Impact of Link Parameters and Channel Correlation on the Performance of FSO Systems With the Differential Signaling Technique

We investigate the effects of link parameters and the channel correlation coefficient on the detection threshold, Q-factor, and bit-error-rate (BER) of a free-space optical system employing a differential signaling scheme. In systems employing differential signaling schemes, the mean value of the signal is used as the detection threshold level, provided that differential links are identical or highly correlated. However, in reality, the underlying links are not essentially identical and have a low level of correlation. To show the significance of the link parameters as well as the correlation coefficient, we derive analytical relations describing the effect of weak turbulence and we determine the improvement of Q-factor with the channel correlation. Further, for the same signal-to-noise ratio, we demonstrate that a link with a higher extinction ratio offers improved performance. We also propose a closed-form expression of the system BER. We present experimental results showing improved Q-factor for the correlated channel case compared to the uncorrelated channel

Comparison of different combining methods for space-diversity FSO systems

This paper gives an overview of different signal combining schemes for free space optical (FSO) communication systems employing spatial diversity (SD) for the case of lognormal weak turbulence atmospheric channels. The receiver configurations are explained and categorized into two domains, namely optical and electrical. Moreover, a new combining scheme in the electrical domain, called “logical” domain combining, is introduced. We present closed-form expressions of bit-error-rate for the combining schemes, considering the typical case of a single-input multiple-output (SIMO) system. Through some numerical analyses, we show that depending on the number of receivers and the turbulence strength, the proposed logical combining method outperforms the other ones