The implementation of wide band radio transmission may occur in two parallel ways: designing a new system with larger bandwidth, or constructing a cooperative system based on existing systems. The former is a spectrum aggregation scheme based on multi-carrier transmission within one system, while the latter fulfills spectrum aggregation based on multi-carrier transmission among systems. On the one hand, the advantage of the multi-carrier transmission scheme within a system and among systems is that the existing Radio Frequency (RF ) techniques and elements can be sufficiently utilized, so system costs can be decreased. On the other hand, spectrum aggregation among systems, especially among heterogeneous air -interfaces, can not only expand the basic bandwidth functions, but also fulfill efficient spectrum usage for Frequency Division Duplex (FDD ) system under the unbalanced Uplink/Downlink (UL/DL ) service scenario. It can also efficiently guard band usage between FDD and Time Division Duplex (TDD ) systems. Commercially, this has significant implications for operators.