“…In Table 2, the principal aspects considered in two main categories (i.e., PU's activities and to PU's protections) are concluded together, to present and to classify the existing works and methods towards PU's design. [15], [27], [32]- [33] Transmission Power [3], [22], [26], [34] PU interference [28], [29], [34] PU's protections Detection Constraint [35]- [39] Collision Avoidance [32], [40]- [41] Traffic Load [42]- [44] QoS Provisioning [18], [45]- [47] [10], [35]- [37], [48] mainly apply a lower bound of detection probability to guarantee the accuracy of detection, and consider other targets (e.g. EE, throughput, PU's spectrum utilization, etc.)…”