Abstract
Cognitive radio (CR) is an emerging technology for enhancing the efficiency of wireless spectrum utilization. In cognitive radio systems, secondary users (SUs) are allowed to access the frequency bands of a primary user (PU) by spectrum sensing. However, a single SU usually cannot provide robust sensing performance due to the effects of hidden nodes, shadowing, and fading channels. In order to overcome this problem, cooperative spectrum sensing techniques have been proposed in the literature. In conventional cooperative spectrum sensing schemes, the required bandwidth of feedback channels become larger with the SUs increased. Moreover, all the SUs are required to have an identical probability of false alarm, that is, the CR system supposes that all the sensing nodes have the same sensing reliability even though some of them are suffering from a deep fade. In this case, the detection performance will be degraded. This thesis investigates how to reduce the average number of sensing bits as well as to improve the detection performance. A two-step detection method for cooperative spectrum sensing is first proposed for improving the bandwidth efficiency over feedback channels. The basic idea is that if one or more reliable SUs have been sifted out during the first step by a strict decision threshold, then the CR system declares that the PU is active at once and no second-step operations are required; otherwise, the detection process will get into the second step, where the SUs transmit their soft information to the cognitive radio base station for further detection. In this way, unnecessary feedback bits transmitted from those unreliable SUs can be avoided. In order to improve the detection performance, a credibility method for cooperative spectrum sensing is also proposed. This method evaluates the sensing reliability of SUs based on previous sensing results, and then adjusts the probability of false alarm of each SU. Assigning a higher probability of false alarm for a more reliable SU, we can have better detection performance for cooperative spectrum sensing. With the credibility method and the two-step detection method, we can improve the spectrum utilization and the bandwidth efficiency of CR systems simultaneously. To verify the effectiveness of the proposed cooperative spectrum sensing scheme, the probability of false alarm, the probability of detection, and the average number of sensing bits are analyzed under Rayleigh fading channels, where the results are close to those obtained from simulations. All these support that the proposed approach outperforms the double threshold detection method in terms of the detection performance and the average number of sensing bits.