Mode Selection Policies In RF-based Cognitive Radio Networks

With the rapid increase of wireless devices,spectrum scare problem has been more serious under the current policy.The cognitive radio technology,which can improve the rate of spectrum utilization by sharing the licensed spectrum,introduce the concept of primary user and secondary user.The primary users have the license of a specified channel,while the secondary users can perform communication through the spectrum holes.As an improving spectrum management policy,cognitive radio has got the public recognition in communication field.Besides of the spectrum resource,energy is another indispensable element for wireless nodes.Energy harvesting technology can convert the ambient nature sources or radio frequency signals to electrical energy and power communication devices.This technology can prolong the lifetime of devices and enable a potentially perpetual operation without any extra supplies.Meanwhile,it reduces the consumption of fossil fuels and helps control environmental pollution problem.The combination of cognitive radio and energy harvesting technology improve both the spectrum utilization and the energy efficiency.However,it also introduces a tradeoff between the time allocated for energy harvesting and that for communication.Aiming to maximize the secondary throughput,we research on the mode selection problem of secondary users in RF-powered cognitive radio network.Based on the single channel and multiple channels network separately,we propose several optimal mode selection policies by formulating this problem under a partially observable Markov decision process framework.In single channel condition,we introduce the cooperative communication pattern and propose two optimal mode selection policies based on its flexibility.In one-dimension decision policy,the secondary users automatically request for cooperative communication and adapt the forward power to the cooperative threshold.While in the two-dimension policy,the secondary users have an authority to select spectrum sensing and cooperative communication simultaneously.This work solves the energy overflow problem in overlay communication mode and the energy un-efficiency problem in underlay communication mode,which improve the communication quality of both the primary network and the secondary network.Simulation results show the priority of our policies and the effects of several parameters.To avoid the competition among the secondary users,we formulate a centralized entity(CE)to execute the mode selections in multiple channels network.The CE performs adaptive channel assignment and secondary modes at the start of each slot.It works with a priority mechanism based on the left energy ratio and imperfect spectrum sensing of secondary users.It transforms a multiple mode selection problem into several single channel mode selection problems.Finally,we compare a newly proposed imperfect sensing policy with the previous two policies from the view point of the secondary network and single member separately.