To improve spectral utilization, the Federal Communication Commission (FCC) has already allowed CR or secondary users to opportunistically reuse vacant frequency bands, especially vacant TV broadcast bands, provided that they do not cause any harmful interference. To this end, cognitive radio (CR) has been viewed as a potential solution and has attracted considerable interests, attentions, and enthusiasms in recent years. Because CR is considered low-priority user in accessing a license channel band, a fundamental requirement is to avoid or minimize any interference to a potential primary user (PU) in its vicinity. A key challenge in this requirement is the reliable detection of the PU signal. Therefore, prior to accessing a licensed spectrum band, the secondary user should employ sensing techniques to make a reliable decision on whether the PU signal is present.
First, aiming to provide an in-depth and comprehensive analysis of the spectrum sensing technologies, this dissertation presents a detailed literature review of the recent research developments and open research issues for spectrum sensing technologies. More specifically, the interests and discussions of the dissertation are primarily focused on cooperative spectrum sensing technologies in the CR network, especially when the received signal is subject to severe channel fading effects and the detector is in the absence of a priori knowledge of the channel and the primary signal.
Next, we propose a selection probability based cooperative spectrum sensing scheme that exploits historical observations to exclude nodes receiving low-strength primary signals. In cooperative spectrum sensing, secondary nodes can cooperate based on their local sensing observations so as to detect primary signals in a more reliable manner. However, because of the low strength of received primary signals, observations from some secondary users may contribute little to global decisions at the fusion center. To reduce energy consumption and sensing overhead, cooperation is recommended only with a subset of nodes receiving high-strength primary signals. Simulation results demonstrate that the proposed scheme can exploit location advantages and shows almost the same detection performance as cooperative spectrum sensing with accurate node selection.
Then, a conservative cooperative spectrum sensing scheme is presented. The proposed scheme can utilize the licensed channel for transmitting local sensing decision with limited interference to primary users. It is shown that the new scheme can save the common reporting channel without sacrificing ROC performance and mitigate total interference probability compared with conventional cooperative sensing scheme.
Afterwards, a bandwidth-efficient transmission scheme based on spectrum sensing results without a common reporting channel is proposed for CR networks. A secondary relay node is ready to assist a secondary source node for both spectrum sensing and data transmission. The secondary source node determines when it needs to cooperate with the relay node by checking the reporting sensing results from destination and relay nodes, which improves the bandwidth efficiency of secondary data transmission. Furthermore sensing time is optimized to maximize the expected bandwidth efficiency under the constraints of spectrum sensing requirements. Finally, the content of the whole dissertation is summarized, and several future research directions of CR technology are indicated.