Performance of Scintillation Mitigation for Linear Polarization Shift On-Off Keying Transmission in Free-Space Optical Communications

• Main Authors: Yan-Qing Hong, Won-Ho Shin, Sang-Kook Han
• Format: Article
• Language: English
• Published: IEEE 2020-01-01
• Series: IEEE Access
• Subjects: Linear polarization shift on-off keying; turbulence-induced scintillation effect; linear polarizer; logarithmic amplification; free-space optical communications
• Online Access: https://ieeexplore.ieee.org/document/9139930/

This study investigates the performance of scintillation mitigation for linear polarization shift on-off keying (LPS-OOK) transmission in free-space optical (FSO) communications. Scintillation effect is a critical issue for FSO communication in vertical links. The LPS-OOK signal with orthogonal linear x- and y-polarizations for bits `1' and `0' is modulated and transmitted from the transmitter end. The bits `1' and `0' suffer the scintillation effect in turbulence channel. Based on the orthogonal polarization characteristics of LPS-OOK, linear polarizer (LP) is used to mitigate the scintillation effect on bit `0' by blocking its polarization without the distortion of bit `1'. Further, owing to the nonlinear gain characteristics of logarithmic amplification, it is used to equalize the intensity fluctuation of bit `1'. Finally, the equalized LPS-OOK is distinguished using fixed threshold decision (FTD) instead of a complex symbol-by-symbol estimation of adaptive threshold. The performance of the proposed technique was verified experimentally for various degrees of the turbulence channel. A Mach-Zehnder modulator (MZM)-based turbulence channel simulator was adopted to accommodate turbulence-induced scintillation effect. The experimental results confirmed that the proposed technique that employed FTD has an improved bit-error-rate (BER) performance compared with that of OOK that employed an adaptive threshold decision (ATD) under various degrees of turbulence channels.