nications against eavesdropping attacks [242]. There are two major categories of secure communication techniques that guard against eavesdropping. One focuses on traditional cryptographic techniques and the other is the physical layer security. Cryptographic techniques involve encryption and decryption of information at the transmitter and receiver. In the physical layer security method, the secrecy rate can be achieved by the mutual information difference between the legitimate user and the eavesdropper. However, the security rate can be limited since it depends on the difference between the channel condition from the transmitter to the legitimate receiver and that from the transmitter to the eavesdroppers. Many promising techniques have been proposed to address this issue, including artificial noise (AN) and cooperative jammer (CJ) [203]. The advantage of physical layer security over cryptographic is that it can achieve secure communications without extra overhead caused by protecting the security key and can therefore be used in relatively simple communication systems. 11.1.5 Privacy Issues in Database-Assisted SS Systems According to [121], there are several differences between security issues and privacy issues. Security issues refer to unauthorized/malicious access, change, or denial of data. Privacy issues refer to the unintentional disclosure of sensitive information from some open-access data. The former is usually the work of malicious attackers who wish to disturb the system. In the latter, malicious users usually only collect information that does not immediately cause direct harm to the system. The goals of security protection are confidentiality, integrity, and availability. The goals of privacy protection are anonymity, unlinkability, and unobservability. Ensuring privacy in SS networks is very important. In some SS systems like SAS in the CBRS band, the IUs can be radar devices and military ships carrying very sensitive infor- mation. Opening these incumbents' exclusive spectrum to sharing could u