Outage analysis of a spectrally efficient scheme for multiuser cognitive relaying networks with spectrum sharing constraints

This paper investigates the outage performance of a spectrally efficient scheme for multiuser cognitive relaying networks under spectrum sharing constraints. In the proposed setup, we consider a secondary network composed by one source node communicating with the best one out of L available destinations, selected according to the channel quality of the direct links. If the selected direct link drops below a given threshold, a half-duplex decode-and-forward relaying transmission strategy is invoked, and then the selected secondary destination employs a maximal-ratio combining technique to combine the received signals from the secondary source and relay. In our analysis, 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. A closed-form expression for the outage probability is obtained and validated by Monte Carlo simulations. Moreover, an asymptotic analysis is carried out and reveals that the achievable diversity order equals L+1.