Spectrum-sharing capacity enhancement with distributed relaying

We consider a spectrum-sharing network where n secondary relays are used to increase the secondary rate. We propose a distributed amplify-and-forward (AF) relaying scheme combined with relay selection and clustering. For the proposed method we obtain a closed-form expression for the secondary rate, which shows that the rate grows proportionally to log n. Remarkably, this growth rate is on the same order as that achieved in the absence of a primary system and its imposed constraints. We find the optimal power strategy for the relays subject to primary constraints: to maximize the secondary rate, each of the selected relays need to transmit with power proportional to n^-1 (thus the sum of relay powers is bounded).