Unified error performance of a multihop DF-FSO network with aperture averaging

In this paper, the performance of a multihop decode-and-forward (DF) based freespace optical (FSO) network is investigated over a unified exponentiated Weibull distributed atmospheric turbulence channel with pointing error and both types of detection techniques, i.e., heterodyne detection and intensity modulation/direct detection (IM/DD). We consider the aperture averaging receivers at both the intermediate relays and the destination to diminish the disadvantageous impact of atmospheric turbulence. First, we derive the new unified expression of the probability density function of the instantaneous signal-to-noise ratio (SNR). Based on the derived statistics, the unified closed-form expression of the average symbol error rate (SER) for the considered multihop network employing differentially modulated M-ary phase shift keying data, is obtained by utilizing a transition probability matrix. It is shown that increasing the hop count by 1 provides similar performance gain under both detection schemes. Further, a small increase in aperture size results in greater improvement under moderate turbulence rather than strong turbulence. The mutual information of the considered network is obtained with binary differential data, and it is demonstrated through results that, in the low-SNR region, the heterodyne receiver provides less mutual information as compared with the IM/DD receiver. However, beyond a particular SNR value (which depends on the hop count, aperture size, and turbulence strength), the heterodyne receiver attains more mutual information in comparison with the IM/DD receiver. The unified expression of outage probability for the DF-based multihop FSO network is also derived in terms of Meijer's G function. In addition, the asymptotic SER performance is examined in order to get impactful insight of network performance at high SNRs. All analytical results derived are verified through simulations.