BACKGROUND
Emerging programmable radio technology such as the frequency and waveform agile radios available as part of the Joint Tactical Radio System (JTRS) program promise to open up new opportunities for robust and secure military communications. These software defined radios (SDR) will become even more powerful with the addition of electromagnetic environment sensing technologies that are being developed through the Defense Advanced Research Projects Agency (DARPA) NeXt Generation (XG) Communications research program. DARPA is developing the XG technology to allow multiple users to share use of the spectrum through adaptive mechanisms that allow opportunistic spectrum access in terms of time, frequency, code, and other signal characteristics. DARPA's goals are to enable an increase of a factor of ten in the usage of spectrum [1].