[Paper Review] On the Performance of a Relay-Assisted Multi-Hop Asymmetric FSO/RF Communication System over Negative Exponential atmospheric turbulence with the effect of pointing error

Mohammad Ali Amirabadi|arXiv (Cornell University)|Aug 26, 2018

Optical Wireless Communication Technologies36 references4 citations

TL;DR

This paper proposes a relay-assisted multi-hop asymmetric FSO/RF communication system to enhance long-range, high-reliability links under severe atmospheric turbulence and pointing errors. By combining short-range RF relays with long-range FSO links and employing amplify-and-forward relaying with user selection, the system achieves low BER and outage probability with minimal power and complexity, showing insensitivity to user and relay count variations.

ABSTRACT

In this paper, a multi-user multi-hop hybrid FSO / RF system is presented. This structure is consisted of two main parts. The main motivation of presenting this structure is communication in long-range impassable links or some specific atmospheric conditions under which RF connection becomes easily disrupted. Although these effects could be mitigated by consuming more power or adding processing complexity, but a small user mobile phone cannot deserve more complexity or power supply. The fact that FSO and RF links are complementary of each other brings a new solution in mind; an access point that amplifies received signal via short-range RF link and forwards it to via long-range FSO link, could solve the mentioned problem. This scenario is exactly implemented at the first part of the proposed structure. At the second part, a multi-hop hybrid parallel FSO / RF link is implemented to connect source and destination Base Stations. It is the first time that in a multi-hop FSO / RF system, multi-user scheme, signal selection at each hop, known and un-known CSI in amplify and forward relaying, and saturate atmospheric turbulence with the effect of pointing error are considered. New expressions are derived in closed-form for Bit Error Rate (BER) and Outage Probability of the proposed structure and verified by MATLAB simulations. The proposed structure has advantages of FSO, RF, relay-assisted, and multi-user systems at the same time. Results indicate that it has low dependence on number of users and number of relays. Therefore, it is suitable for areas with varying population and long-range links. This structure offers independent performance without additional power consumption, processing latency, and complexity.

Motivation & Objective

• Address the challenge of maintaining reliable long-range communication in harsh atmospheric conditions where RF links are disrupted.
• Overcome the limitations of standalone FSO and RF systems by leveraging their complementary strengths.
• Design a scalable, low-power, low-latency communication system suitable for mobile users and dynamic user populations.
• Analyze the impact of negative exponential atmospheric turbulence and pointing error on system performance in a multi-hop relay environment.
• Develop closed-form expressions for key performance metrics to enable efficient system design and optimization.

Proposed method

• Employ a hybrid architecture with short-range RF links for user-to-relay communication and long-range FSO links for relay-to-destination transmission.
• Implement multi-user selection diversity at each relay to improve system reliability and spectral efficiency.
• Use amplify-and-forward (AF) relaying with both perfect and imperfect channel state information (CSI) to model realistic system conditions.
• Model atmospheric turbulence using the negative exponential distribution and include pointing error effects via a gamma-gamma fading approximation.
• Derive closed-form expressions for Bit Error Rate (BER) and Outage Probability using statistical channel models and moment generating functions.
• Validate analytical results through extensive MATLAB simulations under varying numbers of users, relays, and turbulence levels.

Experimental results

Research questions

• RQ1 How does the combination of FSO and RF links improve performance in long-range, high-turbulence environments with pointing errors?
• RQ2 What is the impact of user selection diversity on BER and outage probability in a multi-hop FSO/RF relay system?
• RQ3 How does imperfect CSI affect the performance of amplify-and-forward relaying in this asymmetric hybrid system?
• RQ4 To what extent does the system performance depend on the number of users and relays?
• RQ5 Can closed-form expressions for BER and outage probability be derived under negative exponential turbulence and pointing error conditions?

Key findings

• The proposed system achieves low Bit Error Rate (BER) and outage probability even under strong negative exponential atmospheric turbulence and pointing errors.
• Performance remains stable and largely independent of the number of users and relays, indicating high scalability and robustness.
• The system maintains low complexity and power consumption, making it suitable for mobile and resource-constrained devices.
• Analytical expressions for BER and outage probability are derived in closed-form, enabling fast performance evaluation and system optimization.
• Simulations confirm the accuracy of the derived expressions across various turbulence and pointing error conditions.
• The use of user selection at each relay significantly improves diversity gain and system reliability without increasing hardware or processing load.