Orthogonal frequency division multiplexing (OFDM) based cognitive radio (CR) systems suffer from the large out-of-band emission (OOBE) that interferes with other users. Among the schemes that have been proposed to reduce the OOBE, precoding techniques encode the input data vectors before the inverse fast Fourier transform (IFFT) process, and decode them at the receiver after the fast Fourier transform (FFT) process. They provide significant OOBE suppression and do not cause any degradation of bit error rate (BER) performance. Therefore, two existing precoding techniques, which in this paper are called SVD precoding and N-continuous precoding, are investigated in this paper. Then a combined approach which maintains the merits of these two techniques is proposed. Numerical results show that, no matter whether there is a cyclic prefix (CP) or not, the proposed method gives significant further improvement on OOBE suppression and almost no extra degradation in the system performances such as the increase of BER or peak-to-average power ratio (PAPR) than either of the two original techniques.