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Recent studies suggest that spectrum congestion is mainly due to the inefficient use of spectrum rather than its unavailability. Dynamic spectrum access (DSA) and cognitive radio (CR) are two terminologies used in the context of improved spectrum efficiency and usage. The advent of CR has created a paradigm shift in wireless communications and instigated a change in FCC policy towards spectrum regulations. Spectrum overlay and underlay techniques fall within the hierarchical DSA model and enable primary and secondary users to coexist while improving overall spectrum efficiency. As addressed here, spectrum overlay exploits unused (white) spectral regions while spectrum underlay exploits underused (gray) spectral regions. Similar to UWB transmissions, underlay approaches utilize much more spectrum than overlay approaches and operate below the noise floor of primary users.