io towers) Transmitter Actively transmit signals Data queue Backscatter signals Energy Gateway storage Smart jammer Figure 11.6 Anti-jamming attack in AmBC-CRN. Source: [190] (© [2019] IEEE). proposed method enabled the transmitter to transmit data even under jamming attacks. It was also observed that the more power the jammer used to attack the channel, the better the network performed. 11.2.4 State-of-the-Art Methods of Defense Against Intercept/Eavesdrop In eavesdropping attacks, an attacker tries to intercept private information from the legalized communication system. The basic principle of all defense methods is keeping leakage of information at an acceptable level. The encryption methods aim to totally block this leakage, while the physical layer security methods maintain the leakage rate under the required threshold by using different methods such as providing a higher channel difference, introducing friendly jammers, and/or adding artificial noise. Many ML-based works have been proposed to defend against eavesdropping attacks. 11.2.4.1 RL-Based Anti-Eavesdropping Methods A multilevel Stackelberg game-based secrecy transmission of CRN under an eavesdropper attack was considered in [169]. To protect the achievable rate, some SUs acted as the trusted decode and forward relays. Moreover, to proactively protect the legitimated transceivers, some SUs offered friendly jamming services and requested corresponding service charge prices. Furthermore, an advanced encryption method was adopted to increase the effective security level when users accessed the primary spectrum in the presence of eavesdroppers. By this means, the achievable rate was maximized and the consumed power minimized. Finally, a fuzzy-based MDP Outcome Prediction (MDPOP) Q learning algorithm was proposed to eradicate eavesdropping occurrence in CRNs. In [82], a DRL-based relay selection for secure buffer-aided CRNs was investigated. Considering that an eavesdropper keeps intercepting the signals from the source and relays, the relay selection pr