Time and Spatial Jitter Influence on the Performance of FSO Links with DF Relays and OC Diversity Over Turbulence Channels  
[PERSON]  
1 Department of Electronic Physics and Systems, Department of Physics, National and Kapodistrian, University of Athens, 15784 Athens, Greece; [EMAIL_ADDRESS] (PJ.G. ); [EMAIL_ADDRESS] (E.R) 1  
[PERSON]  
1 Department of Electronic Physics and Systems, Department of Physics, National and Kapodistrian, University of Athens, 15784 Athens, Greece; [EMAIL_ADDRESS] (PJ.G. ); [EMAIL_ADDRESS] (E.R) 1  
[PERSON]  
2 Division of Combat Systems, Naval Operations, Sea Sciences, Navigation, Electronics & Telecommunications Sector, Hellenic Naval Academy, 18539 Pravens, Greece; [EMAIL_ADDRESS] 2  
[PERSON]  
3 Electronics-Telecommunications and Applications Laboratory, Physics Department, University of Ioannina, 45110 Ioannina, Greece; [EMAIL_ADDRESS] 3  
[PERSON]  
1 Department of Electronic Physics and Systems, Department of Physics, National and Kapodistrian, University of Athens, 15784 Athens, Greece; [EMAIL_ADDRESS] (PJ.G. ); [EMAIL_ADDRESS] (E.R) 1  
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Footnote  
However, there are several unavoidable drawbacks that shadow the performance of FSO systems. Atmospheric turbulence is a well-known problem, responsible for the scintillation effect, i.e., the rapid fluctuations of signal level, observed at the receiver. The strength of this effect is related to the prevailing weather conditions of the pathway across the channel, where unequally sized air-flow eddies affect the propagation of the optical beam pulses [4; 5; 6; 7; 8; 9; 10]. Another critical factor concerning the link's performance is the so-called spatial jitter (SJ), commonly known as pointing errors (PEs), related to any misalignments between the transmitter and the receiver. PEs are caused by occasional displacements and vibrations of either link's ends, due to many possible reasons, like building sways, wind loads, and small quakes, inducing the beam's spatial jitter around the detector's aperture center, which implies further irradiance fluctuations of the received signal. Therefore, this effect deteriorates the performance of FSO links, especially the long ones [11; 12; 13; 14]. Equally important, the time jitter is also related to detection errors, caused by the arrival of out-of-sync pulses. Receivers have to detect the incoming signal pulse in a given time slot interval, reciprocally proportional to the bit rate of the system. However, time-jittered pulses are likely to be misdetected at adjacent time slots, inducing bit flips and, consequently, bit stream errors. This effect is heavily responsible for limiting the total throughput of the links, particularly unavoidable in high-data-rate links, in which the time slot is quite short and comparable to the pulse's time jitter [15; 16; 17; 18; 19]. Nevertheless, all three effects have been exhaustively studied and many statistical models have been proposed in the literature to estimate their impact.