Amplify-and-forward multihop non-line-of-sight ultraviolet communication in the gamma–gamma fading channel

Abstract

In this paper, we propose amplify-and-forward-based multihop non-line-of-sight (NLOS) ultraviolet (UV) communication with subcarrier intensity modulation to mitigate turbulence-induced fading. Atmospheric turbulence is one of the main impairments in the performance of UV links, as it causes random fluctuation of the received signal. An aggregated UV channel model of both path loss and turbulence-induced gamma–gamma fading is considered. We derive the probability density function (PDF) of the end-to-end received signal-to-noise ratio (SNR). With this PDF and the random coding error exponent, an exact solution to the optimal number of hops in the proposed multihop UV system is presented. Over the turbulence-induced gamma–gamma fading channel, a closed-form expression for the moment-generating function (MGF) of the inverse of the instantaneous received SNR is also derived. Based on the derived MGF, two closed-form analytical expressions of average bit error rate and outage probability are further derived, together with an upper bound of the ergodic capacity of the proposed system. From the analytical expressions derived, the proposed multihop NLOS UV system is analyzed. Simulation results show that the proposed UV system can effectively mitigate the turbulence-induced fading, thus resulting in an increased coverage distance of the UV system without sacrificing important performance criteria such as the outage probability and the average bit error rate. It is also found that the optimal number of hops decreases with an increasing data rate.