A Mixed Orthogonal Frequency Division Multiplexing (X-OFDM) Waveform for Optical Wireless Communication

Optical wireless communication (OWC), with intensity modulation (IM), requires the modulated radio frequency (RF) signal to be real and non-negative. To satisfy the requirements, this paper proposes a novel mixed orthogonal frequency division multiplexing (X-OFDM) waveform. Hermitian symmetry (HS) of the sub-carriers in frequency domain, guarantees the signal in time domain to be real, which reduces the spectral efficiency to 1/2. For the odd sub-carriers in frequency domain, the signal in time domain after inverse fast fourier transform (IFFT) is antisymmetric. For the even sub-carriers in frequency domain, the signal in time domain after IFFT is symmetric. Based on the antisymmetric and symmetric characters, the X-OFDM waveform is designed for the signal in time domain to be non-negative. With N sub-carriers in frequency domain, the generated signal in time domain is 3N/2 points, where the spectral efficiency is further reduced to 1/3 and the direct current (DC) bias is not needed. Based on the simulations, the X-OFDM waveform achieves much higher power efficiency than traditional waveforms without DC bias, when the normalized bit rate is larger than 1.7 bits/s/Hz.