Existing "command-and-control" spectrum allocations defined by government regulatory agencies prohibit unlicensed access to licensed spectrum, constraining them instead to several heavily populated, interference-prone frequency bands. However, it has been shown that the spectrum is not utilized efficiently across time and frequency. Optimal use can be achieved by dynamic spectrum access (DSA) - the concept of unlicensed users "borrowing" spectrum from incumbent license holders temporarily to improve the spectrum utilization. Cognitive radios offer versatile, powerful, and portable...