Figure 2 elucidates the proposed UAV-to-ground terminal FSO transmission link using OFDM signals. 20 Gbps binary information is generated using a PRBS and mapped onto 4-QAM symbols using 2-bits/symbol. The 4-QAM signal is directed to the OFDM modulator performing various signal processing, including serial-to-parallel conversion of binary data, inverse fast Fourier transformation (IFFT) algorithm, the addition of cyclic prefix (CP), parallel-to-serial conversion, digital-to-analog conversion, and signal filtering. The OFDM component specifications are 1024-IFFT points, 512-subcarriers, 15 dBm average power, and 32-CP. The OFDM electrical signal is in-phase quadrature modulated at 7.5 GHz frequency and then optically modulated over a laser beam from continuous wave laser using a Mach-Zehnder modulator (MZM). The optical beam bearing information is then transported towards to ground station through the free-space channel. At the ground station, direct detection is used for retrieving data. The PIN-photodetector converts the optical beam to the electrical signal, further demodulated using OFDM and quadrature demodulator sections. Finally, the information is decoded using the 4-QAM sequence decoder.

The free-space channel between the UAV and ground terminal is mainly affected by attenuation offered by external climate and turbulent atmospheric conditions. The path loss \({h}_{l}^{p}\) can be estimated using exponential Beer-Lambert Law [15]:

$${h}_{l}^{p}=\text{exp}(-\sigma {L}_{p})$$

1

where \(\sigma\) signifies the specific coefficient of attenuation for external climate conditions and \({L}_{p}\) is the transmission distance from the UAV and ground station, which is given as:

where \({H}_{p}\) signifies the height of the UAV from sea level and \({\zeta }_{p}\) is the zenith angle which is defined as the angle between UAV and ground station. Atmospheric turbulence \({h}_{a}^{p}\) is a random phenomenon due to random fluctuations in the atmospheric refractive index structure profile due to inhomogeneities in the external pressure and temperature conditions, leading to optical turbulent eddies. These optical eddies vary the phase and intensity of the optical signal at the receiver plane and increase the BER. We have used the Gamma-Gamma turbulence model in the reported work given as [16]:

$${f}_{{h}_{a}^{p}}= \frac{2{\left(\alpha \beta \right)}^{\frac{\alpha +\beta }{2}}}{{\Gamma }\left(\alpha \right){\Gamma }\left(\beta \right)}{{(h}_{a}^{p})}^{\left(\frac{\alpha +\beta }{2}\right)-1}{K}_{\alpha -\beta }\left(2\sqrt{\alpha \beta {h}_{a}^{p}}\right)$$

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where \({K}_{\alpha -\beta }\)(.) signifies the modified-Bessel function having \((\alpha -\beta\)) order, \({\Gamma }\) signifies the Gamma function, \(\alpha ,\) and \(\beta\) signify the number of large-scale and small-scale eddies given as [17]:

$$\alpha = {\left\{\text{exp}\left[\frac{0.49{\sigma }_{R}^{2}}{{\left(1+1.11{\sigma }_{R}^{12/5}\right)}^{\frac{7}{6}}}\right]- 1\right\}}^{-1}$$

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$$\beta = {\left\{\text{exp}\left[\frac{0.51{\sigma }_{R}^{2}}{{\left(1+0.69{\sigma }_{R}^{12/5}\right)}^{\frac{5}{6}}}\right]- 1\right\}}^{-1}$$

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where \({\sigma }_{R}^{2}\) signifies the Roytov variance and is expressed as [18]:

$${\sigma }_{R}^{2}=2.25{k}^{\frac{7}{6}}{sec}^{\frac{11}{6}}\left({\zeta }_{p}\right){\int }_{{h}_{0}}^{{H}_{p}}{C}_{n}^{2}\left(h\right){(h- {h}_{0})}^{5/6}dh$$

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where k signifies the wavenumber,\({h}_{0}\) represents the height of the ground terminal, \({C}_{n}^{2}\) signifies the refractive index structure parameter, which can be expressed using the Hufnagel-Valley model as [19]:

$${C}_{n}^{2} \left(h\right)=0.00594{\left(\frac{\omega }{27}\right)}^{2}{\left({10}^{-5}h\right)}^{10}\text{exp}\left(\frac{-h}{1000}\right)+2.7 \times {10}^{-16}\text{exp}\left(\frac{-h}{1500}\right)+ {C}_{n}^{2}\left(0\right)\text{exp}\left(\frac{-h}{100}\right)$$

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where \(\omega\) signifies RMS value of air velocity, \(h\) signifies the height from the sea level, \({C}_{n}^{2}\left(0\right)\) is the ground value of \({C}_{n}^{2}\) which is \(1.7 \times {10}^{-14} {m}^{-2/3}\).