“…When multiple antennas are available at the SU, the detectors can overcome the aforementioned limitations, such as the maximum-minimum eigenvalue (MME) [21], covariance absolute value (CAV) [22], covariance Frobenius norm (CFN) [22], arithmetic-to-geometric mean (AGM) [23], scaled largest eigenvalue (SLE) [24], locally most powerful invariant test (LMPIT) [25], Hadamard ratio test [26], volume-based detection (VD) [27], [28] and eigenvalue moment ratio (EMR) [29] methods have been proposed without any prior knowledge and can deliver desirable performance. Whereas, these detection methods focused mainly on time-invariant channels (i.e., channel state remains unchanged during a sensing period), and may not perform well in time-varying channels.…”