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The sidelobe of noncontiguous orthogonal frequency-division multiplexing (OFDM) signals is required to be deeply suppressed in the licensed user's band in cognitive radio (CR) systems. To this end, we propose a novel method of adding extended active interference cancellation (EAIC) signals to suppress sidelobes and to shape the spectrum of the CR-OFDM signal with a cyclic prefix (CP). For simplicity, we called the proposed scheme EAIC-CP. The key idea of the proposed EAIC-CP is to employ some cancellation signals consisting of tones spaced closer than the interval of OFDM subcarriers to cancel the sidelobes of OFDM signals. Moreover, we derive the optimal cancellation signals to minimize the total sidelobe power subject to a self-interference constraint. Numerical results show that, when the guard bandwidth is equal to one OFDM subcarrier interval, the EAIC-CP scheme offers more than a 45.0-dB sidelobe suppression with unnoticeable signal-to-noise ratio (SNR) loss at a symbol error rate (SER) from 10-2 to 10-3 for 64 quadratic-amplitude modulation (64QAM). Moreover, the EAIC-CP scheme can achieve high spectrum efficiency with low implementation complexity.