Capacity of Optical Wireless Channels in Atmospheric Turbulence with Transmission Power Adaptation based on Fading Reciprocity

An optical wireless channel through the earth's atmosphere undergoes turbulence-induced signal fading, which causes detrimental effects on the communication performance. Transmission power adaptation is a method for mitigating the turbulence-induced effects on optical wireless communications. Fading reciprocity can be used to make a transmitter attain the instantaneous channel state information without the help of a dedicated feedback link. The instantaneous channel state information is necessary for implementation of transmission power adaptation scheme. Statistical models for the observed instantaneous channel state at a transmitter by use of fading reciprocity were developed. The capacity of optical wireless channels with transmission power adaptation based on fading reciprocity was theoretically examined by utilizing the obtained models. The optimal weighting function for transmission power allocation was determined under various conditions. The achievable spectral efficiency (ASE) of an optical wireless channel using transmission power adaptation scheme with various channel parameters was determined. The transmitter stops sending out optical power sharply as the instantaneous channel state fades below a given critical value when good fading reciprocity can be maintained; however, the said sharp stop does not exist any longer when the fading reciprocity is harmed significantly by detection imperfection.