The free space optical (FSO) and more specifically the radio on free space optical (RoFSO) communication systems are becoming very popular because they can achieve high bit rate transmission with low operational and installation cost. The main disadvantage of such systems is their dependence on the atmospheric conditions and more specifically the randomly time varying characteristics of the propagation path through the atmosphere which is, mainly, caused by the turbulence effect and affects significantly the system's availability and performance. On the other hand, the use of optical fiber systems, offer higher bit-rates and security level but their cost is larger. In this work, the performance of a hybrid dual hop optical communication system is investigated. This system consists of a RoFSO communication link which is connected with an optical fiber link part using a regenerator between them. In both links, the modulation technique that is used is the orthogonal frequency division multiplexing (OFDM) with either a 4 or 16 QAM format. The main phenomena that are taken into account are the atmospheric turbulence, which is modelled with K distribution, the nonlinearities of the laser diode which could be modelled by Volterra series and the biasing with the nonlinear clipping at the optical fiber segment. For this system, closed form mathematical expression for the estimation of its BER is derived and numerical results are presented for realistic parameter values.
