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This paper proposes to use Fast Fourier Transform (FFT)/Inverse Fast Fourier Transform (IFFT), instead of vector-matrix multiplication, to implement the spreading/despreading in Carrier-Interferometry Orthogonal Frequency Division Multiplexing (CI/OFDM) and Pseudo-Orthogonal Carrier-Interferometry OFDM (PO-CI/OFDM). That can improve the signal processing efficiency of CI/OFDM and PO-CI/OFDM systems by about 2N/log2N and 2N/(1+log2N) times respectively and dose not make any difference to the system function and performance. Moreover, the efficiency benefits will increase with the increase of the number of sub-carriers. In addition to that, we point out that the transmitter of CI/OFDM is actually technically equivalent to that of a single-carrier system with cyclic-prefix and the receiver of CI/OFDM is a typical OFDM receiver with CI despreading. Hence the low Peak-to-Average Power Ratio (PAPR) property and high anti-fading performance of CI/OFDM system can be well explained.