Spatial diversity for QAM OFDM RoFSO links with nonzero boresight pointing errors over atmospheric turbulence channels

Abstract

ABSTRACTRadio-on-free-space-optical (RoFSO) communication systems have attracted a lot of technological interest in the last few years, due to their high performance characteristics with relatively low installation and operational cost. At the same time a lot of research has been carried out on them. This specific technique facilitates the transmission of radio frequency signals with the aid of optical carriers through the atmosphere; its effectiveness is already known from the Radio over Fibre (RoF) links. However, besides their advantages, the performance of free-space optical (FSO) communication links depends strongly on the atmospheric conditions, due to the fact that the propagation path is the atmosphere. In the present work, we consider the transmission of OFDM radio signals, with QAM modulation format, through an FSO link, using spatial diversity of the receivers. For the estimation of the system performance the atmospheric turbulence effect has been modelled by the gamma-gamma distribution, the accuracy of which spans from weak to strong turbulence conditions. Furthermore, the pointing errors effect is studied with the aid of a new generalized distribution that takes account the nonzero boresight displacement of the optical beam from the optical receiver centre. In addition, different spatial jitters are taken into consideration for the two vertical axes of the receiver. Thus, accurate mathematical expressions are extracted, which lead to the estimation of the average BER and the outage probability of a spatially diverse QAM OFDM RoFSO link. Finally, the theoretical results obtained from the derived expressions are verified through the corresponding numerical simulations which are presented at the numerical results section of the manuscript.