an more effectively protect IUs from harmful interference while guaran- teeing their privacy. The database can provide a more stable service for SUs than CRN [75]. Much research has been devoted to increasing licensed systems' capacities by the extension of Long-Term Evolution (LTE) over the unlicensed spectrum band. Several concepts have been proposed, such as LTE in unlicensed bands (LTE-U), License Assisted Access (LAA) in LTE Advanced (LTE-A), and LTE Wireless Local Area Network (WLAN) 10.1 Background Aggregation (LWA). By allowing LTE users to operate on the unlicensed band without causing any harmful interference to original users such as WiFi devices, coexisting technologies can have a great impact on the spectrum access in the immediate future [144]. The emergence of IoT networks presents new challenges to wireless communica- tion design from both spectrum and energy aspects. Supporting communication with power-limited IoT devices, Ambient Backscatter Communication (AmBC) has attracted extensive attention as a promising technology for SS communications. In AmBC systems, backscatter devices can use surrounding signals from ambient RF sources to communicate with each other. By modulating and reflecting surrounding ambient signals, the backscatter transmitter can transmit data to the receiver without consuming new spectrum resources. The receiver can decode and obtain useful information after receiving the signal. Therefore, the AmBC system does not require a dedicated frequency spectrum, and the number of RF components is minimized at backscatter devices. Those devices can transmit data with sufficient harvested energy from RF sources [188], which can also improve system energy efficiency significantly. In summary, as shown in Figure 10.1, spectrum sharing originated from the concept of opportunistic access. Database-supported access frameworks on a specific licensed frequency band were then developed to connect new users to the unused licensed band without degrading the performance of IUs to improve the SE