Research On Physical Layer Technologies In Cognitive Radio

To solve the contradiction between the spectrum resource shortage and the low spectrum usage, Dynamic Spectrum Allocation (DSA) shall be adapted in the spectrum management, which will allow multiple wireless communication system to share the spectrum without interfering each other. Cognitive Radio (CR) is a kind of important resolution of DSA, which senses the idle spectrum of the wireless environment and then utilizes it. In the last few years, urged by the market requirement, the research of CR is warmed and expanded. Under the background, this dissertation takes the physical layer technologies of CR as the research field.Spectrum sensing plays the key role for CR to implement the DSA, whose technologies can be categorized to basic spectrum sensing and enhanced spectrum sensing. In the aspect of basic spectrum sensing, the cyclostationarity spectrum sensing, which will identifies the coexistent signals from different systems. Focusing on the cyclostationarity spectrum sensing using spectrum correlation function, an approximate distribution is proposed to enable the quantitative analysis of cooperative spectrum sensing and multi-antenna spectrum sensing in this research which can be classified to the enhanced spectrum sensing technologies. Cooperative spectrum sensing combines the detection results from multiple radios to resist the wireless channel fading encountered by a single radio and obtain a more correct sensing. The performance should balance the reliability and efficiency of sensing. Using the average Signal-to-Noise power Ratio (SNR) of signal from licensed system received by rental system and the detection probability demanded by licensed system to model the requirement of licensed system upon rental system, the quantitative relation of detection performance and the detection parameters in cooperative spectrum sensing, such as the number of radios involved in the cooperation and the sensing time of each radio, are investigated. It studies the optimization problem of cooperative detection efficiency under the restrictions of probabilities of detection and false alarm for the cooperation, and then provides an optimization algorithm which figures out the proper values of detection parameters to optimize the system performance in application. According to the analysis of the cooperative spectrum sensing based on cyclostationarity, the cooperative detection efficiency is upgraded with the number of radios is started to increase, however the efficiency is reduced after the radio number reaches certain point.Multi-antenna spectrum sensing utilizes the spatial diversity provided by the multiple antennas to resist the channel fading. To achieve the full spatial diversity, the cognitive radio should combine the multi-antenna signal previously to the detection. An approach named multi-antenna combining spectrum sensing is presented, which obtains some information of the channel frequency response of multi-antenna and perform the pre-detection combination by exploiting signals' cyclostationarity. The closed-form expressions are derived for the detection probability and average SNR of the approach over frequency selective channel, which indicates that the proposed approach is capable of achieving full spatial diversity in spectrum sensing. The simulation results also show that the combination using equal-gain combining suffers little from channel estimation error than the one using maximal-ratio combining, and thus the former has high practicability.Since the frequency bands used by cognitive radio will be discontiguous and changed frequently with time, the channel estimation shall adapt to these special characters and maintain its performance. The channel estimation technology of OFDM-based cognitive radio is investigated, and a method of exploiting the correlation of discontiguous bands in frequency domain is proposed to improve the performance of channel estimation in cognitive radio. The close-form expression of mean square error is deduced and verified in simulation. By simulation experiments, the proposed method is compared with the method of estimating the channel of each band separately. It shows that the stronger is the correlation between discontiguous bands, the higher is the performance gain of the proposed method. The influence of pilot pattern, bandwidth of licensed system and channel coherence bandwidth are also investigated while studying the performance of proposed method.