In this paper, we study the matched filter based spectrum sensing in a more reasonable cognitive radio (CR) scenario when the primary user (PU) could possibly work on more than one transmit power levels. Besides the traditional sensing target where the secondary user (SU) should decide the presence of PU, an additionally new target here could be also recognizing the power levels of PU, which achieves more "cognition" for CR. We derive the closed form solutions for decision regions and several performance metrics, from which some interesting phenomenons are observed and the related discussions are provided. Numerical examples are presented to corroborate the proposed studies.