Queuing Theory Based MAC-layer Methodology of Cognitive Radio

Nowadays, the increasing scarcity of spectrum resources has become thebottleneck of the development of wireless communications industry. However, plenty ofexperiments indicate that static allocated frequency is underutilized in both time andspace domain, which causes low utilization of spectrum resources. Under such situation,cognitive radio is emerging as a promising technology to alleviate the stress onspectrum resources. It senses radio environment, reconfigures operation parameters ofcommunication system in real time and accesses unused spectrum opportunistically.Cognitive radio makes it possible to reallocate the crowded spectrum resources.Cognitive user uses the unoccupied spectrum by chance, and vacates channel whenthe primary user appears. Cognitive user shares spectrum resources with the primaryuser, which brings new challenges to the traditional communication queuing schemes.The challenges include:(1) queuing service problem with different priority users;(2)allocation problem of time-varying available service resources.Queuing theory is a mathematical theory and method of study stochastic servicesystem, which provides a powerful tool for analyzing performance of cognitive radiosystems (such as waiting time, queue length and degree of congestion, etc) and fordesign mechanisms to improve the quality of service. In this background, based onqueuing theory, this dissertation focuses on analyzing how primary user behavior withspecific statistic properties impacts quality of service in cognitive radio system.Furthermore, optimization schemes are proposed to improve cognitive radio systemperformance. The contents are as follows:First of all, spectrum sensing optimization problems in MAC layer of cognitiveradio are proposed in the dissertation. Including:(1) This chapter model primary user behavior pattern by renewal theory; alsopropose the optimization problem of cognitive spectrum sensing period aiming formaximum spectrum opportunity utilization. Furthermore we bring in the interference probability to primary users as the constraint condition of the optimization problem.Under the ideal sensing and imperfect sensing situation, exponetial distribution areadopt as an example of primary user behavior. Moreover, we analyze the relationshipbetween optimal sensing period and statistics of primary user behavior.(2) A proactive sensing algorithm is proposed to focus on the short-time statisticsof channel utilization. Moreover a utility function is set to dynamically determine whichsensing mode to use.Secondly, this dissertation studies the cognitive access control problems in thedelay insensitive system:Chapter four investigates continued-time two-dimensional birth-and-death processwith priority in cognitive radio system. To simplify analysis, one-dimensional queuingmodel without priority is derived to equalize the complicated process. And astep-queuing scheme based on channel reward in multichannel situation is proposed,whose block probability and force termination probability performance determined bythe queuing-structure coefficient (QSC). Analytical results and simulation confirm theequivalent queuing model without priority valid. Also some diverse access scheme arecompared in performance, step-queuing scheme presents a lower block probability andan adaptive force termination probability.Lastly, this dissertation studies spectrum handoff in delay sensitive cognitive radiosystem:(1) This chapter proposes a novel virtual queuing (VQ) scheme to schedulespectrum handoff requirements sent by multiple secondary users. Unlike theconventional fist-come-first-serve (FCFS) scheduling, resuming transmission in theoriginal channel has higher priority than switching to another channel. It costs lessbecause of avoiding expenses for exchanging signaling of frequent spectrum switches.(2) This chapter investigates how to achieve call admission control (CAC) forguaranteeing call dropping probability QoS due to handoff timeout in cognitive radio(CR) networks. When primary user (PU) appears, spectrum handoff should be initiatedto keep secondary user’s link maintenance. We characterize the handoff delay on theeffect of primary user’s behavior and the number of secondary users in CR networks. And user capacity under certain QoS requirement is derived to be the guide line of CAC.The analytical results show that call dropping performance can be greatly improved byCAC when a large amount of secondary users arrives fast as well as the VQ scheme isverified to reduce handoff cost compared to existing methods.