A new emerging radio technology called cognitive radio significantly increases the utilization of the current frequency bands through spectrum-sharing approaches. Electromagnetic waves (that is, radio waves) are essential to the operation of wireless transmission systems, with frequencies in the range 3 Hz–300 GHz. Traditionally, large parts of the radio frequency bands have been licensed to authorized users by government. This static spectrum allocation mechanism causes frequency bands to be insufficient at various times and locations. Practical measurements show that most of the licensed bands are either unused or partially used in different geographical areas for much of the time. In other words, spectrum bands are used sporadically. Figure 1 shows spectrum usage in the range from 1 Hz to 6 GHz, which was measured by the Berkeley Wireless Research Center (BWRC).