| With the development of wireless communication technology,radio spectrum becomes more and more crowded.The current static spectrum management policies have caused a severe dilemma of spectrum scarcity versus under-utilization.Accordingly,Cognitive Radio(CR)technology,which allows dynamic spectrum access flexibly,has been widely deemed as a promising technology in future wireless communications networks.Orthogonal frequency division multiplexing(OFDM),which offers a high flexibility in radio resource management,is deemed as an appropriate air interface of a CR system.The combination of cognitive radio and OFDM technology can give full play to the advantages of both sides.Therefore,how to dynamically allocate resources severs as a hot issue to achieve the high performance of CR systems.In view of the system models with different optimization objective,this thesis studies resource allocation algorithm in OFDM-based cognitive radio.The main contents and contributions of this dissertation are summarized as follows:1.A spectrum-efficient resource allocation model for the CR-OFDM system with perfect spectrum sensing is formulated,which considers the interference between the PUs and SUs and aims to maximize the capacity of CR system.In order to avoid the high computational complexity,we develop a distributed algorithm to obtain the suboptimal solution,including subcarrier allocation algorithm and suboptimal power allocation algorithm.Simulation results show that our algorithm can obtain the system capacity with low complexity larger than conventional power allocation algorithm and close to the optimal power allocation algorithm,which can serve as a suboptimal resource allocation algorithm in CR-OFDM systems.2.An energy-efficient resource allocation model for the CR-OFDM system with imperfect spectrum sensing is formulated.We develop a distributed algorithm including subcarrier allocation and optimal power allocation algorithm.The subcarrier allocation algorithm is based on the maximization of the energy efficiency.The fractional programming method is adopted to allocate the power and we proposed a power assignment scheme with sub-gradient algorithm.The simulation results that the proposed algorithm can achieve higher energy efficiency than the one maximizing the capacity of the CR system.Meanwhile,the resource allocation scheme can ensure the normal communication of each PU compared to the one without considering sensing errors.3.A resource allocation model based relay-aided for CR-OFDM system with imperfect spectrum sensing is formulated.AF protocol is adopted.We aim to maximize the energy efficiency of the CR system and formulate the objective function.Then the optimal and suboptimal power allocation algorithm are proposed.Simulation results indicate that under the condition of imperfect spectrum sensing the system performance declined inevitably but the optimal power allocation algorithm can also achieve the ideal energy efficiency.Compared to the one maximizing the capacity,the proposed algorithm can improve the energy efficiency of the CR systems.Meanwhile,the suboptimal power allocation algorithm can also obtain the good performance with low complexity. |
