Fast spatial orthogonal frequency-division multiplexing based on two-dimensional discrete cosine transform for a pixelated optical wireless communication system

Pixelated two-dimensional (2-D) optical wireless communication systems, with intensity-modulation and direct-detection (IM/DD), are capable to transmit data at high data rates by modulating the data into image intensities at the transmitter. Frequency division modulation (FDM) and space-domain modulation (SDM) are two common modulation schemes in a 2-D pixelated system. Generally, FDM is preferred because FDM benefits from the capability of controlling the spectrum of transmitting signals so that the output signal could be shaped according to the characteristics of a particular optical channel. However, the computational complexity of FDM is relatively high and complex computation is needed at both the transmitter and the receiver. In this work, a fast orthogonal frequency-division multiplexing (FOFDM) modulation which employs 2-D inverse discrete cosine transform (IDCT2) instead of 2-D inverse Fourier transform (IFFT2) is presented. As the image intensities must be real and positive, two schemes are hereby introduced to generate unipolar image intensities. The proposed FOFDM is simulated and discussed in terms of key characterisations including the signal spectrum, spectral efficiency, peak to average power ratio (PAPR), bit error ratio (BER) and computational complexity. Compared with traditional spatial orthogonal frequency-division multiplexing (OFDM) based on IFFT2, the proposed FOFDM has similar BER performance, twice the spatial efficiency for the same constellation size, lower computational complexity and a much simpler structure in the transmitter.