Admission and power control schemes play important roles in deploying cognitive radio technology to cellular networks. In this paper, we propose a method to calculate the power scale factor, introduce two pre-admission control schemes, and reformulate the problem as a multidimensional knapsack problem. In addition, we propose a novel admission and power control scheme called JAPC-MKP. Simulation results show that our proposed JAPC-MKP can very closely approach the optimal results from the optimization software MOSEK, and greatly outperform the existing schemes in most cases.