简介:Thispaperinvestigatestheperformanceofanunderlaycognitiverelaysystemwheresecondaryusers(SUs)sufferfromaprimaryoutageprobabilityconstraintandspectrum-sharinginterferenceimposedbyaprimaryuser(PU).Inparticular,weconsiderasecondarymulti-relaynetworkoperatingintheselectiondecode-and-forward(SDF)modeandproposeabest-relayselectioncriterionwhichtakesintoaccountthespectrum-sharingconstraintandinterference.Basedontheseassumptions,theclosed-formexpressionoftheoutageprobabilityofsecondarytransmissionsisderived.Wefindthataflooroftheoutageprobabilityoccursinhighsignal-to-noiseratio(SNR)regionsduetothejointeffectoftheconstraintandtheinterferencefromthePU.Inaddition,weproposeageneralizeddefinitionofthediversitygainforsuchsystemsandshowthatafulldiversityorderisachieved.Simulationresultsverifyourtheoreticalsolutions.