Research On Key Techniques Of Spectrum Resources Management In Cognitive Radio Networks

The total wireless spectrum available has been currently allocated to designated agency butmuch of the allocated spectrum is largely underutilized, which leads to the spectrum resourcecrisis. Cognitive radio is a key enabling technology capable of collecting unused part of thespectrum opportunity and serves as a remedy for plausible spectrum shortage problem. Cognitiveradio incorporates intelligence into cognitive users/devices through consistent monitoring ofsurrounding wireless environment, then identifies spectrum holes in the domain of time, spaceand frequency etc., and allows cognitive user to access in an opportunistic way. All theprocedures aforementioned should takes interference to primary users into consideration,threshold of interference caused by cognitive user should be met and ideally no interference at allis desirable. As a promising solution for current spectrum crisis, cognitive radio has been widelyacknowledged and closely followed by academic institutions and industrial agencies.Internationally, cognitive radio is generally regarded as a “Next Big Thing” for the possibilityand feasibility of solving the spectrum-related problems nowadays.Since cognitive user opportunistically exploits spectrum bands when primary users areabsent, immediate vacancy from temporarily-accessed channels should be performed as soon asthe primary user returns. Spectrum opportunity fluctuation, however, is hard to capture due to therandom behavior of primary service traffic, which poses many brand-new challenges forspectrum management in cognitive radio networks. In supporting and finishing nationalhigh-tech exploration-oriented project-‘Optimization techniques of wireless resource allocationin cognitive radio networks based on behavior prediction’, this thesis investigates the spectrummanagement techniques of cognitive radio networks in terms of four successive problems,namely identifying spectrum opportunities, collecting and scheduling of identified opportunities,sharing hierarchically with primary users and vacant timely from on-transmitting channels, allpursuing at improving spectrum utilization. We identify the above four questions to fourfunctions within spectrum management framework, namely spectrum sensing, spectrum decision,spectrum sharing and spectrum mobility, all of which taking end-to-end goal of primary user intoconsideration. With maximizing in mind, we perform systematical research on the four functionsof spectrum management with constrained interference to primary users, main contributions andinnovations of this dissertation are summarized as follows:(1) An efficiency-reliability tradeoff algorithm for spectrum sensing is proposed to optimizesensing parameters such as sensing duration and sensing period. Based on Neyman-Pearsoncriterion, spectrum sensing model of maximizing spectrum utilization is formulated. Theproposed sensing model offers sufficient QoS guarantees to primary users by means of posingstrict restriction on missed detection probability. Instead of optimized a single parameter, wemodel the sensing problem in a two-dimensional manner and joint optimization of both sensing duration and sensing period is conducted. Energy-based detection method is employed to obtaindetailed mathematical analysis together with theoretical deduction and spectrum utilization isoptimized. Differing from methods adopted in most popular literature focusing on fixed falsealarm rate to maximize detection probability, we fix probability of missed detection to minimizeprobability of false alarm using Neyman-Pearson criterion, which causes no explicit harm toprimary users within the define of specified value. For a given configuration of cognitive radionetworks, i.e., target detection probability, received signal-to-noise ratio of cognitive user,primary user behavior, optimal sensing duration and sensing period can be obtained withspectrum utilization greatly improved and spectrum reliability safely guaranteed. Simulationresults reveal that the proposed tradeoff algorithm proves to be efficient with sufficientprotection to primary user.(2) A cooperative spectrum sensing method is proposed with noise power fluctuationwireless channel fading taken into consideration. Noise power uncertainty and channel fadingdegrades the performance of spectrum sensing, cooperative spectrum sensing, which gives betterreliability performance compared with local sensing, is adopted to mitigate the adverse effect ofchannel fading and noise power fluctuation. Since radio frequency front-end of secondary user isexposed to a number of interference such as thermal noise of transceiver, interference fromprimary users and peer secondary users, we model the overall effect of the aggregatedinterference as additive white Gaussian noise with average power fluctuated. Furthermore,fading characteristics of wireless channel is also incorporated into mathematical modeling andclosed-form expression of detection probability are conducted on Rayleigh channel, Ricianchannel and Nakagami-m channel with noise power exponentially distributed. To further offsetthe performance degeneration caused by noise power fluctuation and improve the reliability ofspectrum sensing, cooperative sensing based on D-S combination theory is employed. ImprovedGaussian distribution function is borrowed to serve as basic probability assignment function anddecision criterion is obtained to determine whether the licensed channel is busy or not.Simulation results show that although sensing reliability cannot be mitigated, cooperativesensing still gives better reliability performance compared with local spectrum sensing.