The radio on free space optical (RoFSO) communication systems are gaining popularity due to their high data rates, license free spectrum and adequate reliability at installation and operational costs which are much lower than comparable technologies. A significant disadvantage of these systems concerns the randomly time varying characteristics of the propagation path mainly caused by the turbulence effect. On the other hand, the optical fiber communication systems offer links with higher data rates but with expensive infrastructure and installation cost. In this work, we study the BER performance of an optical communication system which consists of a RoFSO link that is connected with an optical fiber link through a regenerator node. The signal propagates, in both links, using the OFDM technique with QAM format and the dominant impairments which have been taken into account are the atmospheric turbulence, modelled with the negative exponential distribution, the nonlinear responsivity of the laser diode which can be modelled with a Volterra series and the biasing with nonlinear clipping noise. For this setup, closed form mathematical expression for the estimation of system's BER has been derived and the corresponding numerical results are presented for common link parameters.
