In an overpopulated city, on a small land mass, you can only stretch the living space so far before inhabitants are stacked one on top of the other. Such is the electromagnetic spectrum, a finite space in which the airwaves are literally bulging at the seams. It's now becoming more possible, however, to make those "living spaces" in the spectrum more efficient, thanks to a new Defense Advanced Research Projects Agency (DARPA) dynamic spectrum access (DSA) radio technology. The DSA radio technology developed under DARPA's Next Generation (XG) program is the focus of Technology Editor George I. Seffers' article "Defense Spectrum Access Bursts Into Airwaves" in this issue of SIGNAL Magazine. The technology uses electromagnetic spectrum so effectively that it has gained White House attention and is being touted for possible commercial use both domestically and internationally. Commercial and military demand for spectrum has swelled immensely. Cell phones, wireless e-mail, digital audio and HDTV all compete for the same spectrum the military requires for C4ISR technologies. With so many demands on the spectrum, U.S. military forces face unique access issues in each country in which they operate. The U.S. Federal Communications Commission (FCC) regulates commercial spectrum use, and potential international usage is going to completely book the schedule of the XG program office. Public awareness about spectrum demands received a boost in June from President Obama's memorandum outlining a four-part strategy to free up 500 megahertz of spectrum by decade's end. The administration also released a fact sheet specifically mentioning DARPA's XG program and the National Science Foundation's Enhancing Access to Radio Spectrum program, which supports research enabling more people to share spectrum. Despite burgeoning demands on available spectrum, however, only about five percent is used at any given time. As a result, DSA technology could take advantage of up to 95 percent of unused spectrum. In densely populated areas with congested spectrum, the DSA likely would do little good because there is not enough free spectrum available. Still, the DSA will be a big improvement anywhere underutilized spectrum exists. DSA software monitors the spectrum for unused frequencies, which the DSA-enabled radio will automatically jump to even if that portion of the spectrum is not owned by the Defense Department. If the spectrum owner powers up a communications device, the DSA senses that device coming online and automatically jumps to another band of unused spectrum without disrupting the DSA user's communications or interfering with others' communications efforts. Larry Stotts, XG PM and deputy director of DARPA's Strategic Technology Office, emphasizes that spectrum allocation is a true challenge:
The FCC's spectrum allocation chart is a real messy kind of thing. Not only is every piece of spectrum allocated, sometimes it's allocated two or three times to different people, and they have to have playground skills to get along. So we said, wouldn't it be neat if we could take one of our electronic warfare sensors and listen for the absence of energy? We call that 'white space' because there's nothing there. If the spectrum is unoccupied, I will grab it for my own use. We have a radio that we've been developing over the last 10 years that will take any spectrum chunk and just add it to what is available because the more spectrum you have, the higher the data rate you have.
The JTRS program office also is interested in the DSA technology, which already has been demonstrated on the EPLRS and on AN/PRC-148 and AN/PRC-152 handheld, push-to-talk radios. The aim is to find the best ways to increase data sharing while dealing with the spectrum crunch. DARPA's dynamic spectrum access radio technology is but one solution in the overall quest to increase electromagnetic spectrum efficiency-but it's considered state of the art. The DSA can be likened to a traffic cop in that it directs the flow of spectrum traffic based on usage and volume. Cooperative efforts among international entities in the military, public and private sectors is integral to making this technology a continued success. Read the complete article and share your views on the DSA's impact and further potential.