Performance analysis and optimization of a UAV relay-assisted FSO communication systems

This paper investigates and optimizes unmanned aerial vehicle (UAV) relay-assisted free-space optics (FSO) communication systems under the effects of pointing errors (PEs) and atmospheric turbulences. The incorporation of UAVs as buffer-aided moving relays in the conventional FSO (CFSO) relay-assisted systems is proposed for enhancing the performance of PEs through AT. Using M-PSK (phase shift keying) and M-QAM (quadrature amplitude modulation), the impact of PEs on transmission quality is evaluated in this work. We evaluate and enhance the symbol error rate, outage probability (OP), and signal-to-noise ratio (SNR) for the UAV-to-ground station-based FSO communications systems. The spatial diversity-based relay-assisted CFSO systems can enhance the performance of the UAV-UAV FSO links. Numerical results show that to achieve a bit-error-rate (BER) of \(\le {\text{10}}^{\text{-5}}\), the required SNR is ≥ 23 dB when the wind variance of the UAVs \({\sigma }_{\alpha }^{2}\) increases from 0 to 7 mrad with FSO link distance L = 2000 m. The required SNR is ≥ 25 dB when the wind variance \({\sigma }_{\alpha }^{2}\) is 1 mrad at a OP of \({\text{10}}^{\text{-}\text{6}}\). To obtain an average BER of \({10}^{-6}\), the SNR should be 16.23 dB, 17.64 dB, and 21.45 dB when \({\sigma }_{\alpha }^{2}\) is 0 mrad, 1 mrad, and 2 mrad, respectively. Using 8-PSK modulation without PEs requires 23.5 dB at BER of \({\text{10}}^{\text{-8}}\) while 16-QAM without PEs requires 26.5 dB to maintain the same BER of \({\text{10}}^{\text{-8}}\). Compared with 16-QAM without PEs, the SNR gain of 8-PSK without PEs is 3 dB.