The performance of orthogonal frequency division multiplexing in the weak turbulence regime of free space optics communication systems

Radio on free space optics—RoFSO—has gained momentum in research because of its cost effectiveness and efficiency in transferring data at a high rate that is comparable to that for optical fiber media. While the transmission data rate is limited in fiber due to dispersion and nonlinearity, such effects do not prevail in FSO communication links. The data rate depends mainly on the switching speed of the optoelectronic devices. With the characteristics of free space being random in nature, the performance of RoFSO is primarily governed by atmospheric conditions. In this paper, we evaluate the performance of the orthogonal frequency division multiplexing (OFDM) signal in free space and compare against its counterpart radio frequency (RF) wireless communication systems. Simulations have been done on the atmospheric conditions by means of modeling the scintillation effect using log-normal distribution. The performance of the proposed system under two different base-band modulations, namely OFDM–PSK (phase shift keying) and QAM (quadrature amplitude modulation) in weak turbulence conditions is studied. It is found that PSK performs better than QAM. Also the M-ary performance analysis shows that 3–5 dB improvement in the signal to noise ratio is obtained for OFDM based FSO transmission compared to RF based wireless transmission. (paper)