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Based on decode-and-forward (DF) protocol, this work focuses on the adaptive power allocation and outage performance of underlay cognitive radio and opportunistic relaying (UCR-OR) systems with direct path between cognitive source and destination. The UCR-OR systems suffer from the interference of multiple primary user (PU) pairs. Under the outage constraint of PUs and the cognitive peak transmit power limit, we first obtain the adaptive power allocation schemes for secondary transmitters. Secondly, we obtain the exact closed-form expression to the outage probability of UCR-OR systems by using appropriate mathematical proof. Finally, to obtain a clear insight and to highlight the effect of system parameters on the performance of UCR-OR systems, the asymptotic closed-form expression of outage probability is achieved with the assumption of high cognitive transmit power. The presented simulations show that, due to the adaptive power allocation employed, the outage probability of UCR-OR systems is decreasing with PUs' transmit power P p  when P p  is less than a specific value PP*Open image in new window. Only when the value of P p  is greater than PP*Open image in new window the outage probability is increasing gradually with the increase of P P . When the transmit power of PUs is very high, the outage probability of UCR-OR systems tends to one. That is to say, in this case, the increase of PUs' transmit power degrades severely the performance of UCR-OR systems. Besides this, it is also found that the diversity gain of UCR-OR systems is proportional to the number of cognitive relays. The parameters of PUs only affect the coding gain of UCR-OR but not the diversity gain.

eurasip journal on wireless communications and networking

Adaptive power allocation and outage performance of cognitive best relay cooperation systems with multiple primary transceiver pairs and direct path between cognitive source and destination