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In this paper, a low complexity power loading algorithm that maximizes the downlink transmission capacity of the cognitive radio (CR) user while keeping the total interference introduced to different primary user (PU) bands below a specified threshold for an OFDM-based cognitive radio system is proposed. Our suboptimal algorithm is based on 2^nd order interference tracking and nulling mechanism to allocate power at CR's sub-carriers. The performance of our proposed suboptimal schemes are compared with the performance of the classical power loading algorithms, e.g., water-filling, 1^st order interference tracking, nulling, and other suboptimal schemes. Numerical results show that our algorithm has low complexity but obtains a higher channel capacity than some previous suboptimal algorithms. We dedicated also that for a given interference threshold, the 2^nd order interference tracking mechanism has dynamic …