Researches On Cooperative Spectrum Sensing Technology In Cognitive Radio

With the rapid development of wireless communications, the competition for the unlicensed spectrum has kept growing intensively. While, much research indicates that most licensed spectrum often keeps idle and has low utilization efficiency. Obviously, the spectrum crisis and the low spectrum utilization efficiency constrain the development of wireless communications. As a potential solution to spectrum sharing, cognitive radio (CR) technology has attracted more and more attention with the opportunistic spectrum access. Spectrum sensing is an essential technology to make sure that the operation of the CR system has no harmful interference to the normal primary user. In order to improve the reliability of spectrum sensing, cooperative spectrum sensing has been proposed. However, it is shown that, when the reporting channels between cognitive users and BS become very noisy, the cooperative sensing obtains no advantages. In practice, the reporting channels are always subject to the fading effects. In this thesis, our studies are based on non-ideal reporting channels.In this thesis, we make the improvement to some existing schemes and propose some innovation ideas on cooperative spectrum sensing in cognitive radio, not only list the theoretical analysis in detail, but also display the performance simulation and corresponding result analysis. The main contributions are described as follows.Firstly, Cluster-based cooperative spectrum sensing for cognitive radio under bandwidth constraints is studied.Considering that the control channels are bandwidth limited, we can just allow the secondary users who have enough information in each cluster to send the sensing results to cluster head with the scheme of energy detection based on double thresholds in order to decrease the average number of sensing bits in the process of spectrum sensing. And then, the sensing performance and the number of the sensing bits of this scheme are investigated in detail. Simulation results show that, the average number of sensing bits decreases greatly at the cost of a little loss in the spectrum sensing performance.Secondly, a robust multi-user cooperative spectrum sensing based on the space-time block coding (STBC) scheme is taken into consideration. Considering that the performance of the STBC scheme will be influenced when the inter-channel between the secondary users within the virtual cluster is very noisy, a robust CR users’clustering scheme is proposed to overcome the limitation of the fixed clustering, where we make good use of the inter-channel information between the users. In this thesis, we provide the flow charts of the new scheme. Simulation results and analysis show that the proposed scheme has more robustness and the sensing performance can be improved significantly.Thirdly, cooperative spectrum sensing with different sensing durations in cognitive radio system is discussed.In order to make full use of the idle time when the users are waiting for their reporting orders, cooperative spectrum sensing with different sensing durations is proposed to enhance the system performance with the method to improve the time efficiency. While, it is notable that, because of the various detection channel conditions, the sensitivity between the sensing performance and the sensing time is different. Considering this issue, we set the clusters with high sensing sensitivity adopt different sensing durations to reduce the unnecessary energy consumption. Simulation results show that the proposed scheme can improve the sensing performance significantly.