Adaptive Modulation and Coding for Free Space Optical Communication
A major problem in long-range wireless optical links is atmospheric turbulence due to inhomogenities in the temperature and presure. This results in severe fluctuations of the received intensity of the optical signal, known as fading or scintillation. Over the last decade, various fading mitigation techniques have been proposed to improve the reliability of wireless optical links. A common assumption in all those techniques is that only the receiver has channel state information (CSI) while the transmitter has no knowledge of the channel. For very slowly-varying fading channels as in the case of atmospheric turbulence-induced fading, it is possible to reliably estimate the channel state information and send it back to the transmitter. The transmitter can use this information to vary the transmission parameters such as power, modulation size, code rate etc. according to the channel conditions. This is known as link adaptation.
In this thesis, we investigate an adaptive modulation and coding scheme for wireless optical communication systems with M-ary pulse position modulation over log-normal turbulence channels under peak power constraints. Specifically, we consider three schemes: i) Adaptive modulation for uncoded wireless optical communicaiton systems, ii) An adaptive modulation scheme for coded wireless optical communication systems, and iii) Joint modulation and coding adaptation. Optimal adaptation strategy that maximizes the efficiency of the system at a targeted bit-error rate and outage probability under peak power constraint is determined. Our simulation results demonstrate significant performance improvements of the proposed adaptive schemes over non-adaptive counterparts.
