Nowadays, unmanned aerial vehicle (UAV) communication systems are commonly considered as one of the key enabling technologies for 6G. The hybrid free space optical (FSO)/radio frequency (RF) system has the advantages of both FSO and RF links to improve communication system performance, and the relay-assisted system adopts multi-hop transmission and cooperative diversity methods to extend communication coverage. Thus, a joint consideration of UAV-assistedUAV assisted relay in hybrid FSO/RF transmission is meaningful. In this paper, we aim to analyze the performance of UAV-assisted multi-hop parallel hybrid FSO/RF communication systems with and without pointing errors (PE) in terms of Bit Error Rate (BER) and outage probability. In our considered system, the FSO sub-link adopts the Exponential Weibull turbulence model and the RF sub-link suffers the Nakagami fading model. With these, new mathematical formulas of both BER and outage probability are derived under the UAV-assisted hybrid FSO/RF with different modulation methods. Through numerical evaluationnumerical simulations, the performances of UAV-assisted hybrid FSO/RF systems are analyzed under different weather conditions, modulation methods, optical receiver aperture, RF fading parameters, pointing errors, and relay structures. The results demonstrate that (1) compared to hybrid FSO/RF direct links, UAV-assisted hybrid FSO/RF systems can further improve system performance; (2) the performance of UAV-assisted hybrid FSO/RF systems varies with different relay structures; (3) large receiver aperture and RF fading parameters can further improve the communication performance of hybrid FSO/RF direct links and UAV-assisted hybrid FSO/RF systems.

Due to the influence of the atmospheric environment and pointing errors, the performance of free space optical communication is greatly limited. In this paper, we propose a parallel multi-hop hybrid free space optical (FSO)/radio frequency (RF) system to improve the system performance. The FSO sub-link and RF sub-link are modeled by Gamma–Gamma turbulence with pointing errors and Nakagami-m distributions, respectively. Based on the selective combination scheme, the probability density function (PDF) and cumulative distribution function (CDF) of the output signal-to-noise ratio (SNR) of the hybrid FSO/RF one-hop or direct link are obtained. Then, the PDF and CDF of the output SNR of the parallel multi-hop hybrid system are derived with the decoded forward (DF) protocol considered. Finally, the expressions of the average bit error rate (ABER) and outage probability are derived for the parallel multi-hop hybrid system, the hybrid FSO/RF direct link, and the FSO-only direct link. The results show that the parallel multi-hop hybrid system can effectively mitigate the negative impact of atmospheric turbulence and pointing errors and can significantly improve the system performance.

To improve the performance of fee-space optical communication systems, this paper analyzes the performance of a relay-aided hybrid fee-space optical (FSO)/radio frequency (RF) cooperation system based on a selective combination and decoding forward transmission scheme. In this system, the FSO sub-link experienced Málaga turbulence with pointing errors and the RF sub-link suffered Nakagami-m fading. Firstly, the probability density function (PDF) and cumulative distribution function (CDF) of the end-to-end output signal-to-noise ratio (SNR) of the relay-aided hybrid FSO/RF system are derived. Then, using the extended generalized bivariate Meijer’s G-function (EGBMGF) and the approximate analytical formula of the generalized Gauss–Laguerre integral, mathematical expressions of the end-to-end average bit error rate (ABER) and outage probability of the relay-aided hybrid FSO/RF system with different subcarrier intensity modulation and different detection schemes are derived. Through a simulation analysis of the system, the results show that compared with the other three modulation technologies, the hybrid FSO/RF direct link and relay-aided hybrid FSO/RF system with coherent binary phase shift keying (CBPSK) modulation have the best bit error performance. Compared with direct detection, the hybrid direct link and relay-aided hybrid system with coherent detection can significantly improve the communication performance. Increasing the RF fading parameter m can further improve the bit error and outage performance of the hybrid direct link and relay-aided hybrid system; the hybrid direct link can significantly mitigate the degradation of communication performance in the FSO system caused by pointing errors, and the relay-aided hybrid system can further improve the communication performance; under weak turbulence conditions, the impact of pointing errors on the performance of the relay-aided hybrid system can even be ignored. The greater the total number of paths in the relay-aided hybrid system, the better the communication performance of the system; however, the more hops, the worse the performance of the system. The outage probability of the hybrid direct link and relay-aided hybrid system are very sensitive to the decision threshold, and the larger the decision threshold, the worse the outage performance. The transmission distance of different hybrid direct links has little impact on the performance of hybrid direct links and relay-aided hybrid systems. Improving the signal-to-noise ratio of RF sub-links significantly improves the performance of hybrid direct links and relay-aided hybrid systems.

Hybrid free space optical (FSO)/radio frequency (RF) system has attracted extensive attention because of its advantages of both the FSO and RF links. From the viewpoint of overall system performance, this paper presents a systematic analysis method of communication performance and security performance of the hybrid FSO/RF system with the Málaga turbulence channel and the $\alpha -\mu$ fading channel. The hybrid FSO/RF system adopts the diversity method of maximum ratio combining (MRC) to receive signals. The new expressions of communication performance parameters (i.e., the bit error rate, the outage probability, the ergodic channel capacity) of the only FSO system and the hybrid system are obtained. Then, the new expressions of the security performance parameters (i.e., the security outage probability and the strictly positive secrecy capacity) of the hybrid system with the FSO or RF links eavesdropping are derived, respectively. Our derived analytical expressions present an efficient tool to investigate the impact of system parameters on the overall performance of the hybrid system, namely modulation scheme, turbulence intensity, pointing errors, target rate, and eavesdropper output signal-to-noise ratio. The simulation results show that compared with the only FSO system, the hybrid system can significantly improve the communication performance of the system; the communication performance of the hybrid system using coherent binary phase shift keying (CBPSK) modulation is obviously better than the other two modulation technologies; with the deterioration of atmospheric environment (increasing turbulence intensity and pointing errors), the communication performance and security performance of the hybrid system will decline; both RF link eavesdropping and FSO link eavesdropping have a greater impact on the security performance of the hybrid systems; whether it is FSO link eavesdropping or RF link eavesdropping, the reduction of target rate and output signal-to-noise ratio of the eavesdropper can improve the security performance of the hybrid system.

The performance of Low-Density Parity-Check (LDPC)-coded Orthogonal Frequency Division Multiplexing (OFDM) is investigated over turbulence channels in underwater wireless optical communications (UWOC). The relation between the bit error ratio (BER) and parameters such as the scintillation coefficient, signal-to-noise ratio (SNR), length of LDPC code, and order of OFDM is quantified by simulation. Results show that while the OFDM with subcarrier quadrature amplitude modulation (QAM-OFDM) has slightly better anti-turbulence performance than the OFDM with subcarrier phase shift keying modulation (PSK-OFDM), the LDPC-coded QAM-OFDM has a much better performance than the QAM-OFDM and the LDPC-coded PSK-OFDM, and, at SNR = 12, it decreases the BER by four orders of magnitude compared to the 16QAM-OFDM system when the scintillation coefficient ${\sigma }_{\xi }^2$= 0.05.