Hybrid spectrum sharing in cognitive two-way relay networks

In this paper, we study hybrid overlay/underlay spectrum sharing in cognitive two-way relay networks with an opportunistic relay selection scheme. Different from the traditional interference power constraint, we adopt primary users´ (PU) outage constraints to guarantee the QoS requirement of a primary system. The exact closed-form expression for the outage probability of a secondary system is derived over Rayleigh fading channels. Our results show that there exists a critical threshold for Primary Transmitter´s (PT) transmit power. The outage performance of the secondary system is almost unchanged before PT´s transmission power reaches the threshold. However, the outage performance degrades quickly when the power exceeds the threshold. Another important observation is that a PU´s outage constraint can lead to outage saturation phenomenon. There is outage performance tradeoff between the primary system and the secondary system, which is closely related to the transmit power limit of the secondary system.