Performance analysis of CSMA in an unslotted cognitive radio network with licensed channels and unlicensed channels

We consider a carrier sense multiple access(CSMA) in an unslotted cognitive radio network under a mixed spectrum environment of licensed channels and unlicensed channels. In this system, primary users use only licensed channels, while secondary users use unlicensed channels and opportunistically can use licensed channels unused by primary users temporally. If an arriving secondary user(SU) does not find any idle channels, then the SU either enters the retrial group with probability q for later retrial or gives up its service and leaves the system with probability 1 q. SUs in the retrial group retry independently after exponentially distributed random time. We analyze the system by continuous time Markov chain(CTMC) with level dependent QBD structure and obtain the steady state probability of the system by matrix analytic method. In numerical results, we compare the performance of two systems with retrial(q > 0) and without retrial(q = 0). It is shown that the retrial phenomenon of SUs has an impact on the performance of SUs in cognitive radio networks.