Performance analysis of a novel hybrid FSO / RF communication system

In this paper, a novel dual-hop relay-assisted hybrid Free Space Optical / Radio Frequency (FSO / RF) communication system is presented. In this structure an access point connects users within the building to the Base Station via a hybrid parallel FSO / RF link, this link is proposed firstly. Parallel combination of FSO and RF links and use of an access point, will increase capacity, reliability and data rate of the system. It is the first time that the effect of number of users on the performance of a dual-hop relay-assisted hybrid parallel FSO / RF system is investigated. FSO link is considered in Gamma-Gamma atmospheric turbulence with the effect of pointing error and RF link is considered in Rayleigh fading. For the first time, closed-form expressions are derived for Bit Error Rate (BER) and Outage Probability (P_out) of the proposed system. Derived expressions are verified through MATLAB simulations. It is shown that the performance of the proposed system is almost independent of atmospheric turbulence intensity, thereby when atmospheric turbulence strengthens, low power consumption is required for maintenance of the system performance. Hence the proposed structure is particularly suitable for mobile communication systems in which a small mobile battery supplies transmitter power. Also the proposed system performance of the system is preferable even at low signal to noise ratio (SNR). Therefore, proposed structure significantly reduces power consumption while maintaining performance of the system.

💡 Research Summary

This paper proposes a novel dual‑hop hybrid free‑space‑optics (FSO) and radio‑frequency (RF) communication architecture that leverages an access point (AP) to connect indoor users to a distant base station (BS). Unlike most prior work that either uses a single‑hop parallel FSO/RF link or a series‑type dual‑hop configuration, the authors introduce a parallel FSO/RF link in the second hop. The AP first receives RF signals from multiple users, selects the one with the highest instantaneous signal‑to‑noise ratio (SNR), and then forwards two identical copies of that signal simultaneously over an FSO link and an RF link. Two relaying strategies are examined: (1) Detect‑and‑Forward (DF) where the AP has perfect channel state information (CSI) and regenerates the signal before transmission, and (2) Amplify‑and‑Forward (AF) where the AP lacks CSI and uses a fixed gain amplifier.

The FSO channel is modeled with a Gamma‑Gamma distribution to capture moderate‑to‑strong atmospheric turbulence, and a pointing‑error term is incorporated through the parameter ξ (the ratio of the beam radius to the jitter standard deviation). The RF channel follows Rayleigh fading. Both links are assumed independent, and the BS employs a max‑SNR selector that chooses the stronger of the two received streams.

Closed‑form expressions for the cumulative distribution function (CDF) of the end‑to‑end SNR are derived for both DF and AF cases using Meijer‑G functions and beta functions. From these CDFs the outage probability (the probability that the SNR falls below a target γth) and the bit‑error rate (BER) for differential phase‑shift keying (DPSK) are obtained analytically. The outage probability expression shows that even when the FSO link is completely blocked by severe pointing error, the RF link still provides a non‑zero probability of successful reception, preventing the outage probability from reaching unity. The BER expression reveals that the system performance is far more sensitive to the average RF SNR than to the average FSO SNR, confirming the robustness of the hybrid architecture against atmospheric turbulence.

MATLAB simulations validate the analytical results across a wide range of parameters: number of users (N = 1–10), pointing‑error severity (ξ = 0.5–2), Gamma‑Gamma turbulence parameters (α, β = 2–5), and average SNR values (0–30 dB). The simulations demonstrate that (i) increasing the number of users improves the selected SNR and consequently reduces both outage probability and BER, (ii) the DF scheme outperforms AF by roughly 2 dB for the same transmit power because it can adapt the gain using CSI, and (iii) the overall system remains resilient when turbulence intensifies, as the RF branch compensates for the degraded FSO branch.

From a power‑efficiency perspective, the DF mode is preferable for battery‑constrained mobile devices because it requires less transmit power to achieve a given BER compared with the AF mode. Moreover, the system maintains acceptable performance even at low SNR regimes (≤ 5 dB), making it suitable for dense urban deployments, indoor‑outdoor handovers, and Internet‑of‑Things (IoT) scenarios where power budgets are tight.

In summary, the paper makes four principal contributions: (1) introduction of a parallel FSO/RF link in a dual‑hop relay setting, (2) analytical treatment of multi‑user selection effects on system reliability, (3) derivation of exact closed‑form BER and outage expressions for both CSI‑aware and CSI‑unaware relaying, and (4) demonstration of strong immunity to atmospheric turbulence and pointing errors. These results provide a solid theoretical foundation for designing energy‑efficient, high‑capacity hybrid optical‑radio networks in next‑generation cellular and wireless systems.