## 4 Enabling Technologies for FSO Communication
### Adaptive Transmission
#### 4.4.2 Adaptive Transmission Techniques
An adaptive symbol rate, where the symbol duration adjusts based on channel conditions, along with the simultaneous adaptation of both the symbol rate and power, has been proposed to mitigate the effects of scintillation in FSO links [20]. In [14], the challenges of adaptive FSO transmission over AT channels are revisited, considering practical power constraints and modulation sizes. Two scenarios, (1) power adaptation with a fixed modulation size and (2) joint adaptation of power and modulation size, are explored. Also, an adaptive uncoded transmission algorithm that adjusts both transmit power and modulation size based on channel conditions is introduced. The optimization focuses on these adaptive parameters while adhering to peak power constraints. Simulation results indicate that these adaptive algorithms significantly enhance performance, particularly in conditions of intense turbulence. In [222], a coded FSO system utilizing a fixed code rate and M-ary PPM under an average power constraint is investigated. The simulation results show that the proposed adaptive scheme offers considerable performance enhancements in relation to non-adaptive alternatives. In [226], the study examines rate-adaptive FSO communication using experimental data collected from a 1.87 km terrestrial FSO link under various weather conditions. To address the fluctuations in channel gain caused by AT and weather changes, a rate-adaptive communication system is implemented that punctures LDPC codes in a rate-compatible manner. In addition to the standard random puncturing method, an optimized intentional puncturing technique is also utilized. The evaluation results indicate that, unlike uncoded OOK, the rate-adaptive method ensures reliable and efficient FSO communication across different weather conditions and scintillation indices. Furthermore, applying intentional puncturing enhances the system's efficiency in terms of throughput. Moreover, a novel adaptive transmission technique for FSO systems, designed to operate under AT and utilizing subcarrier PSK (S-PSK) intensity modulation, is introduced in [227]. By leveraging the fixed envelope properties of S-PSK, this approach enhances the effective use of the FSO channel capacity by adjusting the modulation order of S-PSK in response to real-time turbulence-induced fading and a specified BER target. Numerical results demonstrate considerable gains in SE without increasing the average transmitted optical power or compromising BER, particularly in conditions of moderate to strong turbulence. Additionally, this variable-rate transmission technique is utilized in MIMO FSO systems, leading to further improvements in SE as the number of transmit and/or receive apertures increases. The performance of an FSO system for ground-to-satellite communication is examined in [228], focusing on the combined impacts of AT and beam wandering using MPSK. To enhance SE, an adaptive transmission strategy is implemented, optimizing channel capacity by adjusting the modulation order of the MPSK scheme based on the current channel conditions and the target BER. Additionally, a single-input multiple-output (SIMO) system employing MRC diversity is utilized to alleviate the impacts of AT and beam wandering. Optical power allocation strategies for an adaptive Wavelength Division Multiplexing (WDM) radio over FSO (RoFSO) systems, utilizing variable wavelengths to reduce the impact of weather turbulence, are introduced in [220; 229]. These strategies optimally distribute transmit power to maximize capacity and enhance the overall effectiveness of the WDM RoFSO system. Thus, within a total power constraint, varying optical powers can be assigned to selected wavelengths, leading to further enhancements in system performance [16]. A practical FSO system featuring an adaptive channel coding and power control scheme is examined in [221].