Alleviation of Jamming in Free Space Optical Communication
Most FSO communications systems operate at a limited range of distinct wavelengths (usually 1300 nm or 1550 nm) making them predictable and easily distorted by an intruder. Anti-jamming techniques have been studied broadly for RF networks, but this field is still a new area for FSO. In October 2019, Bhatnagar et al published a paper on studying and resolving jamming errors of FSO systems using Gamma-Gamma (GG) fading channels. Bit error rate (BER) and pointing error (PE) effects on a single-input single-output (SISO) system were measured analytically to determine the worst-case jamming conditions numerically. The tests were also performed under different conditions of atmospheric turbulence (AT) with different PE parameters of jamming noise. To combat some of the adverse effects caused by jamming, a multiple-input single-output (MISO) system was considered. The MISO system proved to have better performance in terms of BER than the SISO system; however, the analytical results for both were fully explored in the paper.
Two important parameters that can mitigate the effect of jamming on a communication system are field-of-view (FoV) and phenomenon-operating frequency. As previously discussed, the operating frequencies of FSO systems are currently limited; therefore more focus is currently placed on widening the FoV of the system. A wider FoV helps reduce PE while maximizing the acceptance rate of the transmitted optical signal. Unfortunately, a jammer is more easily placed inside a wider FoV which can cause the jamming signal to be trapped inside the photodetector.
Common jamming technology can fall under two categories. Constant jammers remain in the on state and constantly impede any signal being transmitted jammer’s power supply is exhausted. Pulse jammers alternate on and off based on an activating signal. The randomness of this jamming activity causes the signal to become unpredictable at the receiver end. Different jamming methods of FSO systems have previously been studied; however, this study was the first to take AT and PE and their effect on the BER of both SISO and MISO FSO systems into account.