ABSTRACT
The current communication systems are characterized by a static spectrum alloca-
tion policy according to the spectrum allocation bodies around the world. As shown
in Figure 1.1, a considerable portion of spectra have been assigned to services such as
TV/broadcasting, radio, navigation, and so forth, and quite a few spectra (labeled as
white in Figure 1.1) have not been assigned, which results in the scarcity of spectrum
resources. Hence, few spectrum resources are currently available for future wireless
applications. For example, in China, authoritative predictions indicate that the spec-
trum bandwidth requirement for 4G communications is 1360∼1600MHz. However,
according to the decisions of a world radiocommunications conference in 2007, only
about 448 MHz bandwidth is available for 4G communications [2].
