Outage Analysis of a Spectrum-Sharing Cognitive Relaying Network with Improved Spectral Efficiency

The outage performance of a spectrally efficient scheme for diversity exploitation in multiuser cognitive relaying networks under spectrum sharing constraints is investigated. In our analysis, we consider a secondary network composed by one source node communicating with one out of L available destinations, which is selected according to the channel quality of the direct links. If the transmission through the selected direct link drops below a given threshold, a half-duplex decode-andforward relaying transmission is then invoked. For this purpose, it is first selected a new secondary destination so as to maximize the signal-to-noise ratio resulting from employing a maximal-ratio combining technique on the received signals from the secondary source and relay. In addition, assuming an underlay spectrumsharing approach, the overall transmit power at the secondary network is considered to be limited by the maximum tolerable interference power at the primary user receiver, as well as by the maximum transmit power available at the secondary nodes. An exact analytical expression for the outage probability of the proposed scheme is derived and validated by Monte Carlo simulations. Furthermore, closed-form expressions derived from the asymptotic analysis of the outage probability reveal that the proposed scheme achieves full diversity order, equal to L + 1.