Abstract:The cognitive Multiple-Input and Multiple-Output(MIMO) technology integrated by the cognitive radio and MIMO, can improve the system performance and spectrum efficiency. The system capacity characterizes the maximum transmission rate of the communication system, namely, the communication system performance. In basis of the traditional MIMO system capacity derivation, the channel transmission rate theory limit under user interference is given after introducing the interference between the primary users and the secondary users. And then, the capacity loss caused by the interference between users is simulated and studied. The influence of the interference threshold on the primary user is analyzed.