# Post-Flood UAV-Based Free Space Optics Recovery Communications with Spatial Mode Diversity
###### Abstract  
The deployment of unmanned aerial vehicles (UAVs) for free space optical communications is an attractive solution for forwarding the vital health information of victims from a flood-stricken area to neighboring ground base stations during rescue operations. A critical challenge to this is maintaining an acceptable signal quality between the ground base station and UAV-based free space optics relay. This is largely unattainable due to rapid UAV propeller and body movements, which result in fluctuations in the beam alignment and frequent link failures. To address this issue, linearly polarized Laguerre-Gaussian modes were leveraged for spatial mode diversity to prevent link failures over a 400 m link. Spatial mode diversity successfully improved the bit error rate by 38% to 55%. This was due to a 10% to 19% increase in the predominant mode power from spatial mode diversity. The time-varying channel matrix indicated the presence of nonlinear deterministic chaos. This opens up new possibilities for research on state-space reconstruction of the channel matrix.  
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capacity and wide coverage. This requires the design of satellite constellations comprising multiple satellites across several countries for reliable coverage in the flood-stricken area, and to backhaul unflooded cells via satellite [12; 13; 14]. High-altitude platforms (HAPs) such as airships, aerostats, and balloons have been used to ferry antennas in the affected area, for relaying data to neighboring ground base stations to a large number of users [15]. HAPs are located in the stratosphere, enabling a wide coverage and accommodating a large number of users [16]. Compared to satellites, HAPs provide a higher area throughput and resource utilization [15]. HAPs can also distribute the recording of the orbital paths of satellites and monitor the probability of a collision between satellites [17]. Data transmission from a HAP involves the control of the HAP flight trajectory and the transmit power of the HAP antenna [18; 19; 20].  
Figure 1: UAV-based spatial mode diversity free space optics (FSO) system for post-flood recovery communications.  
Recently, the deployment of unmanned aerial vehicles (UAVs) during natural disasters has become increasingly prevalent for forwarding information from the affected area to undamaged ground base stations during rescue operations [21]. UAVs generally have limited onboard energy [22]. Typically, several UAVs are connected in topologies such as a mesh, star, or cluster. UAVs are linked to each other in order to reach the undamaged ground base station, in conjunction with routing and positioning algorithms [23; 24; 25; 26].  
The remainder of the paper organized as follows. Section 2 provides an overview of free space optics (FSO) for post-flood recovery communications and related work on spatial mode diversity using FSO. Section 3 elucidates the novelty and contributions of the paper. Section 4 describes the design of a UAV-based spatial mode diversity FSO system for post-flood recovery communications. Section 5 reports on the performance analysis of the proposed system.