“…In this context, several studies are investigated. Some papers optimised sensing time (Liu, Liang, & Zhang, 2013;Shokri-Ghadikolaei, Abdi, & Nasiri-Kenari, 2013) independently and some considered a fixed duration frame and maximised the transmission efficiency by defining it as the ratio of transmission time to frame duration (Balieiro, Yoshioka, Dias, Cordeiro, & Cavalcanti, 2013;Chang & Senadji, 2012;Choi & Yoo, 2013;Errong PEI et al, 2013;Hosseini, Amindavar, & Nadarajah, 2014). In the previously mentioned studies, there is nothing about interference caused by PU returns.…”