In this letter, we consider a cognitive radio cellular system for downlink transmission using Multi-carrier code-division-multiple-access (MC-CDMA). The bottleneck of conventional system is the non-availability of spectrum which is necessary for transmission of multi-media services such as picture transmission, video signal reception etc. We assume that CR system can recognize sporadic idle band spectrum and we will make use of spectrum to implement multi-carrier modulation and carrier frequency generation. We simulate the performance of CR based MC-CDMA system for Rayleigh fading channel. We also compare the BER performance with conventional MC-CDMA. We observe that SR based MC-CDMA obtain robust performance in the scenario of multi-user interference.