Improving Performance of Integrated Ground-HAPS FSO Communication Links With MIMO Application

In this paper, we investigate the performance of multiple-input multiple-output (MIMO) free-space optical (FSO) communication systems, encompassing both slant path (downlink and uplink) and horizontal links. The FSO links span ground-to-high altitude platform system (HAPS), HAPS-to-ground, and HAPS-to-HAPS configurations. To model the atmospheric turbulence-induced fading, pointing error impairments, and angle-of-arrival (AOA) fluctuations, we employ the Gamma-Gamma distribution, Hoyt distribution, and Rayleigh distribution, respectively. We then derive the probability density function (PDF) of the end-to-end signal-to-noise ratio (SNR) under the combined effect of turbulence, AOA fluctuations, attenuation due to absorption and scattering, and generalized pointing error impairments. Capitalizing on this PDF expression, we present novel results for fundamental performance metrics, including outage probability, average bit-error rate (BER) of on-off keying (OOK) modulation scheme, and ergodic capacity expressed in terms of the Meijer's G function. Additionally, we provide highly accurate asymptotic results for the outage probability and the average BER at high SNR, obtained in terms of simpler functions. Furthermore, we demonstrate that the integration of MIMO configurations in either the transmitter, receiver, or both significantly enhances the performance of FSO communication links. To validate our analytical results, computer-based Monte-Carlo simulations are conducted, affirming the reliability of our findings.