Cooperative spectrum sharing via controlled amplify-and-forward relaying

We consider a spectrum sharing protocol in which a secondary system operates on the same spectrum as a primary system, without adversely affecting the rate of the primary system. The protocol comprises of a two-phase transmission. In the first phase, the primary transmitter transmits a primary signal to the primary receiver, which is also received at the secondary transmitter and receiver. The secondary transmitter amplifies the received primary signal and generates a linearly weighted combination of this signal and the secondary signal. The weight is a variable power allocation factor α (0 ⩽ α ⩽ 1). This composite signal is then broadcast in the second transmission phase. We analyze the achievable rates for the primary and secondary systems, and determine α such that the rate of primary system with this spectrum sharing protocol is no worse than that in the absence of the secondary system. We show that the spectrum sharing protocol can improve the rate of primary system by an appropriate choice of α, while at the same time achieve secondary spectrum access.