| At present,radio frequency(RF)communication technology is still the first choice for traditional communication,but the radio frequency spectrum as a wireless signal transmission medium is increasingly tight.The problem of spectrum scarcity has become one of the most important problems in the field of wireless communication in recent years,and cognitive radio Cognitive radio network(CRN)is a spectrum sharing method that can effectively solve the limitation of spectrum resources.In addition,free space optical(FSO),which communicates in atmospheric channels via laser signals,can provide cost-effective communications without requiring any spectrum licenses.However,in the long-distance transmission process,it is easily affected by bad weather such as snow,fog,haze,etc.,which will seriously affect the communication and even interrupt it.The underlay cognitive mixed RF-FSO system studied in this paper effectively improves the link communication performance and broadens the communication range,further improves the utilization of the radio frequency spectrum,and alleviates the dilemma of the scarcity of spectrum resources.In order to alleviate the attenuation effects of multipath effect,path loss,aiming error and atmospheric turbulence on the performance of the mixed system,and at the same time make the theoretical research results more close to the actual engineering application,this paper combines space diversity technology and space-time coding technology.Two underlay cognitive mixed RF-FSO systems are proposed and their performances are analyzed.The main contents of the research are as follows:First,under the κ-μ shadow/M distribution mixed statistical model whose statistics are closer to reality and has excellent generality,the space diversity underlay CRN mixed RF-FSO system is proposed.The switch-and-examine combining(SEC)technology is used at the receiving end of the single input multiple output(SIMO)FSO link,which comprehensively considers the effects of atmospheric turbulence,path loss and pointing error on the atmospheric channel.The Meijer G closed expression of the outage probability of the mixed system is deduced,and the outage performance of the SIMO underlay cognitive mixed RF-FSO system is simulated and analyzed by different channel fading parameters,peak transmit power,interference temperature,the number of receivers and other key parameters.Finally,the accuracy of the expression is verified by Monte Carlo simulation.Secondly,the underlay cognitive mixed RF-FSO system with multiple input multiple output(MIMO)orthogonal space-time block codes(OSTBC)is proposed for both the RF link and the FSO link,and the closed expression for the outage probability of the mixed system is derived.The outage performance of MIMO-OSTBC underlay cognitive mixed RF-FSO system by key parameters such as peak transmit power of secondary user,fading parameters,the number of SU transmitting antennas and relay receiving antennas,and the relative speed of primary user are simulated and analyzed,respectively.Finally,Monte Carlo simulation verification is carried out,and the results are fitted.This research work has certain guiding significance for the engineering practice of future the mixed underlay cognitive RFFSO relay systems. |
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