(3) A delay-optimal spectrum decision method with traffic load balancing is proposed. Onfinding multiple available spectrum opportunity, secondary user needs to decide which channel ismost suitable to access according to specified optimization criterion. All the traffic load rushed tothe best channel, if no control measure is taken, and the best channel would soon turn out to bethe most crowded, underutilized channel (Hotspot channel effect). The proposed methodbalances aggregated traffic load to multiple spectrum opportunities according to availability ofeach channel. Sensing errors, spectrum heterogeneity and outage effect of channel areincorporated into the formulation and analysis of the problem. Firstly, a probability-basedspectrum decision is proposed to evenly distribute traffic load of secondary user and detaileddeduction based preemptive-resume priority queuing theory is performed. Probability allocationvector of spectrum opportunity is obtained to minimize residual time of secondary user. Secondly,to overcome the lack of real-time knowledge of channel state, a sensing based spectrum decision method is proposed. In order to guarantee seamless communication for secondary user, thesensing based method allows secondary user to freely select channel after collision with primaryuser occurs. Finally, based on the aforementioned two spectrum decision method, adelay-optimal spectrum decision is presented with traffic load reasonably balanced. Base stationof secondary user selects suitable decision method to realize optimization of residual time.Simulation results reveals, compared with random decision method, our proposed method givesbetter delay performance as well as load balancing effect.(4) An access control method in the spectrum-overlay sharing environment is proposed.Based on sensing results and decision policy, the most available channel is selected according tospecified optimization criteria. In practice, due to sensing errors and other underlying layerrelated problems, it is inevitable to collide with primary user, causing QoS degeneration for bothprimary user and secondary user. As for the spectrum-overlay sharing mode, once collided, thewhole packet/frame needs to be re-transmitted. So, in order to restrict collision rate, it is of greatsignificance to choose the proper sending instant of secondary user. Borrowed from IEEE802.11Distributed Coordination Function with p-persistent transmission policy, a control access methodis proposed to sufficiently protect the communication process of primary user. Subjected toextended transmission time (ratio) restriction of primary user caused by collision with secondaryuser and average residual time restriction of secondary user, access probability for secondaryuser is obtained and greedy access is prohibited to further protect the transmission of primaryuser. The construction of the optimization problem is of general purpose and no otherassumptions are demanded. Furthermore, solution-solving process of formulated problem isquite generic, arbitrary extent of protection for primary user can be set and access probability canbe thus obtained. Simulation results show that spectrum utilization is improved while QoS ofboth primary user and secondary user is guaranteed.(5) A transmission time optimization method is proposed in the spectrum-underlay sharingenvironment. Unlike spectrum-overlay sharing method mentioned above, the proposed methodfurther relaxes restrictions and conducts research in a spectrum-underlay fashion, which allowssecondary user continue sending with ultra-low transmission power, but interference caused bysecondary transmission to primary user should be strictly restricted to ensure that no explicitharmful disturbance comes into being. Differing from classical interference-controlling measurethat pushes hard restriction on transmission power of secondary user, our proposed methoddefines interference in terms of overlapped transmission time of primary user and secondary user.With guaranteed maximal data rate (channel capacity) of primary user, we aim to maximize datarate of secondary user. We convert quantified interference to signal-to-noise ratio of Shannoninformation formula within the stability condition of secondary user service queue. Relationbetween transmission time of secondary user and overlapped time interference to primary userare derived. Our proposed method models primary user behavior by alternative renewal processand no slot system requirement is demanded. With constraints relaxed and solutionclosed-formed, the proposed method still works even if no prior information of primary userbehavior were available. Theoretical analysis and simulation results verify our proposal that maximal data rate of secondary user can be achieved with maximal data rate of primary userguaranteed.(6) An optimal target channel sequence generation method is proposed in hybrid spectrumhandoff environment. Finishing sensing, decision and sharing of spectrum opportunity, spectrumaccess is permitted and multiple disruptions may occur due to the random return of primary user.Once collision occurs, the interrupted secondary transmission should immediately preserve thecontext of communication process and find another channel available to resume unfinishedtransmission (the extent primary user are exposed to secondary user’s collision is severelyrestricted). To maintain seamlessness and transparency, target channel sequence of secondaryuser experiencing disruption is researched in pursuit of minimal residual time spent in cognitivenetworks. Our proposed method attempts to shorten the average residual time composed ofnormal transmission time of each secondary user and handoff delay caused by multiple spectrumhandoffs. Mathematical modeling of average residual time of each transmission is conductedbased on preemptive-resume priority queuing theory and long-terms statistics of primary userand secondary users as well as spectrum heterogeneity are involved. For the sake ofmathematical heterogeneity, only Laplacian transform solution of average residual time isobtained but optimal target channel sequence is obtained. Furthermore, channel outageprobability is introduced to the analysis and derivation and average residual time with constantoutage probability is then obtained in a Laplacian transform expression, and correspondingoptimal target channel sequence is also obtained. Theoretical analysis and simulation resultsreveal that, compared with random handoff mechanism, our proposed handoff method offersbetter delay performance.