In this paper we propose a novel adaptive scheme for full duplex communication of secondary users (SU) in a cognitive radio network. The secondary network operates adaptively in three modes; Cooperative Sensing (CS), Full Duplex Transmit and Sensing (FDTS), and in-band bidirectional Full Duplex Transmit and Receive (FDTR). In the CS mode, the secondary nodes will detect the signal of a primary user (PU) through a novel cooperative MAC protocol and will decide the system's mode of operation in

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... e subsequent spectrum hole. This adaptive decision is based on dual-threshold detection (DTD) introduced for the first time. When the primary's signal at SU's receivers is weak, the system switches to the FDTS mode to avoid higher collisions probability and long or endless collision durations. In the FDTS mode, one of SUs senses the PU activity continuously whilst transmitting to another node. When the channel conditions allow, the system switches to the FDTR mode, in which the secondary users would communicate bidirectionally in an asynchronous full duplex manner. The novel idea of asynchronous transmission in this mode will result in decreased maximum and average collision durations. Analytical closed forms for probability of collision, average collision duration and cumulative collision duration, as well as throughput of the SU network are derived, and performance of the proposed scheme in terms of the above-mentioned metrics, its effectiveness, and advantages over conventional methods of sensing and transmission are verified via simulations. Index Terms-Asynchronous cognitive radio, cooperative sensing, full duplex communications.