Diversity techniques enhance the performance of high speed wireless channel to transmit data and information. This paper shows the performance analysis of a cognitive radio network with maximal-ratio combining (MRC) in terms of capacity and SNR. For this we propose an efficient water filling based power allocation algorithm to determine optimum power levels for secondary users. The main objective of antenna selection and cognitive MIMO system is to maximize the capacity of secondary system with interference constraints to primary users. So we propose exhaustive search and complexity reduced antenna selection algorithms for reducing the complexity of the system which is important for improving the transmission capacity of MIMO model and the results of algorithms are compared with MRC to show it gives best performance improvement. © 2017 IEEE.