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Multi-carrier systems are one of the best candidates for applying in cognitive radio (CR) networks because of the spectrum shaping and high adaptive capabilities. Since secondary users (SUs) in this structure use a limited number of sub-carriers because of deactivation of the primary users' (PUs) band, the total capacity of CR networks is limited. On the other hand, multiple transmit antennas can be applied to orthogonal frequency division multiplexing (OFDM)-based CR in order to compensate this capacity leakage. This study aims to investigate multiple-input multiple-output (MIMO)-OFDM as one of the best hybrid multi-carrier systems, as a physical layer of CR networks. Considering different conditions to obtain maximum total capacity of CR networks, an optimal power allocation algorithm is scrutinised. Theoretically, it is shown that this proposed algorithm can maximise the total capacity and at the same time, keep the caused interference in PUs' bands in a tolerable range. To simplify the algorithm complexity, we also propose a sub-optimal scheme. The simulation results of the new algorithms are compared with previous methods, which present the enhancement and efficiency of the proposed algorithms. Furthermore, the simulation results show that our proposed schemes can load more power into the CR user's band in order to achieve higher transmission capacity for a given interference threshold.