Cooperative secondary user selection as a relay for the primary system in underlay cognitive radio networks

We consider an underlay cognitive radio network consisting of a single pair of primary transmitter-receiver PTx-PRx and a group of M secondary transmitter-receiver pairs STx-SRx within the transmission range of the primary system. Secondary transmitters are partitioned into two groups K, KεM active secondary transmitters ST_i, i ε {1, 2,..., K} which may transmit data with the co-existence of PTx below a certain interference threshold to the primary receiver and (M-K) inactive secondary transmitters ST_j, jε {1, 2,..., (M-K)} which are in idle state, ready to assist the primary system when the data rate between PTx and PRx over a direct link falls below the target rate R_target. In this paper, we propose a cooperative relay selection scheme where one of the inactive ST_j which achieves the primary target R_target will be selected as a best relay to forward the primary information. Simulation results show the effectiveness of our proposed system.