Multihop Relaying Over IM/DD FSO Systems With Pointing Errors

In this paper, the end-to-end performance of a multihop free-space optical system with amplify-and-forward channel-state-information-assisted or fixed-gain relays using intensity modulation with direct detection technique over Gamma-Gamma turbulence fading with pointing error impairments is studied. More specifically, novel closed-form results for the probability density function and the cumulative distribution function of the end-to-end signal-to-noise ratio (SNR) are derived in terms of the Fox´s H function. Based on these formulas, closed form bounds for the outage probability, the average bit-error rate of on-off keying modulation scheme, the moments, and the ergodic capacity are presented. Furthermore, using the moments-based approach, tight asymptotic approximations at high- and low-average SNR regimes are derived for the ergodic capacity in terms of simple elementary functions. The obtained results indicate that the overall system performance degrades with an increase of the number of hops. The effects of the atmospheric turbulence conditions and the pointing error are also quantified. All the analytical results are verified via computer-based Monte-Carlo simulations.