As seen in the previous chapter, turbulence in the atmosphere causes random fluctuations of the phase and amplitude of the received signal leading to deep signal fades or surges. These fluctuations of the received signal severely degrade the link performance, particularly over a link distance of 1 km or longer or if communication is taking place with moving platform. Moreover, the propagating pulse may experience pulse broadening due to atmospheric scattering that further deteriorates the performance of the received signal and limits the application of FSO to short-range links. Various techniques have been proposed in literature to mitigate the effect of atmospheric turbulence like aperture averaging, diversity, channel coding, adaptive optics, etc.
Aperture averaging technique is the simplest form of spatial diversity where the size of receiver aperture is larger than the fading correlation length. This technique averages the scintillation and thus significantly reduce the atmospheric turbulence especially in strong turbulence. Diversity technique makes use of multiple transmitter or receiver or both, and it can significantly combat the atmospheric fading by creating additional spatial degree of freedom. It allows multiple beams to propagate through different atmospheric spatially coherent cells, and, therefore, the likelihood that all the beams are simultaneously being affected by turbulent atmosphere is reduced than that of single beam. Channel coding like Reed-Solomon codes, LDPC codes, etc., significantly improves the bit error rate of the system ranging from 5 to 15 dB depending upon the strength of turbulence in the atmosphere. In the subsequent sections, all these performance improvement techniques will be discussed in details.
Outage Probability Convolutional Code Maximum Ratio Combine Constraint Length Adaptive Optic System
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