Cooperative spectrum sharing with bi-directional secondary transmissions

In this paper, we propose a new cooperative spectrum sharing scheme that allows bi-directional transmission of the secondary users (SUs) while serving as a relay to the primary link. Specifically, we consider the network consisting of a primary transmitter PT, primary receiver PR and two SUs S1 and S2. Due to the unfavorable propagation condition of the primary link from PT to PR, the primary would like to seek help from the SUs to relay its signal. As a return, secondary terminals can gain opportunity for transmission. We propose a two-slot transmission strategy. In the first time slot, both PT and S2 transmit. In the second time slot, S1 will relay the primary signal as well as transmit its own signal to S2. Both the amplify-and-forward and decode-and-forward relay operations at S1 are considered. We derive the achievable rate regions for the sum-rate of the secondary links versus the primary link. Numerical results show that the proposed scheme can better explore the spectrum opportunity for the SUs as compared to the existing schemes. Furthermore, the improvement in spectrum utilization is not only from allowing the SU to transmit in the first time slot but also from the cooperation of the primary transmitter to control its power.