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.

Previous orthogonal frequency division multiplexing over free space optics (OFDM-FSO) systems relied on signal strength, wavelength, and polarisation to multiplex data streams in order to improve signal quality and feasible connection range. Alternatively, this work leverages on orbital angular momentum (OAM) mode division multiplexing with multiple OAM modes (OAM + 1, OAM +2, OAM +3, OAM +4) encountering different modal coupling effects under atmospheric turbulence. Advanced modulation and coding schemes (QPSK, 16QAM, and 64QAM) are deployed to improve the bit-error rate (BER), packet error rate (PER), and achieve a connection range of 1000 m in a free space optical link. OAM +1 was shown to achieve average BER and PER of 10 -3 at SNR equals to 22 dB and 30 dB respectively.