Research On Key Techniques Of Energy Efficiency In Cognitive And Cooperative Wireless Communications

Fast increasing popularity of powerful intelligent devices and vast penetration of mobile internet business causes explosive volume of wireless traffic,which in turn makes spectrum even more crowded and communication systems more energy-intensive.Therefore,energy efficiency(EE)has been considered as one of the key features in the 5th generation mobile communication and has attracted many research interests.As important means for improving spectrum efficiency,cognitive radio(CR)and cooperative transmission(CT)are also considered as potential techniques to achieve green communication.By introducing CT into cognitive radio networks(CRNs),cooperative cognitive radio networks(CCRNs)can not only improve spectrum efficiency(SE)but also overcome fading effect,and thus attracts much attention from researchers.Most recent research works on CCRNs are mainly focused on improvement of access opportunities and transmission throughput,few of them are concerning on EE.However,it is necessary to study impacts on cooperative spectrum sensing(CSS)and CT in CCRNs when EE is concerned to meet low-carbon communication requirement.Therefore,this dissertation focuses mainly on innovative EE-oriented CSS,CT mechanisms and resource allocation(RA)strategies in CCRNs.Main contributions of this dissertation are as follows:Firstly,this dissertation studies energy-efficient CSS strategies in relay-assisted CRNs.In CSS,more secondary relays(SRs)involving in spectrum detection require more network energy consumption and cause more interference,which means there is a tradeoff between sensing energy consumption and detection performance.To tackle this issue,a novel adaptive energy-efficient sensing(AES)strategy with differential censoring is proposed to improve EE of CSS.We derived generalized-form expressions for detection probability,reporting probability and average sensing energy over Rayleigh fading channels for AES.Then,we study sensing EE maximization problem and show that sensing duration can be optimized to achieve a balance between sensing performance and energy consumption.Numerical results show that our proposed scheme can greatly improve sensing EE while only slightly degrading detection performance as compared to traditional schemes.Secondly,this dissertation studies energy-efficient secondary CT strategies in relay-assisted CRNs.Existing researches on relay-assisted CRNs mostly aim to optimize SE,and these studies rarely consider EE.Therefore,we propose an innovative EE-oriented and best-relay assisted CT scheme(BCT)for relay-assisted CRNs,in which SRs take part in CSS to promote transmission opportunity,and the best SR assists secondary transmission to enhance communication quality in transmission stage,thus to improve secondary EE.Based on BCT,mean energy efficiency(MEE)maximization problem with sensing duration and transmitting power as optimization variables is modeled for fading channels under constraint of minimal secondary outage probability.Using Jensen's inequality,the original optimization problem is decomposed into two relatively independent subproblems which solve sensing duration and power allocation(PA)respectively.And for the two subproblems,we propose an efficient cross iteration based algorithm to obtain the suboptimal solutions.Simulation results show that the proposals can achieve significantly higher EE while enhancing reliability of secondary transmission remarkably compared to non-cooperation single cognitive transmission schemes in high quality of service(QoS)requirements.Thirdly,this dissertation studies EE-oriented RA policy in cognitive relaying links(CRL).Unlike related works in terms of maximizing transmitting power under fixed throughput,we formulate a subcarrier transmission path selection and PA optimization problem with the goal of maximizing system EE in CRL considering secondary QoS and total power budget.Theoretical analysis demonstrates that the EE function of CRL has a globally unique optimal solution.And then,the nonconvex problem is transformed into convex programming problem by discussing EE optimization problem in different situations,and a step-by-step strategy based on binary search assisted ascent(BSAA)method is proposed.Simulation results show that our proposed strategy can realize the optimal EE while ensuring the reliability of secondary transmission.In addition,due to spectrum sensing errors in practice,we formulate a PA optimization problem with imperfect sensing for secondary users(SUs),while restricting interference to primary user(PU).The optimization problem is non-convex and it is transformed to a parametric convex form by using the fractional programming and Lagrange dual method.And then,a two-layer embed iterative PA algorithm with low complexity is proposed to obtain the EE-oriented PA policy in CRL under imperfect sensing.Finally,this dissertation studies energy-efficient CT scheme in heterogeneous CRNs(Het-CRNs).In CT,improvement of primary system transmission efficiency is at cost of SR's energy consumption,which is unfair for SR when there is no rewarding introduced to SRs.Besides,existing research has an implicit assumption that PU needs SRs cooperation which is not always necessary.Therefore,we propose an innovative EE-oriented ‘win-win' CT scheme for Het-CRNs,namely energy-efficient cognitive cooperation(ECC),in which primary system releases a part of its radio spectrum to secondary system in exchange for SR's relay transmission of PU's traffic.Then,SU's traffic flow can be split into the unlicensed spectrum and the released licensed spectrum to improve EE and overall system throughput in Het-CRNs.It is a nonconvex optimization problem to maximize EE for SUs under constraints of power limitation and throughput requirements from both PUs and SUs.By employing parametric fractional programming and golden segmentation search(GSS)method,an efficient iteration based RA policy is developed.Simulation results show that the proposed strategy can gain significantly higher EE without sacrificing performance of primary transmission,and thus,a ‘win-win' goal is achieved.