Reliable wireless communications today requires careful allocation of specific portions of the electromagnetic spectrum to individual radio networks. While pre-allocating spectrum is effective in benign environments, radios remain vulnerable to inadvertent interference from other emitters and intentional jamming by adversaries.

On March 19-20, 2014, 15 teams from around the country demonstrated new ways to help overcome these challenges by participating in the final event of the DARPA Spectrum Challenge-a national competition to develop advanced radio techniques capable of communicating in congested and contested electromagnetic environments without direct coordination or spectrum preplanning. After two intense days of competition, teams from Tennessee Technological University and Georgia Tech Research Institute and an independent team of individuals emerged as the overall winners, earning a total of $150,000 in prize money.

"The sophistication of the solutions that the teams developed really impressed us," said Yiftach Eisenberg, DARPA program manager. "The teams showed that radios can learn to coexist and communicate reliably by autonomously sensing and adapting to congested electromagnetic environments-paving the way for new spectrum-sharing applications for the Department of Defense and commercial industry."

The final event took place at DARPA's offices in Arlington, Va. Eighteen teams had previously participated in the Spectrum Challenge preliminary event in September 2013. Three teams that participated in the preliminaries were unable to complete their ambitious designs in time for the final event. The competitors at the final event represented the top 15 teams out of the 90 teams that initially registered. Academic institutions from around the country comprised 12 of the 15 teams, while the remaining three teams were individual radio hobbyists and practitioners working on their own time.

Both the preliminary and final events included two separate tournaments, each with its own goals:

+ Cooperative tournament: In each match, three teams attempted to effectively share the spectrum while transmitting random data files from their source radio to their destination radio over the same 5 MHz UHF band. A team's match score was its total packets delivered plus the higher of the two other teams' delivered packets-thus motivating cooperative behavior.

Teams could not coordinate in advance on how to share the spectrum; instead, they had to develop and implement algorithms to enable their assigned software-defined radios to dynamically communicate at a high rate while leaving spectrum available for the other two teams to do the same. This event tested conditions encountered during military operations involving multiple units and coalition partners, and also has possible future commercial applications.

Tennessee Tech Telecom, a team from Tennessee Technological University, won the cooperative tournament and a $50,000 prize. Efficient Spectrum, a team of individuals from Centreville, Va., came in second and earned a $25,000 prize:

+ Competitive tournament: In each match, two teams sought to dominate the spectrum, with the winner being the first to transmit all its files of random data (or to successfully transmit the most packets in three minutes) from a source radio to a destination radio. Teams had to develop and implement algorithms to enable their assigned software-defined radio to dynamically communicate at a high rate in the presence of competitors' signals within the same 5 MHz UHF band. This event tested conditions directly applicable to military communications, where radios must deliver high-priority data in congested and often contested electromagnetic environments.

Efficient Spectrum-the runner-up in the cooperative tournament-won the competitive tournament and added another $50,000 to its earnings. A team from Georgia Tech Research Institute (GTRI) took home $25,000 as the second-place finisher:

+ As in the preliminaries, DARPA provided all teams with the same radio hardware to ensure that each team would win or lose based on its software algorithms alone. All the matches again occurred on the ORBIT testbed at Rutgers University's WINLAB, which streamed the proceedings live to big screens in DARPA's conference center, where contestants as well as representatives from across the government and industry eagerly watched the event unfold.

"The Spectrum Challenge exceeded expectations for stimulating new technologies and attracting new talent," said Dan Kaufman, director of DARPA's Information Innovation Office, which oversaw the Spectrum Challenge. "As competitors alternately battled over and worked to share the spectrum, it was easy to see how these advanced capabilities could prove invaluable in a wide range of military and civilian applications that seek to maximize the value of this precious and finite resource."

"It was immensely satisfying to see the enthusiasm of the participants and how quickly they grasped the complexities of operating in dynamic spectrum environments," Eisenberg said. "We want to continue to work with industry and academia to build upon the achievements we have seen so far."