Cooperative Spectrum Sensing In Cognitive Radios

With the rapid development of wireless communications, new technologies, new equipments and new services in a variety of environments (home, office, public areas, etc.) grow explosively, leading to the spectrum overcrowding. Meanwhile, the spectrum scarcity has become increasingly prominent while most of the licensed spectrum stays idle. Cognitive radio has been proposed as an attractive means to improve the spectrum efficiency by utilizing the licensed band opportunistically. Cognitive radio becomes one of the key technologies for future wireless communications.Spectrum sensing is the foundational technique of cognitive radio. Cooperative spectrum sensing by multiple secondary users is a hot spot in the field of spectrum sensing, since it can improve the detection performance, handle hidden primary user problem and reduce the detection time. This thesis studies the weighted combination algorithms for cooperative spectrum sensing, cooperative spectrum sensing based on energy probability distribution (EPD) and cooperative spectrum sensing for Digital Multimedia Broadcasting-Terrestrial (DMB-T) system. The innovations are listed as below.Firstly, the algorithm of weighted one-bit hard combination for cooperative spectrum sensing is proposed, since the optimal cooperative spectrum sensing scheme indicates that the larger observation energy value with a larger weight could improve the detection performance. Two thresholds are adopted to divide the possible energy value into three weighted regions, and no information is transmitted to the data fusion center when the energy value falls into the middle region. When the probability of false alarm is constrained to be constant, the objective is to maximize the probability of detection. The optimization problem is simplified by separating the weight of the middle region into several intervals. When4cooperativer users experience independent and identically distributed (i.i.d.) Rayleigh fading with same average SNR using100samples, the probability of false alarm is no larger than0.01and the probability of missed detection is no larger than0.1, the proposed optimal one bit hard combination exhibit almost the same detection performance with the equal gain combining (EGC) algorithm. And compared with the "OR" rule algorithm, it can acheive about0.5dB performance gain and39%fewer average reporting bits are required. Secondly, multiple-threshold scheme for cooperative spectrum sensing based on the number of cooperative secondary users is investigated. The thresholds separate the energy value into several regions and each region is assigned a fixed weight for data fusion. An algorithm is proposed to find the optimal thresholds that maximize the probability of detection for a given probability of false alarm, which can also be applied to other designed weights in order to find the optimal thresholds.When more than3cooperativer users experience i.i.d. Rayleigh fading with same average SNR using100samples, the proposed multiple-threshold scheme has even better detection performance than the EGC scheme.Thirdly, cooperative spectrum sensing based on energy probability distribution (EPD) is proposed. In such a scheme, each cooperative secondary user records the number of the received samples whose energies are above a given threshold. The correlation between the sum of the recorded numbers and the EPD of the noise is used at the data fusion center to reach a final decision on whether there is a primary user. The detection performance of the cooperative EPD spectrum sensing is analyzed theoretically. When there are only a few samples to perform spectrum sensing, it is easy to quantify the EPD results because the local detection statistics are integers. The EPD algorithm is also analyzed to distinguish the primay signals with different EPD.Finally, cooperative spectrum sensing for Digital Multimedia Broadcasting-Terrestrial (DMB-T) system in China is investigated. There are three frame modes for DMB-T system, and only one mode is used based on the channel situation in practice. The probabilities of false alarm and missed detection of energy detection for DMB-T systems are analyzed theoretically. For frame header mode2, the matched filter method is utilized to perform local spectrum sensing. For frame header mode1and3, extended pseudonoise sequences (PN) are proposed to be the local correlation sequences to perform correlation based spectrum sensing. Simulation results show that the performance of the proposed schemes is much better than that of the present PN correlation (PNC), cyclic extension correlation (CEC) and energy detection schemes. Based on the proposed local correlation spectrum sensing, weighted one bit hard combination, multiple-threshold combination based on the number of cooperative secondary users, EGC and "OR" cooperative spectrum sensing schemes are studied for DMB-T primary signals. When the sensing channels are i.i.d. Rayleigh fading with same average SNR using1frame signal, the probability of false alarm is no larger than0.01and the probability of missed detection is no larger than0.1, the optimal one bit combination scheme improves the performance gain by0.9dB and1.9dB with3and7cooperative secondary users respectively, and the weighted combination scheme based on the number of cooperative secondary users improves the performance gain by1.1dB and3.0dB, respectively.