# Post-Flood UAV-Based Free Space Optics Recovery Communications with Spatial Mode Diversity
## 2 Free Space Optics for Post-Flood Recovery Communications  
Free space optics is an attractive solution during floods when conventional base stations spanning several cities have been damaged, providing connectivity to the flood-stricken area and ensuring minimal disruption in unaffected areas. Free space optics provides a high data bandwidth and rapid deployment, without spectrum licensing costs [27; 28]. In addition, the light signals can penetrate easily through water droplets [29]. Although satellites and HAPs cover a larger area, the deployment time is longer [30]. Motivated by the rapid deployment and agility of UAVs for recovery communications [22; 30], a UAV-based FSO is proposed as a wireless access technology for rapid recovery communications during floods. Several UAV-based FSO strategies have previously been developed for recovery communications between base stations for recovery communications, focusing on downlink scheduling [31], and the placement of FSO transceivers [32; 33], divergence angle [34], or hybrid radio frequency (RF)/FSO links [35; 36]. An unveiled challenge to UAV-based FSO systems is to maintain a high SNR between the base station and UAV-based FSO relay. This is largely unattainable due to minute but rapid UAV propeller and body movements, which result in swift fluctuations in the beam alignment and water scattering from flood waters. Consequently, this would lead to frequent link failures. To address this issue, spatial mode diversity is proposed for improving the resilience of the system from UAV movements. Harnessing spatial modes as independent information carriers using space division multiplexing (SDM) techniques has recently gained traction for tackling the impending data capacity crunch [37]. Spatial modes provide an additional degree of freedom in wireless communications. Characteristically, in a SDM system, independent data-carrying beams are structured on distinct spatial modes that can be multiplexed at the transmitter aperture for co-transmission and demultiplexed at the receiver aperture, with minimal interference [37]. To increase the resilience of an FSO system, multiplexing in the amplitude, frequency, polarization, and time domains have been employed. Recently, a new degree of freedom based on spatial modes is being explored. The simultaneous transmission of several spatial modes can be realized without the separation between apertures, thus reducing the device footprint. Individual spatial modes encounter different refractive index perturbations from atmospheric turbulence, despite propagating in the same path [38; 39]. Spatial modes may be leveraged for transmission of independent data streams to improve link reliability and prevent network interruption. Several approaches have been demonstrated for spatial mode diversity in FSO systems. In [40], spatial mode diversity was employed in conjunction with multiple transmitter-receiver aperture pairs to improve FSO link reliability under atmospheric turbulence. Three aperture pairs were used and each aperture pair utilized a Gaussian beam and an OAM beam in the uplink and downlink direction simultaneously for carrying the same data stream. In [41], spatial mode diversity from a three-mode photonic lantern coupling FSO receiver was designed in conjunction with a digital maximal ratio combining to enhance the worst signal-to-noise ratio (SNR) by more than 10 dB and to mitigate the interruption probability under atmospheric turbulence. In [42], the three-mode diversity reception in free space optical links from low-Earth orbital satellites to a ground terminal was demonstrated by mode coupling to a few-mode fiber for tracking power variations under different elevation angles.