The Study Of Optimization Of Mac-layer Parameters Based On Spectrum Sensing Algorithm

In order to improve spectrum usage, cognitive radio (CR) has recently been introduced to share the licensed spectrum by dynamic spectrum access. Dynamic access requires CR user to sense the spectrum periodically to find unused licensed bands. The performance of the spectrum sensing is of significant importance to avoid interference to the primary users. On the base of the protection to the primary users, our research focuses on the maximum of spectrum usage.This thesis describes some constraints of spectrum sensing in cognitive radio. Based on energy detection, this thesis studies the optimization of sensing time. Our objective function is to minimize average channel-search time by optimizing the sensing time during channel-search. When average channel-search time is minimized, periodical sensing time is optimized to maximize spectrum usage. Simulation results show that there exists optimal channel-search sensing time to minimize average channel-search time and optimal periodical sensing time to maximize spectrum usage.In single-channel environment, based on the traffic pattern of CR users in each period, spectrum utilization and collision rate to the primary users are derived in single-user and multi-user environments. Under the constraint of the collision rate to the primary users, the optimal data transmission time can be found to maximize the spectrum utilization. The theoretical optimal data transmission time is derived by using Taylor approximation in single-user environment. By considering the collisions among CR users in multi-user environment, MAC access is introduced to control the users in a single channel. The simulation shows that the optimal data transmission time can be found to maximize the spectrum utilization and MAC access can increase the utilization meanwhile decrease the collision rate to primary users, tradeoff between spectrum utilization and the capacity of CR users can be considered.In multi-channel environment, CR users contend for an exclusive use of all channels and sense each channel periodically so as to discover more idle channels. This thesis optimizes sensing periodicity vector to maximize channel usage of CR user and derives the formulation of idle channel usage. When the idle channel pool is empty and CR user has to switch channel, three different channel-search sequences are introduced and overhead latency ratio(OLR) is defined to analyze the tradeoff between channel-search overhead and channel-switch latency. Simulation results show that optimal sensing periodicity vector can be found to maximize channel usage, channel-switch latency decreases with the increasing number of channels when waiting time is fixed, OLR can be used to select proper waiting time by analyzing the requirements of real time and sensing overhead.