## 4 Enabling Technologies for FSO Communication
### Spatial Diversity
#### 4.6.2 MISO, SIMO, and MIMO Free-Space Optical Systems  
To improve the reliability and performance of FSO communication systems under varying atmospheric conditions, advanced configurations such as Multiple-Input Single-Output (MISO), SIMO, and MIMO are employed. This section delves into these configurations, exploring their architectures, benefits, and the unique challenges they address in the realm of FSO communications. By leveraging multiple Txs and/or Rxs, these systems aim to alleviate the impacts of AT and enhance link robustness and capacity. Transmit Diversity FSO Systems  
Utilizing optical arrays, akin to antenna-array technology in microwave systems, is viewed as a strategy for addressing the fading effects of FSO channels. In particular, MISO (transmit diversity) FSO systems are based on spatial diversity methods with multiple lasers at the Tx end, all of which are pointed towards a distant photodetector. In these systems, the sources and the detector are positioned such that the Rx can simultaneously observe all Txs [264]. Also, this diversity technique has been used to improve PLS in FSO systems [60]. In FSO systems utilizing IM/DD, heuristic STC designs, including RC and orthogonal space-time block codes (OSTBCs), have been suggested as simple alternatives to the MISO PSO diversity scheme [265; 266; 267]. In the RC system, the same information signal is simultaneously transmitted from all the transmit apertures (laser sources). This is highly effective for reducing fading at the Rx [1; 263], and the superiority in RC performance has been shown in [268]. For instance, research indicates that the performance of Alamouti code is inferior to that of RC over FSO links [266; 269; 270]. Additionally, it has been demonstrated that OSTBC yields worse performance than RC in FSO systems utilizing IM/DD and OOK. Furthermore, studies have shown that RC outperforms OSTBC in a MIMO FSO system using SIM [268; 269; 271]. Additionally, when CSI is accessible at the Tx, a transmit diversity technique can be employed that selects the optical path with the highest scintillation value. Under IM/DD, this selection transmit diversity (STD) method, also referred to as transmit laser selection (TLS) can achieve full diversity while outperforming other alternatives, such as STCs and RCs. These alternatives implement transmit diversity without relying on CSI, meaning they do not require any feedback from the Rx to the Tx [264; 272]. In scenarios where the Tx has imperfect CSI, various transmission strategies are explored by proposing an extension of the STD scheme to situations where a feedback with a limited number of bits is available [272]. In this scheme, the transmit apertures are divided  
Figure 11: Generic spatial diversity setup. into various partition sizes, and one partition is selected for signal transmission based on feedback bits, utilizing RC. However, optimizing the partition selection is quite complex and necessitates the numerical solution of an infinite integral [273]. Additionally, more intricate signaling schemes can be employed to enhance coding gain alongside the benefits of diversity. For example, a combination of STD and space-time trellis coding (STTC) for MISO FSO communication systems using IM/DD over strong AT channels is analyzed in [274]. Additionally, since a feedback-based beamforming scheme enhances channel-adaptive signaling in wireless communication systems and significantly improves performance, an imperfect quantized feedback-based beamforming approach for a generalized MISO FSO system operating over \(\Gamma\Gamma\) fading channels with PE is proposed in [275]. Similarly, a simplified version of the STD scheme, which utilizes only one-bit feedback regarding the instantaneous CSI available at the Tx in a MIMO FSO system using SIM BPSK and functioning under \(\Gamma\Gamma\) fading, is introduced to mitigate the high sensitivity to feedback errors associated with the STD method [273].