will perform the joint military utility assessment.

The AJCN system consists of a mission payload segment and a control and reporting segment. The mission payload segment is software driven and can be retasked or reprogrammed via the control and reporting segment in real time to meet changing mission requirements. In its full operational capability, it is envisioned that the AJCN would be inherent in numerous terrestrial and airborne platforms throughout the battlespace to form a constellation of multifunctional radio frequency (RF) nodes.

The AJCN concept evolved from a DARPA program called the airborne communications node, which developed technologies for a modular, scalable, software-defined radio that could bridge and translate voice and/or data between two or more dissimilar radios. In 2000, this technical challenge was taken a few steps further to pursue a software-defined RF system. The premise was that if a software-defined radio could instantiate such capabilities as a communications modem, digital signal processing or networking through software, it would stand to reason that this type of radio could be designed to perform other RF processing as well, such as signals intelligence collection, electronic warfare and psychological operations broadcast. Additionally, with enough processing capability, a software-defined radio could run multiple RF processes concurrently.

This expanded AJCN concept required several technological accomplishments to make it feasible, such as wideband interference cancellation, a hardware-independent multimission software framework, a security architecture that supports multiple security levels, and advanced networking and data translation capabilities. Wideband interference cancellation is necessary to receive and transmit multiple signals simultaneously in the same or adjacent frequency bands. This is especially crucial for the fidelity of wideband signals intelligence collection while performing other RF functions in the very high frequency/ultrahigh frequency bands.

Interference mitigation is accomplished in the AJCN in several stages. Antenna placement, relative to the platform and other antennas, is the first stage of limiting interference. The second stage is an active signal cancellation process that adds the inverse of the transmitted signals at the appropriate level to the receive channels. The third stage uses digital processing to steer a null gain in the direction of the antennas that are producing interfering signals. Null steering also is beneficial in eliminating interference from off-board signals such as high-power television and radio towers or jammers. In the laboratory, the narrowband interference cancellation system decreased the interfering signal an average of 80 decibels to bring all signals within the dynamic range of the AJCN processing receiver.

The hardware-independent multimission software framework is the heart of the AJCN. The node incorporates Diamond, a commercial software package that enables the system to have a truly modular, scalable open architecture that supports hardware component swapping in a plug-and-play fashion. With Diamond, the AJCN can be scaled to fit the size, weight and power constraints of almost any platform. Diamond provides flexibility in system design, supporting a variety of operating systems such as Solaris, Windows and Linux as well as running multiple instances of software-defined radio architectures such as the Joint Tactical Radio System (JTRS) SCA 2.0, SCA 2.2 and non-JTRS architectures. The node also uses a universal software interface for field programmable gate array chips that eliminate the need to port system software to run on different or new generations of the chips.

Performing signals intelligence collection, communications, electronic warfare and/or psychological operations broadcast simultaneously from the same payload requires a security architecture that establishes multiple separate security levels that are networked to and managed by a trusted agent. Additionally, the security level and crypto key for the red side processors must be changed as the RF missions change.

The AJCN is leveraging the JTRS-led effort in developing and certifying the Sierra II cryptographic module built by the Harris Corporation. The Sierra II chip will be the trusted agent to manage a library of crypto keys and several virtual private networks (VPNs) and red side processors that would establish secure enclaves as needed for each required security level. The AJCN technology demonstration will illustrate the VPN security architecture with three to four different security levels operating simultaneously and with the ability to reconfigure dynamically.

The AJCN requires an advanced networking capability and data translation service to enable the transmission of information through the constellation to any system in the appropriate message format, encryption, network protocol and RF waveform. This any-to-any gateway capability is accomplished by leveraging advanced networking technologies.

While the concept for the AJCN grew out of the software-defined radio development efforts, the core hardware architecture is that of a signals intelligence system with added computing power and a multichannel transmit capability. For the demonstration, the receiver front end includes several 25-megahertz tuners that are tunable across the low-band range. The tuners can be stitched together to provide a wideband stare capability, or they can be tuned through a frequency range to perform a narrowband search. The receiver front end can collect and process signals intelligence data while simultaneously receiving and processing communications signals in the same frequency band. Essentially, once the RF energy is collected and sampled, many programs and algorithms can be utilized to distinguish and process the signals in the digital domain.

The AJCN can be tailored to fit the size, weight and power constraints of many platforms. Larger payloads contain a greater number of apertures, tuners, processors and transmitters that in effect permit larger aggregate bandwidth to receive and more transmit channels to be established at any one time. Smaller AJCN payloads will have less bandwidth and channel capacity, but essentially will have the same functional capabilities because of the common software framework and RF applications.

The vision of the AJCN after the demonstration and transition phases are complete is to deploy a constellation of nodes throughout the battlespace at high and low altitudes as well as on the surface. The constellation will form a dynamic mobile network of sensors, relays and attack platforms integrated with command and control centers, dedicated sensors and weapon systems.

