We consider the communication needs of cognitive radios, in order to ensure the unimpaired operation of licensed users and secondary users' communications isn 't intermitted. This paper proposes a STBC MC- CDMA cognitive radio systems and a hand-off technique for secondary users. We design a novel power control game algorithm named NCPG to efficiently use spectrum resources. Simulation results show that our algorithm can meet the different SIR requirements and yields significant improvement in throughput under the same power consumption.