Cognitive Radio Prepares for Action

Signal, Apr 2008 by Kenyon, Henry S

Swart networking system navigates frequency selection without human intervention.

An experimental radio technology could provide U.S. warfighters with assured access to voice, data and video communications, The prototype systems use an advanced wireless networking capability to link troops with larger networks such as the Global Information Grid. The radios also are capable of sensing the electromagnetic environment and selecting frequencies that are not in use automatically.

Maintaining connectivity on modern battlefields has become challenging with the introduction of new communications and jamming technologies. These systems often work within close proximity to each other, putting pressure on frequency allocations in an already crowded spectrum band. A new communications system offers a way to provide individual soldiers with vital information while navigating through a thicket of signals and data transmissions.

The goal of the Defense Advanced Research Projects Agency's (DARPA's) Wireless Network After Next (WNaN) program is to create a flexible architecture for military communications (SIGNAL Magazine, July 2006, page 17). A key aspect of this effort is to develop and test an inexpensive handheld radio capable of selecting its own frequencies and forming small networks within a larger battlefield network.

The WNaN program is an extension of DARPA's successful Dynamic Spectrum program that focused on solving many of the spectrum management and access issues affecting the U.S. Defense Department. Preston F. Marshall, WNaN's program manager, Arlington, Virginia, shares that the effort is taking a different approach to dynamic spectrum allocation. At the beginning of the program, researchers questioned industry firms about the factors that make military radios expensive. Marshall says that this discussion allowed scientists to understand some of the intractable performance and cost problems of building highperformance communications systems.

Citing lessons learned in DARPA's Next Generation (XG) program (SIGNAL Magazine, December 2003, page 45), Marshall maintains that the goal is not to develop a radio that can work well in any arbitrary environment, but to develop one that can adapt and select the environments in which it works best and bring other similar radios into its network. "We use XG, not just to pick frequencies, but to pick environments in which the radio is not overly stressed or challenged. Then we can start to attack some of the driving economics of high-performance military communication," he says.

In developing WNaN, scientists explored two aspects. The first was determining whether affordable nodes could be built, and the second was to find out whether it was possible to create a network capable of adapting and scaling into hundreds or thousands of nodes. When the program was launched, Marshall notes that it was understood that if the network was built, there must be hardware to exploit it.

DARPA issued a broad agency announcement to determine the kind of radios competing firms would build. Marshall explains that the only requirement the agency insisted on was that the WNaN radios be able to find available areas in the spectrum and that they have four transceivers/channels in each handset. The multiple transceivers overcome network issues such as interference or overload by switching among the three other networks the node is connected to. "This is a core principle of the Internet-never be dependent on any one communication path," he says.

The WNaN program's network development phase was launched in mid-2007. Marshall notes that this effort is an extension of DARPA's research into cognitive radio. Although the path is evolutionary, he hopes the technology's effect is transformational. Besides solving spectrum issues, XG technology is a critical component for cognitive radio because it allows designers to compromise on some aspects of hardware performance.

DARPA researchers determined which technical aspects could be solved by hardware and which were limited by physics. Marshall admits that physical laws constrain certain aspects of radio development, such as front-end linearity and amplification of some wave forms. This limitation means that a cognitive radio will not use certain waveforms unless it is absolutely necessary. He adds that these considerations provide power and cost savings and permit the use of commercial parts in the system.

Marshall shares that several manufacturers are developing radio frequency integrated circuits for commercial worldwide interoperability for microwave access, or WiMAX, systems. Each of these inexpensive microchips is a super heterodyne receiver. But these receivers do not have the performance typically associated with a military radio. "With WNaN, we think we can build a military network that can have the same reliability but run across these relatively less robust chip sets," he says.

The WNaN radio also is known as the $500 radio. The term refers to the targeted cost of each individual unit when it is in full production. The $500 radio is designed as a handheld unit that can be worn on a soldier's hip or carried in a rucksack. By the end of the program, the goal is to have reduced the device's size to that of a large cellular telephone or personal digital assistant. Marshall says that after evaluating the devices, the services will provide feedback about the need for additional features such as data displays. The program has two cycles in which to include feedback from the field to modify the devices. "Our interest in the technology was how we could apply networking adaptation to allow us to use lower cost, lower performance components and still achieve very high quality military communications," he says.