Performance analysis of SCM-FSO links for transmission of CDMA signals under Gamma-Gamma turbulent channel

In this paper, we investigate the performance of the sub-carrier multiplexed-free space optics (SCM-FSO) for transmitting code division multiple access (CDMA) signals in terms of carrier-to-noise-plus-interference ratio (CNIR) and bit error rate (BER), under Gamma-Gamma turbulent FSO channel. We propose an analytical model for optimization of the SCM-FSO link and a closed-form BER expression taking into account the nonlinear effects of the SCM optical link as well as the strong atmospheric turbulence of the FSO channel. We emphasize how the choice of optimal optical modulation index (OMI) can increase the received CNIR and also its dependency on the attenuation of the FSO channel. We then discuss the effect of atmospheric turbulence on the received BER. It is shown that, for a high quality CDMA signals reception over the SCM-FSO system, the link has to be engineered to perform optimally in terms of received optical power, number of carriers and OMI.