Interference-free proactive channel gain estimation in cognitive radio

Recently, a proactive estimation method has been proposed to obtain the cross-channel gain from cognitive transmitter (CT) to primary receiver (PR), where CT acts as a full-duplex amplified-and-forward (AF) relay for primary users. However, since the cognitive and primary users usually have no cooperation, it has no guarantee that the direct and relay signals are synchronized, i.e, the time delay between direct and relay signals may be randomly large or small. This may cause interference to PR. In this paper, we analyze the impacts of the small and large time delays on proactive estimation, and propose an interference-free method to estimate the cross-channel gain. Simulation results demonstrate that the proposed method obtains about 5% estimation error with about 99% interference-free probability.