| Wireless technology is favored since it could access communication networks flexibly and easily. According to the report of global mobile company Erikson, the number of mobile phone users has reached7.5billion in2013, which has exceeded the current world population of7billion. Wireless communication has become the mainstream of service technology and its scale is expanding continuously, and thus people have to develop cloud technology in the public network to handle the large amount of data transmission. With the shift of network users to mobile Internet and the development of the technology of Net of Things, the higher demands about capacity and reliability of wireless transmission were put forward. Therefore, it is the primary issue of how to improve the spectrum efficiency and use limited spectrum resource to carry these businesses as the further development of wireless communication technology.Fixed spectrum allocation policy is the chief cause of low spectrum efficiency. In2003, Federal Communications Commission (FCC), International Telecommunication Union (ITU) and other spectrum institutions found that the spectral efficiency for all regions was very low. In order to improve spectral efficiency and the adaptability in spectrum environment of cognitive system, wireless networks need to have the capability of sensing. In1999, the United States proposed the Cognitive Radio standard of using spectrum through spectrum sensing, which allowed the unauthorized user to access to idle spectrum of authorized user opportunistically using dynamic management techniques, and to use the spectrum effectively. It is reported that the problem of spectrum shortage could be completely solved, and Cognitive Radio is considered to be one of the core technologies in future wireless network.To use spectrum dynamically, the problem of spectrum sensing and spectrum access should be solved. The former requires the system implementing rapid and accurate detection to idle spectrum; the latter requires users accessing spectrum holes adaptively, and switching to another channel quickly when the primary user was found. Some adverse environmental factors, such as hidden terminal and wireless fading, will make users can not find potential communicate conflicts in the mutual coverage, and make wireless sensing performance reduce greatly; Meanwhile, the quality of service of spectrum sharing system will decline when the network load becomes large. This dissertation deeply explored the theory and technical issues involved in the above two aspects. The main work is listed as follows:First of all, the research background of Cognitive Radio was introduced, which indicated that the reality of the current spectrum crisis and low utilization rate of authorized spectrum. Then the concept, features, standards, projects and prospects of Cognitive Radio technology were described.Second of all, the multi-antenna diversity combining technology and spectrum sensing technology were studied, and a multi-antenna intelligent spectrum sensing method was proposed, which combined the Matched Filter Detection, Energy Detection and Cyclostationary Detection algorithms. The relevant Binary Hypothesis Testing model was established through assuming the presence of Gaussian noise and flat Nakagami fading channel, and the closed expressions of instantaneous detection probabilities were deduced; According to the random statistical properties of amplitude gain under the three multi-antenna combine modes of SC, EGC, MRC, the probability distribution characteristic of instantaneous signal to noise ratio was analyzed separately. By averaging the instantaneous detect probability on the probability density function of instantaneous signal to noise ratio, the exact or approximate average detection probability expression was derived, and the expression of average detection time was also obtained according to the event Probability of every detection algorithm. Subsequently, the ROC characteristics of detection probability and detection time were analyzed by MATLAB numerical calculation tool.Third of all, the protocol and model of Dynamic Spectrum Access were studied. A centred and mixed spectrum access scheme based on losing service polling scheduling was proposed, in which Second Users judge the active state of primary user according to the results of sensing and interfere temperature and then access channel by a probability. By constructing the MAC layer queuing model under Nakagami channel, the closed expression of average service rate of Second User was analyzed. In order to ensure the quality of service for Primary Users, we specified the Primary User who has the maximum tolerable delay, in which the theoretical maximum Average Service Rate was analyzed. Because the business type of Second User was diverse, the case of multi-queue structure in second user system was further investigated, in which two scheduling schemes was considered:the first one was probabilistic scheduling scheme, and the access probability of each queue was obtained according to Lagrange Multiplier Method; the second one was the losing service polling scheduling mechanism, in which we supposed that the customer will leave in the three traditional polling models of Gated service, Limit1service and Exhaustive-Gated two levels priority service. The mathematical model and function relationship was established using the analytical method of embedded Markov chain and Probability Generating Function, and the expression of system parameters, such as Average Queue Length, Average Cycle Time, Average Waiting Time, were precisely parse out. Subsequently, the ROC characteristics above were analyzed by MATLAB numerical calculation tool.Finally, based on OPNET software, Cognitive Radio sensing and access simulation platform was designed according to multi-antenna spectrum sensing and polling spectrum access scheme above, which included three aspects:in physical layer, the process of signal fading, additive Gaussian channel, generate primary user signal types were build with Simulink; in MAC layer, node module, data sources generating module and queue module were build with OPNET. Physical layer and MAC layer exchange information through OPNET and MATLAB co-simulation engine interface. Then we used the experimental platform to test and research the relevant parameters by setting a simulation scenario, including spectrum utilization, Average Queue Length and Average Waiting Time. |
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