## 4 Enabling Technologies for FSO Communication
### MIMO FSO Systems  
A highly effective approach to address the dispersive nature of FSO channels involves utilizing multiple lasers and photodetectors, similar to the RF MIMO wireless communication systems. This configuration can enhance channel capacity linearly, scaling with the number of antennas [281, 47,148]. MIMO FSO systems are linked to either RC [265,268] or TLS [264,272]. In the case of RC, the same information symbol is sent from all apertures, allowing for implementation without CSI or feedback. Conversely, the TLS approach involves activating the laser that provides the highest irradiance at the Rx, necessitating both a feedback link and the acquisition of CSI [241]. MIMO FSO systems are particularly adept at mitigating AT fading by averaging its effects, thereby increasing the reliability of FSO links [91; 282]. Furthermore, the arrangement of multiple sources and detectors can indirectly assist with pointing issues. For instance, a horizontal rooftop transmit/receive array is likely to experience cyclostationary fading rather than complete pointing loss in the event of significant sway [265; 283]. In a MIMO FSO system, the laser sources and photodetectors are arranged such that the fading encountered between source-detector pairs is statistically independent, allowing for the exploitation of diversity benefits from the MIMO channel [244; 265; 148]. Additionally, because of the incoherence of the lasers across the array, their powers combine effectively at the Rx array. In practice, however, MIMO FSO systems may not consistently deliver the theoretical gains they promise. This limitation arises in situations where the assumption of independent channel fading across all links in the MIMO FSO system fails, often due to the proximity of the devices [244; 265]. In this context, the limited distance between adjacent optical sources at the Tx or neighboring photodetectors at the Rx can easily lead to spatially correlated fading, which reduces the efficiency of the MIMO FSO system [284; 285]. Also, because both the path loss and fading statistics of the channel depend on distance, a significant number of transmit and/or receive apertures is necessary for long-range links to attain the desired performance improvements. This requirement increases the complexity of MIMO FSO systems [244]. The performance of MIMO FSO systems has been explored in various previous studies. For instance, the BER performance of FSO links under LN AT with spatial diversity is examined in [263]. In [286], the study investigates MIMO FSO systems operating in strong turbulence channels, utilizing a single IT approximation for the distribution of the sum of independent IT variates and employing EGC at the Rx. Additionally, ref. [287] presents infinite series representations for the error performance of MIMO FSO systems over independent and identically distributed (i.i.d.) IT fading channels. Furthermore, in [288], the \(\alpha-\mu\) distribution is employed to approximate the sum of arbitrary i.i.d. IT variates. This approach leads to simple yet accurate approximations for the OP and average bit error probability (ABEP) of MIMO FSO systems that operate over i.i.d. IT channels with EGC at the Rx. For MIMO-FSO systems operating over correlated IT channels, numerous studies have utilized a simplified channel correlation model, such as the exponential correlation model [263; 289]. However, this exponential correlation model, originally developed for RF communications, is inadequate for accurately describing the correlated AT channels. Consequently, recent research has focused on exploring the characteristics of fading correlation induced by AT [258; 290; 291]. Specifically, studies [258; 290] employ wave optics and Monte Carlo simulations to elucidate the effects of turbulence strength, link span, aperture diameter, and separation on channel correlation for both receive and transmit diversity systems. However, since simulation methods can only focus on specific configurations, extending the results to general cases can be challenging.