The node will support the desired information superiority and dominance capabilities outlined in Joint Vision 2020 and address numerous joint and service-specific mission needs. As a multifunctional RF system, the AJCN provides seamless interoperability, an alternative to satellite communications for over-the-horizon communications, enhanced ISR capabilities, improved mission flexibility and greater platform efficiency and utility.

Featuring JTRS and Joint Airborne Signals Intelligence Architecture compliance, waveform bridging and message translation applications within the AJCN will provide an any-to-any communications gateway capability among the services, government agencies and public entities.

Within the scope of the demonstration, the node will establish the ability to bridge data and voice networks that use Link-16, the single channel ground and airborne radio system, the enhanced position location reporting system, HaveQuick II, demand assigned multiple access ultrahigh frequency satellite communications, global system for mobile communications, and land mobile radio as well as very high frequency and ultrahigh frequency AM/FM. As the JTRS Joint Program Office develops new software waveforms, these waveforms will be integrated into the fielded AJCN system.

The AJCN will improve signals intelligence collection through a network-centric approach. Although the node was designed around a signals intelligence architecture, its initial collection capabilities lack the fidelity of world-class systems in use today such as Rivet Joint and Airborne Reconnaissance Low (ARL). An AJCN constellation can effectively compensate for the decreased fidelity by virtue of the larger numbers of apertures and look angles together with a shorter range to the adversary. The AJCN could be deployed in close proximity to the adversary, where adversary RF signals are stronger. A modest number of AJCNs deployed around the battlespace in a network would have a better opportunity to acquire signals and provide multiple lines of bearing to signals of interest that are low-power or are masked by terrain from standoff sensors. As a result, an array of AJCN sensors could augment and extend the capabilities of Rivet Joint, ARL or Prophet to increase their geolocation accuracy significantly.

A prior knowledge of the RF mission requirements over time in an area of operations is not necessary to employ the AJCN effectively. The payload can be reconfigured, re-tasked and/or retuned on the fly in real time to meet changing mission requirements. Control of the payload configuration is accomplished through the control and reporting segment, a software graphical user interface. It can be used to control the payload from any workstation that has connectivity to the platform. The RF missions can be controlled by multiple users with these interfaces. Additionally, one operator can control a number of AJCN payloads by running several interfaces on a single terminal.

The employment of the AJCN concept can realize efficiencies and cost savings. All of the services require an airborne communications node to either extend communications range or provide beyond-line-of-site relay capabilities. On average, these systems would have a duty cycle of 50 percent or less. If an AJCN is employed, the 50 percent of down time could be used to perform other RF functions. Depending on mission priorities, platforms with the node that are primarily dedicated to one mission could be reconfigured on the fly to perform another mission that has been degraded by attrition.

A second area of increased efficiency through the AJCN is in the costs and utilization of satellite communications. The Army and the U.S. Marine Corps are increasing their use of satellite communications to solve their in-theater beyond-line-of-sight connectivity shortfalls. The Army is leveraging Inmarsat for maintaining a common operational picture, and the Marines are making greater use of Iridium telephones for tactical command and control.

The AJCN can decrease dependence on satellite communications in two ways. First, as an airborne bridge/relay, the AJCN can provide the necessary beyond-line-of-sight connectivity between tactical radios to reduce the need for satellite communications. Second, the node can be configured to operate as a surrogate satellite and decrease the requirements for satellite transponders.

The AJCN ACTD is a five-year effort that includes plans to initiate transition to acquisition in fiscal year 2006, following a successful joint military utility assessment. The demonstration is limited in scope, funding and security accreditation. As such, the ACTD payloads will incorporate only a basic set of capabilities sufficient to demonstrate the concept and assess military utility. JFCOM will be conducting the joint military utility assessments through the second quarter of fiscal year 2006. If successful, the transition to a formal system development and demonstration phase in late fiscal year 2006 will occur in parallel with the extended uses evaluation phase of the ACTD.

An existing Air Force program with similar objective capabilities will likely conduct the follow-on development of the AJCN. During the system development and demonstration phase, the Air Force would integrate all of the JTRS communications waveforms as they become certified with message translation applications and implement a full complement of signals intelligence, electronic warfare and psychological operations broadcast capabilities to fulfill multiple service and joint program requirements. The U.S. Defense Department expects that low-rate initial production of AJCN payloads could begin in late fiscal year 2007.

Lt. Col. Robert Walter, USA (Ret.), is a senior engineer/analyst for SRA International Incorporated, providing technical support to the Adaptive Joint Command, Control, Communications, Computers, Intelligence, Surveillance and Reconnaissance Node (AJCN) advanced concept technology demonstration (ACTD). George Duchack is the former Defense Advanced Research Projects Agency program manager and ACTD technical manager for the AJCN. He is president of Sage Solutions Group Incorporated.

Web Resources
Adaptive Joint C4ISR Node (AJCN) advanced concept and technology demonstration (ACTD): www.monmouth.army.mil/cecom/pao/infofacts/websiteprogs2.htm
AirborneCommunications Node: www.darpa.mil/ato/programs/acn.htm
Bridging the Gap: www.darpa.mil/darpatech2004/pdf/bridgingthegap.pdf

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