Performance evaluation of a novel relay assisted hybrid FSO / RF communication system with receive diversity

One of the main problems in mobile communication systems is the degradation of Radio Frequency (RF) connection when mobile user is far from base station. One way to solve this problem is to increase the transmitter power, but the mobile transmitter is not able to supply much power. Another way is to use a relay; among relay schemes, amplify and forward is better for long range communications. Amplify and forward relay is not affordable in terms of power consumption and performance, because it consumes a lot of power inefficiently and enhances the noise. Therefore, in other cases, except in the case of long range links, other relay protocols, such as decode and forward, as well as demodulate and forward, are preferable. In this paper, a novel multi-hop hybrid Free Space Optical (FSO) / RF link is presented; it is made up of two main parts. The first part establishes the connection between the mobile user and source base station, and the second part establishes the connection between the source and the destination base stations. In the first part, a mobile user wants to connect to the source base station via a long range link; therefore, a fixed gain amplify and forward relay with multiple receive antennas is used for communication establishment. In the second part, the source and the destination base stations are connected via a multi-hop hybrid parallel FSO / RF link with demodulate and forward relaying. Considering the FSO link in Gamma-Gamma atmospheric turbulence with the effect of pointing error in moderate to strong regime and the Negative Exponential atmospheric turbulence in saturate regime, and the RF link in Rayleigh fading, new closed form exact and asymptotic expressions are derived for the Outage Probability and Bit Error Rate of the proposed structure. Derived expressions are verified with MATLAB simulations.

💡 Research Summary

This paper addresses the well‑known problem of RF link degradation for mobile users located far from a base station. To mitigate this issue without requiring excessive transmit power at the mobile terminal, the authors propose a two‑stage, multi‑hop hybrid free‑space optical (FSO) and radio‑frequency (RF) communication architecture that exploits both receive diversity and opportunistic relay selection.

In the first stage, a fixed‑gain amplify‑and‑forward (AF) relay equipped with multiple receive antennas connects the mobile user to a source base station. Each antenna experiences independent Rayleigh fading; a selection‑combining (SC) scheme picks the antenna with the highest instantaneous SNR, which is then duplicated. One copy is transmitted directly over the RF link, while the other copy is converted to an optical signal with conversion efficiency η and transmitted simultaneously over an FSO link. The parallel transmission of RF and FSO signals creates a redundancy that protects the system against deep fades or turbulence in either channel.

The second stage consists of a cascade of relay nodes that form a multi‑hop parallel FSO/RF link between the source and destination base stations. At each relay, the received FSO and RF signals are evaluated, and the one with the larger instantaneous SNR is selected (opportunistic selection). The selected signal is demodulated, regenerated, duplicated, and again sent over both an FSO and an RF branch in parallel. This process repeats hop‑by‑hop until the destination is reached. The relaying protocol used in this stage is demodulate‑and‑forward (DF), which avoids noise amplification inherent to AF, reduces power consumption, and has lower computational complexity than decode‑and‑forward (also DF in the sense of simple regeneration).

Channel modeling is comprehensive. The FSO links are described by the Gamma‑Gamma distribution with pointing‑error impairment for moderate‑to‑strong turbulence, and by the Negative‑Exponential distribution for the saturated regime. Both models lead to probability density functions (PDFs) and cumulative distribution functions (CDFs) expressed in terms of Meijer‑G functions. The RF links follow the classic Rayleigh fading model. The authors derive the instantaneous SNR PDFs for each hop, then combine them using the selection‑combining rule for the parallel branches.

Performance metrics are outage probability (the probability that the end‑to‑end SNR falls below a threshold γ_th) and bit‑error rate (BER) for on‑off keying (OOK). By integrating the derived SNR CDFs, the authors obtain exact closed‑form expressions for both metrics. The derivations involve products of Meijer‑G functions and exponential terms; the authors extend integration identities from prior works (especially