Research On Spectrum Sensing Among Cognitive Radios

Cognitive radio is an effective technique that is used to solve the problem of scarceness of spectrum resources and improve the availability ration of spectrum. As a key technique in the development of next generation network, cognitive radio has been drawing more and more attention since it was first proposed in 1999. Prompt discovery of"spectrum hole"is the prerequisite of the feasibility of cognitive radio technique. Spectrum sensing, as a key technique in the cognitive radio system, has recently become the focus of research and its development plays an important role in promoting cognitive radio realization. Therefore, this thesis has made an intensive study centered on improving the performance of spectrum sensing.Energy detection has become the most commonly used method because of its simplicity and ease of implementation. This thesis has made a systematic study on energy detection of a single secondary user(SU). A comparison towards energy detection performance is made between in AWGN channel and fading channel; besides, the influence of noise on energy detection is also analyzed, including the influence of SNR on number of samples and SNR wall in AWGN channel and Rayleigh channel when the noise is uncertain, and then expressions of SNR wall in two different channels are respectively given. The study shows that energy detection is extremely sensitive to the SNR wall, especially in the conditions of fading channels and low SNR; therefore, a conclusion can be drawn that energy detection of a single SU is inapplicable to low SNR because of its limitedness in noise immunity.Cooperative spectrum sensing can effectively overcome the defects in single user spectrum sensing. Based on hard decision fusion scheme, this thesis aims to make an in-depth study on sensing performance under the AND rule, OR rule and counting rule and analyze the advantages and disadvantages of the different merging rules. Two situations are considered in this paper, when local SNR is the same, the number of cooperating nodes will be optimized, thus the proper merging schemes shall acquire better property; while when the local SNR is different, the secondary users with high SNR will be selected to participate in the cooperation to gain better performance.The problem that error code exists in control channel between seconrary users and fusion center in cooperative spectrum sensing is proposed in this thesis. The theoretical derivation and simulation experiment all indicate that error code has a strong impact on sensing performance and extremely low bit-error rate shall enlarge the impact as the number of the collaboration users increases. In this paper, a simple and practical transmitting-receiving protocol is proposed. In theory, the application of this protocol can completely overcome the influence of error code; to a great extent, this protocol can decrease the channel expenses even if there is no error code. In a good local wireless environment, the channel expense can be diminished by 80%, which is of great importance for the improvement of sensing performance.