Analysis of the Effect of Vignetting on MIMO Optical Wireless Systems Using Spatial OFDM

The performance of pixelated multiple-input mul-tiple-output optical wireless communication systems can be impaired by vignetting, which is the gradual fall-off in illumination at the edges of a received image. This paper investigates the effect of vignetting for a pixelated system using spatial orthogonal frequency division multiplexing (OFDM). Our analysis shows that vignetting causes attenuation and intercarrier interference (ICI) in the spatial frequency domain. MATLAB simulations indicate that for a given constellation size, spatial asymmetrically clipped optical OFDM (SACO-OFDM) is more robust to vignetting than spatial dc biased optical OFDM (SDCO-OFDM). Moreover, for the case of SDCO-OFDM, the very large zeroth subcarrier causes severe ICI in its neighbourhood causing flattening of the bit error rate (BER) curves. We show that this BER floor can be eliminated by leaving some of the lower spatial frequency subcarriers unused. The BER performance can also be improved by applying a vignetting estimation and equalization scheme. Finally, it is shown that equalized SACO-OFDM with 16-QAM has the same overall data rate as equalized SDCO-OFDM using 4-QAM, but requires less optical power.