We consider a carrier sense multiple access(CSMA) in an unslotted cognitive radio network under a mixed\r\nspectrum environment of licensed channels and unlicensed channels. In this system, primary users use only\r\nlicensed channels, while secondary users use unlicensed channels and opportunistically can use licensed channels\r\nunused by primary users temporally. If an arriving secondary user(SU) does not find any idle channels, then the SU\r\neither enters the retrial group with probability q for later retrial or gives up its service and leaves the system with\r\nprobability 1 - q. SUs in the retrial group retry independently after exponentially distributed random time. We\r\nanalyze the system by continuous time Markov chain(CTMC) with level dependent QBD structure and obtain the\r\nsteady state probability of the system by matrix analytic method. In numerical results, we compare the\r\nperformance of two systems with retrial(q > 0) and without retrial(q = 0). It is shown that the retrial phenomenon\r\nof SUs has an impact on the performance of SUs in cognitive radio networks.
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