A novel FFT/IFFT size efficient technique to generate real time optical OFDM signals compatible with IM/DD systems

Optical forms of orthogonal frequency division multiplexing (OFDM) are widely used in intensity modulated direct detection (IM/DD) systems. These forms use the Hermitian symmetry of the frequency symbols to generate real OFDM signals compatible with the optical transmitters characteristics. Thus, to modulate and demodulate N frequency symbols, 2N-point inverse fast Fourier transform (IFFT) and fast Fourier transform (FFT) are needed, resulting in a doubled circuit complexity. In this paper, we propose a novel technique to generate real OFDM signals, using only N-point IFFT/FFT blocks. This technique consists on generating a conventional complex OFDM signal and juxtaposing the real and imaginary parts of the complex signal in the time domain. The proposed technique has a saving of 55% in the number of the FFT/IFFT operations, which results in a great reduction of the power consumption and the occupied chip area. Furthermore, we demonstrate that the proposed technique, with only N-point IFFT/FFT, exhibits the same tolerance to nonlinearity as the conventional real OFDM systems.