e. As shown in Figure 10.3, the major steps in CRN can be summarized as spectrum sensing, spectrum selection, spectrum access, and spectrum handoff [2]. The CR agent first uses the sensing function to monitor the unused spectrum and search for possible access opportuni- ties for SUs. Based on the sensing results, the spectrum selection function helps SUs select the best available channels and the spectrum access mechanisms provide fair spectrum 10 Secure Spectrum Sharing with Machine Learning: An Overview ML based SS LTE-U/ LTE-LAA Spectrum Information sensing optimization methods extraction Spectrum Incumbent Duty cycle selection detection methods selection Spectrum Channel AmBC-CR access selection theory methods Spectrum Distributed handoff methods Figure 10.2 ML-based methodologies for SS. Spectrum Spectrum sensing selection Observation Decision learning steps action Spectrum Spectrum handoff access Figure 10.3 Key steps in CRN. scheduling among vying SUs. Since a channel must be vacated when the PU reappears, the corresponding SU must perform a spectrum handoff function to switch to another available channel or wait until the channel becomes idle again. It is worth noting that most of the existing vertical SS approaches adopted these four steps in their frameworks. 10.2.1.1 Spectrum Sensing Before an SU accesses the licensed channel, it needs to first observe and measure the state of the spectral occupancy (i.e. idle/busy) by performing spectrum sensing. During this proce- dure, the SU needs to distinguish the signal of PUs from background noise and interference. As such, spectrum sensing can be formed as a classification problem. Automatic modulation recognition is a keystone of CR adaptive modulation and demod- ulation capabilities to sense and learn environments and make corresponding adjustments. Automatic modulation recognition can be deemed equivalent to a classification problem, 10.2 ML-Based Methodologies for SS and deep learning (DL) achieves outstanding performance in various classification tasks. S