Study On Key Technologies Of Dynamic Spectrum Access For Cognitive Radio

Spectrum is a precious natural resource. Static spectrum allocation mechanism governed by government regulation leads to low overall spectrum utilization. To improve spectrum utilization efficiency, cognitive radio has been proposed to use spectrum holes which are not utilized by licensed users for the moment. This kind of spectrum utilization is referred to as dynamic spectrum access(DSA). DSA offers an efficient way in utilizing spectrum autonomously and dynamically which helps in mitigating the spectrum scarcity problem and improving spectrum utilization.A lot of problems need to be resolved before DSA comes into reality. This dissertation studies several key technologies of DSA, including spectrum sensing, rendezvous, spectrum handoff and link adaptation, and develops a testbed to test the capabilities of DSA. The main content of this dissertation is summarized as follows.The first part focuses on wideband spectrum sensing. In Nyquist wideband spectrum sensing, a method based on group testing(GT) is proposed. A two step GT based on polyphase filter banks is proposed to decrease the number of binary tests when spectrum occupation rate is not that high, thus decreasing the computational complexity of wideband spectrum sensing. Simulations results validate the effectiveness of this method. In sub-Nyquist wideband spectrum sensing, to conquer the problem of interfering primary users when trusting spectrum sensing result when spectrum is not sparse, reconstruction verification methods relying on a single sensing node are proposed. Specifically, a reconstruction verification method based on correlation between two consecutively reconstructed support sets is proposed for random demodulator based sub-Nyquist sampling. Furthermore, for modulated wideband converter based sub-Nyquist sampling, a reconstruction verification method based on correlation between two consecutively reconstructed subband energy is proposed. Simulations are done to validate these methods.The second part deals with rendezvous. A wideband receiving blind rendezvous without common control channel is proposed. Theoretical analysis of expected time-to-rendezvous(ETTR) and maximum time-to-rendezvous is conducted in static spectrum environment by considering both symmetric and asymmetric channel availability models. Compared with narrowband receiving blind rendezvous methods, the proposed method can decrease rendezvous time greatly. Furthermore, theoretical analysis of ETTR in dynamic spectrum environment is also conducted. Both symmetric and asymmetric channel availability models are considered. Simulation results show that by receiving on multiple channels simultaneously, the rendezvous can be fastened. Compared with traditional wideband receiving blind rendezvous method, the proposed method can reduce the number of signal analysis thus reducing computational complexity of rendezvous.The third part is about spectrum handoff. Two target channel visiting problem models are formulated for a single spectrum handoff: probability of handoff failure minimization problem model and expected handoff delay minimization problem model. For the former problem, a target channel visiting method based on greedy is proposed. Theoretical analysis proofs that this method reaches the minimum probability of handoff failure when the time duration of a channel remaining vacant follows Uniform, Exponential, Rayleigh or Weibull distribution. Simulations are done to evaluate the performance when estimation error exits in estimating channel parameters. For the latter problem, methods based on evolutionary algorithms are proposed. Specifically, particle swarm optimization(PSO) and biogeography-based optimization(BBO) are used as examples to illustrate procedures of these methods. Simulation results show that the methods based on PSO and BBO perform far better than random channel visiting method, and solutions obtained by PSO and BBO are very close to the optimal solutions.The fourth part deals with link adaptation. To meet the need of green communications, the energy consumption minimization link adaptation problem is formulated as an optimization problem to optimize energy consumption with quality of service requirements as constraints. Evolutionary algorithms are proposed to solve this problem. PSO and BBO are used as examples to illustrate the procedure of the algorithm. A novel fitness evaluation mechanism is proposed which takes both the main objective and the constraints into consideration. Simulations are conducted to evaluate the effectiveness of this method.The final part realizes a dynamic spectrum access testbed. Based on software-defined radio architecture, a time division dulplex communication DSA testbed is developed which realizes capabilities of wideband spectrum sensing, blind rendezvous, in-band sensing and spectrum handoff. Some of the key algorithms proposed in this paper are realized in this testbed. Algorithms, protocols and hardware design of the tested are discussed. Experimental results are given. Results illustrate that the testbed realizes main capabilities of DSA. Specifically, the testbed can establish initial link without common control channel and can handover to another vacant channel to resume communication when the current channel is re-occupied by a primary user.