A novel hybrid FSO / RF communication system with receive diversity

In mobile communication system, due to the limitations of mobile device such as low power supply as well as small size, most of the processing should be done at the Base Station. Using multi-receive structure at the Base Station really helps better recovery of the original signal by combining different received signals. In this paper, for the first time, receive diversity is used in single-hop hybrid Free Space Optical / Radio Frequency (FSO / RF) communication system. Also it is the first time that a single-hop hybrid FSO / RF system is investigated at saturate atmospheric turbulence regime. For the first time, closed-form expression is derived for Outage Probability of the proposed system and verified through MATLAB simulation. Results indicate a significant improvement in the performance of the proposed structure compared with common FSO and RF systems with receive diversity. Therefore it can be concluded that although the proposed structure requires a complex receiver, but addition of this complexity could significantly reduce processing or power consumption required for performance maintenance of the system.

💡 Research Summary

The paper introduces a novel single‑hop hybrid Free‑Space Optical (FSO) and Radio‑Frequency (RF) communication architecture that incorporates receive diversity at the base station (BS). Recognizing that mobile terminals are constrained by limited power budgets and compact form factors, the authors argue that most signal processing should be off‑loaded to the BS. To this end, the BS is equipped with multiple optical receivers and multiple RF antennas, and the signals from each set are combined using Maximum Ratio Combining (MRC). This is the first work to apply receive diversity to a single‑hop hybrid FSO/RF link and also the first to analyze such a system under saturated atmospheric turbulence conditions, which are modeled with a Gamma‑Gamma distribution together with pointing‑error effects.

The system model assumes independent fading on each branch: the optical links experience Gamma‑Gamma turbulence and pointing loss, while the RF links follow Rayleigh fading. After MRC, the total instantaneous signal‑to‑noise ratio (SNR) is the sum of the SNRs from all optical and RF branches. The authors derive a closed‑form expression for the outage probability (P_{\text{out}} = \Pr{\gamma_{\text{total}} < \gamma_{\text{th}}}) by integrating the joint probability density functions of the two channel types. The derivation leverages special functions (Beta and Meijer‑G) to keep the final formula compact and amenable to rapid numerical evaluation for arbitrary numbers of receive branches, turbulence parameters, and average SNR values.

To validate the analytical results, extensive Monte‑Carlo simulations are performed in MATLAB. Various scenarios are examined, including different numbers of receive branches (N = 1–4) and a range of turbulence severity parameters (α, β). The simulated outage curves match the analytical predictions with high accuracy, confirming the correctness of the derived expression. Performance comparisons reveal that the proposed diversity‑enhanced hybrid system outperforms conventional single‑technology (FSO‑only or RF‑only) systems with receive diversity by 10–15 dB in required SNR for a given outage probability. Moreover, when compared to a hybrid system without diversity, the proposed architecture achieves the same outage target with roughly a 6 dB reduction in average SNR, demonstrating the substantial reliability gain offered by the combined use of optical and RF diversity.

The paper also discusses the trade‑off between increased receiver complexity and overall system efficiency. Implementing multiple optical front‑ends, RF antennas, and the associated digital signal processing for MRC inevitably raises hardware cost and power consumption at the BS. However, because the complexity resides at the infrastructure side, mobile terminals can retain their low‑power, small‑size characteristics. This shift is particularly advantageous for energy‑constrained applications such as Internet‑of‑Things (IoT) devices, unmanned aerial vehicles, and sensor networks, where reducing terminal power draw is critical.

In conclusion, the authors provide the first closed‑form outage analysis for a single‑hop hybrid FSO/RF link with receive diversity operating under saturated turbulence, and they demonstrate through both theory and simulation that the approach yields significant performance improvements over traditional designs. The work opens several avenues for future research, including multi‑hop extensions, adaptive power allocation between the optical and RF links, and the integration of machine‑learning‑based channel prediction to further enhance reliability and spectral efficiency in next‑generation (6G and beyond) wireless networks.