# Impact of Weather Factors on Unmanned Aerial Vehicles' Wireless Communications
## 3 Results and Discussion
### Atmospheric Weather Variations
The trend of operating Doppler speed range doubling with corresponding doubling of the symbol rate, and hence sub-carrier spacing, is also seen for cases as depicted in Figure 13c,d. This can also be explained by the presence of the term \(\left(\pi N\frac{v_{r}f_{c}}{R_{S}c}\right)^{2}\) in Equation (16) that governs SNR degradation. As \(R_{s}\) is multiplied by a constant, to maintain the same value for the above governing term from Equation (16), \(v_{r}\) must be multiplied by the same constant. Other behavior, such as increased SNR degradation with the increase in \(N\), can also be explained on purely mathematical ground using the above governing term. In summary, the results show that relative motion caused Doppler shift is a major cause of SNR degradation in UAV communication. SNR degradation could be minimized by following several approaches. While speed reduction is one obvious choice, it may not be possible under all circumstances. To mitigate the effect of an increase in relative speed by a factor of \(k\), one of the following rudimentary approaches could be followed:

* Decrease the carrier frequency \(f_{c}\) by a factor of \(k\)
* Increase the symbol rate \(R_{s}\) by a factor of \(k\)
* Decrease the number of sub-carrier \(N\) by a factor \(p\): \(\frac{N}{p}\in Z^{+}\) and \(\frac{k}{p}\leq 1\) 4. ## 4 Conclusions

In this work, we examined the impact of atmospheric conditions, specifically atmospheric gases, clouds, fog, and rain, on bandwidth availability, utilizing signal attenuation and bit error rate across five different modulation schemes, employing weather conditions pertinent to UAV operations. Our findings revealed that atmospheric attenuation increases with both propagation distance and frequency, and precipitation further amplifies this effect. Temperature and pressure variations were found to have a minimal impact on attenuation over short distances. The analysis highlighted that the principal parameters influencing atmospheric attenuation are distance, frequency, and precipitation, resulting in an elevated BER. In the lower frequency (<10 GHz) bands, where currently available UAVs operate, a comparative analysis of the impact of atmospheric gases, clouds, fog, and rain on attenuation reveals that atmospheric gases have the least effect on short-range UAV command and control communications bandwidth. Although clouds and fog contribute to attenuation to a greater extent than gases, their impact is relatively minor in comparison to rain, which significantly affects attenuation due to the larger size of raindrops, resulting in increased scattering and absorption. While lower-order modulation schemes and higher power transmissions result in an increase of maximum operating frequency, a decrease in the same is observed with the reduction of power transmission and utilization of higher-order modulation schemes. Given that the attenuation increases with frequency, the utilization of higher operating frequencies, supported by higher operating power levels and lower-order modulation schemes, results in more pronounced bandwidth suboptimization under adverse weather conditions. The complex interaction between the power transmission, modulation-order, and weather conditions in influencing the maximum operating frequency represents a significant finding in our research, with potential applications in the development of spectrum management algorithms for UAV operation. The key takeaway message for UAV operators is that increment weather conditions and Doppler shift degrade UAV link performance and mitigation is possible using a variety of options, such as using low carrier frequencies, low modulation orders, and low speeds. For link budgeting purposes, at least 50 dB additional path loss should be included. Low frequency bands should be reserved for critical command and communications. Also, for UAV link operation, higher symbol rates should be supported at low carrier frequencies.