Civilian, public-safety and military radio system operators all are seeking additional spectrum that is in short supply. In the civilian world, additional spectrum is needed for next-generation cell phones offering more advanced services and for wireless broadband access. Public-safety agencies are seeking more spectrum to roll out mobile data applications that will make first responders safer and more effective.

Meanwhile, the U.S. military needs more spectrum to foster its new philosophy of network-centric warfare, which calls for every unit on the battlefield — from high-flying aircraft to the soldiers on the ground — to communicate with one another to identify friends and foes, a goal made more challenging by the fact that the military often operates in countries where the necessary spectrum isn't available.

Defense Advanced Research Projects Agency (DARPA), the U.S. Department of Defense's think tank for bold ideas — where the Internet and stealth technology were first hatched — is taking a crack at the worldwide spectrum-efficiency problem through its Next Generation (XG) program, which leverages 2G/3G/4G cellular phone terminology. “DARPA is involved because many more radios are planned to be deployed by the U.S. military in the future,” said James Dunnigan, military analyst and commentator for NBC news. “More radios [mean] more spectrum is needed.”

XG — first conceived in 2002 — is designed to address military and civilian user demand for greatly increasing amounts of spectrum, according to Preston Marshall, XG program manager. “We want to find ways to maximally load spectrum with radios. The key concept is, while spectrum is allocated, a relatively small amount of spectrum is used on a point on the ground,” Marshall said. “The metric within DARPA is to increase by a factor of 10 the number of radios that can be fit into a piece of spectrum, instead of the current method of assigning and de-conflicting spectrum.”

An important element of XG is a customizable policy language that allows any regulator to define a set of spectrum-usage rules that would be loaded into a “smart” radio so it wouldn't interfere with incumbent users. “DOD needs worldwide operability,” said Marshall. “You can download rules so that radios conform to host nation [regulation].”

DARPA is actively promoting global use of the XG policy language. Marshall recently spoke to the International Telecommunications Union to evangelize the concepts behind XG. “Radios could roam the world and always be compliant with local policies,” he said. “Worldwide roaming policies have benefit to all countries if we can demonstrate them.” Already, one Japanese consumer electronics company is in discussion with DARPA about how it might provide input into the development of XG based upon its experience with spectrum, something that is possible because the policy language and definitions are in the public domain for anyone to use, Marshall added. “We want to leverage a lot of thinking outside of DARPA, how you build radios worldwide,” he said.

Also developed under XG is a sensor card designed to scan the airwaves for available frequencies and effectively shrink “thousands of dollars” of spectrum analyzer gear onto a single card, Marshall said. “It's a small 4-by-5 inch card, about a half-inch [thick]. We haven't done any work on reducing the form factor, but it should be something you can make fairly small,” he added. Marshall believes the card eventually could be mass produced. “It's similar in complexity to an 802.11 card. If the commercial world builds it, we get access to that kind of production.”

Under contract to DARPA, Raytheon has been assembling the pieces of XG to demonstrate that the entire concept is viable for military use. “We're focusing on what it means for spectrum access and to improve spectrum efficiency. Under Phase Two [of the contract], we've done a simulation demonstration, implementing XG on a few prototype radio platforms in the lab and demonstrated better than 10 times the use of existing spectrum,” said Scott Sidel, Raytheon's principal investigator. He added that Phase 3 would include a “medium to large-scale” over-the-air demonstration scheduled for fall 2006. XG is expected to wrap up by the end of 2006, with the technology likely to make a rapid transition into the military, if funding is available, according to Marshall.

Raytheon has worked with DARPA on XG since the beginning. According to Sidel, XG radios sense actual spectrum usage and detect what bands are being used. “The machine readable format for [spectrum usage] rules establish the bounds of use for an XG radio, power on the bands and takes into account its location,” he said.

Accordingly, an XG radio's rule set would let it access both unlicensed and licensed spectrum. Raytheon uses the terms “white space” to describe available open spectrum, “black space” to define spectrum that is off-limits for use because of interference concerns and “gray space” to identify frequency regions where other users are also active but where spectrum can be shared with power control and various coding schemes to prevent interference. A “heteromorphic” waveform incorporating defined policy rules would manage the use of white and gray spectrum by assessing variables such as available frequencies, time, location and power.

Marshall is careful not to use the FCC's definition of “cognitive radio” when describing XG. “We have created a framework to build a cognitive radio, [capable of] understanding its environment, what its options are. We won't actually build it. An XG radio would be one that is fully capable of supporting cognitive features.”

Commercial interest in XG is growing, according to Marshall. “We put a lot of effort into working with a lot of companies that are not traditional DARPA suppliers but have an interest in radios,” he said. “Companies want to understand [XG] technology.” He noted that Intel, Cisco Systems, Philips, Verizon, the FCC and National Telecommunications & Information Administration (NTIA) all attended a recent XG workshop. “Intel is very interested in policy language. It would allow them to ship one radio and use the policy language to tailor it to different counties.”

Sidel indicated Raytheon is in discussions with several commercial vendors. “We believe the algorithms are ready now for integration into commercial products. We're talking to radio vendors and system vendors.” Moreover, Dunnigan believes the chances of civilian XG adoption are high. “Look at GPS as an example of a dual-use technology. It began as a military navigation program, but now you find GPS chips in cell phones and GM's OnStar system,” he said.

XG is changing people's attitudes as well as changing technology, according to Marshall. “You have the FCC spectrum task force and the NTIA [now] talking about new methods. There's an incredible willingness to look at what you wouldn't have four years ago.”