Information Theoretic Study of Friendly Jammer Abating an Eavesdropper in FSO Communication

This work is concerned with the physical layer security of the free space optical (FSO) communication network comprising of a transmitter, a receiver, an eavesdropper (Eve), and a friendly reactive jammer that aims to thwart the activity of the Eve. As the key feature of this work, we calculate the discrete-input-continuous-output for memory-less channel (DCMC) capacity of the authentic and eavesdropping channel in the presence of the atmospheric turbulence characterised by $\alpha -\mu $ probability density function (PDF), zero and non-zero boresight pointing error, and partial band or broadband jamming noise. For this purpose, novel PDFs for all-encompassing conditions of FSO channel and the received signals at different nodes of the considered system are formulated. Thereafter, we devise schemes to wane the effect of a vicious Eve by formulating novel closed-form expressions of average secrecy capacity for both strategies. Our derived analytical expressions can be used as tools to examine the DCMC capacity and secrecy capacity for different turbulence regimes of FSO link, severity of boresight errors, and activity of a friendly jammer. The mathematical results are affirmed by Monte-Carlo simulation. We exhibit that deploying a friendly jammer to assist enhancing the security of an FSO system is very conducive as it makes the Eve to hike its signal-to-jamming-noise ratio by almost 10 dB when the jammer’s activity increases from 10% to 100% of the time frame of data transmission.