## 4 Enabling Technologies for FSO Communication
### Spatial Diversity
#### 4.6.1 Overview  
It has been shown that the performance of SISO FSO links is significantly impacted by strong turbulence, falling short of the typical BER targets for FSO applications within practical SNR ranges. This highlights the need for the implementation of robust fading-mitigation techniques [252]. Historically, various techniques have been employed in FSO systems to address the detrimental effects of the atmospheric channel. These include error control coding combined with interleaving [165; 253] and multiple-symbol detection through MLSD [47; 212]. However, both methods present certain practical challenges [252]. For example, relying solely on an ECC is not an effective strategy for mitigating fades, as it necessitates a significantly large interleaver, around 100 Gb for a 10 Gb/s channel [254; 255; 256]. Moreover, when the link functions as the PHY layer of a data network, the upper layer protocols struggle to perform optimally in the presence of these fades and the prolonged delays associated with interleaving and deinterleaving, resulting in reduced throughput and poor efficiency [254; 233]. It has also been noted that channel coding is effective primarily in weak turbulence conditions or when a large aperture size is utilized. In reality, coding is not efficient in countering fading. Its advantages become more pronounced only in low fading channels or when the effects of turbulence are significantly mitigated, for instance, through techniques like aperture averaging, AO, or spatial diversity [233]. Conversely, MLSD is hindered by high computational complexity [47; 252]. Another promising approach is the implementation of spatial diversity, a well-established technique in RF systems. By utilizing multiple apertures at either the Tx or Rx, spatial diversity can significantly enhance performance by introducing additional DoF in the spatial dimension. This method is particularly appealing for reducing the frequency and duration of signal fades [256]. It is especially critical in strong turbulence channels, where SISO links demonstrate extremely poor performance (i.e., exceeding \(10^{-3}\) in the SNR range of 30-50 dB) [252]. Furthermore, spatial diversity minimizes the likelihood of temporal blockage of laser beams caused by obstructions, enabling communication over longer distances even in challenging weather conditions [252]. Furthermore, unlike spatial diversity in wireless systems, spatial diversity in atmospheric optical systems can be implemented compactly, as the coherence length is typically on the order of centimeters. This means that multiple Txs or Rxs can be positioned just centimeters apart to experience nearly independent channel fades [254; 255; 255]. Consequently, effective fading reduction can be achieved through spatial diversity techniques, such as employing multiple beams at the Tx (MISO) [257; 258], using multiple apertures at the Rx (SIMO) [259; 233; 245], or utilizing a combination of both (MIMO) [260; 261; 244]. Among these approaches, using multiple lenses at the Rx is the simplest, offering a balance between gain and system complexity in achieving spatial diversity [233; 262]. The overall spatial diversity system configuration, illustrated in Figure 11, consists of \(M\) Txs and \(N\) Rxs, where \(M,~{}N\geq 1\) and at least one of \(M\) or \(N\) is greater than one. The outputs from the Rxs can be combined in various ways to generate the final observations, which are used to determine whether a 0 bit or a 1 bit is transmitted [91; 255]. In various application scenarios, it is reasonable to assume that the Tx subapertures are spaced apart by more than a coherence length, which is particularly significant for ground-to-space links [254; 255]. FSO links utilizing transmit and receive diversity can be effectively modeled as equivalent SISO systems by appropriately scaling the channel variance. In essence, spatial diversity leads to a reduction in channel variance.