Outage probability of a multi-relay cognitive network with an uncertain number of forwarding relays

In this paper, we focus on a Cognitive Relay Network (CRN) where only the information of the average channel gains is available at the secondary source. A distributed multiple relay selection scheme is proposed for the secondary transmission. Since the secondary source does not fully control the selection but lets each secondary relay decide by itself, it doesn´t know which and how many relays will participate in the forwarding. This brings a serious uncertainty in the number of forwarding relays. We analyze this uncertainty and derive the closed-form expression of the outage probability of the secondary transmission under the uncertainty. Finally, we evaluate the proposed relay selection scheme by means of simulation. The simulation shows the appropriateness of our analytical model. In addition, although having lower interference constraint at each relay, the performances of the proposed multi-relay CRN scheme are shown to be strictly superior to the single relay CRN. Moreover, the simulation results suggest that it is necessary to spend the general overhead for secondary networks on knowing the exact QoS requirement of the primary user (i.e. the interference violation probability) to apply our proposed relay selection scheme with the statistical total interference constraint.