Research On Cogniitve Link Spcctrum Efficiency Exploiting The Adaptive Modulation

Due to improve the efficiency of spectrum utilization, cognitive radio allowssecond users to opportunistically access idle spectrum bands according to channelstate information after sensing the idle licensed spectrum band. Adaptive modulationand coding technology is an enabling technology for improving the transmissionefficiency in which modulation schemes and M-ary methods are adaptively selectedaccording to sensing the channel state information. Adaptive modulation and codingcan be recommended for the improving of efficiency of spectrum utilization andtransmission rate in the cognitive radio networks. Thus, this paper gives research indetail on the application of adaptive modulation and coding to dynamic access modesin cognitive radio, and the main contents are as follows:1、Considering the hybrid dynamic access mode including the underlay andinterweave sharing modes in cognitive radio, we present a novel optimizationframework of the average spectral efficiency of the cognitive radio in which theprimary and cognitive users are equipped with the adaptive modulation and coding(CR-AMC). This framework is modeled as an optimization problem of transmissionpower of the secondary user with the constraints of the minimum average spectralefficiency of the primary link, the limited average transmission power of thecognitive link and average packet error rates of both links, to maximize the averagespectral efficiency of the cognitive link, and gives the theoretical proof on theconvexity of the optimization problem. The numerical results show that the proposedhybrid mode is superior to the single access mode by means of the adaptivemodulation and coding from the aspect of the average spectral efficiency of thecognitive link.2、The joint optimization model of transmission rate and power is constructedbased on CR-AMC. The cognitive transmitter adjusts adaptively the transmissionpower according to the channel states and their respective modulation mode, tomaximize the average spectral efficiency of the cognitive link. The numerical resultsshow that, given the overall average transmission power of the cognitive link, theaverage spectral efficiency of the cognitive link with the joint optimization of transmission power and rate is higher than the respective spectral efficiency with theoptimized transmission power.3、To study the effect of delay sensitivity of channel on the average spectralefficiency of the cognitive link based on CR-AMC and introduce the delay parameterto compute, we study a novel framework of the maximization of effective spectralefficiency of the cognitive link with the constraints of dual quality-of-service (QOS),namely, average packet error rates and time delay, of both links. The numerical resultsshow the effect of the primary activity, PER, etc, on average spectral efficiency ofcognitive link, and that the effective spectral efficiency of cognitive system can beeffectively improved by adjusting the time delay index.