[Paper Review] On Secure Mixed RF-FSO Systems With TAS and Imperfect CSI

Hongjiang Lei, Haolun Luo+5 more|arXiv (Cornell University)|Sep 5, 2018

Wireless Communication Security Techniques40 references4 citations

TL;DR

This paper investigates physical layer security in mixed RF-FSO dual-hop relay systems with multiple-antenna eavesdroppers and imperfect channel state information (CSI), proposing four transmit antenna selection (TAS) schemes to enhance secrecy performance. It derives closed-form expressions for PDF and CDF over Nakagami-m and Málaga fading channels, analyzes the impact of outdated RF CSI and imperfect FSO CSI, and establishes bounds on effective secrecy throughput, showing that outdated CSI significantly degrades secrecy performance and that increasing transmit antennas offers limited gains.

ABSTRACT

In this work, we analyze the secrecy outage performance of a dual-hop relay system composed of multiple-input-multiple-output radio-frequency (RF) links and a free-space optical (FSO) link while a multiple-antenna eavesdropper wiretaps the confidential information by decoding the received signals from the resource node. The channel state information (CSI) of the RF and FSO links is considered to be outdated. We propose three transmit antenna selection (TAS) schemes to enhance the secrecy performance of the considered systems. The secrecy outage performance with different TAS schemes is analyzed and the effects of misalignment and detection technology on the secrecy outage performance of mixed systems are studied. We derive the closed-form expressions for probability density function (PDF) and cumulative distribution function (CDF) over Málaga channel with imperfect CSI. Then the closed-form expressions for the CDF and PDF of the equivalent signal-to-noise ratio (SNR) at the legitimate receiver over Nakagami-$m$ and Málaga channels are derived. Furthermore, the lower bound of the secrecy outage probability (SOP) with different TAS schemes are derived. Besides, the asymptotic results for SOP are investigated by exploiting the unfolding of Meijer's $G$-function when the electrical SNR of FSO link approaches infinity. Finally, Monte-Carlo simulation results are presented to testify the correctness of the proposed analysis. The results illustrate that the outdated CSI shows a strong effect on the secrecy outage performance. In addition, increasing the number of antennas at the source cannot significantly enhance the secrecy performance of the considered systems.

Motivation & Objective

• To analyze the secrecy outage performance of mixed RF-FSO dual-hop relay systems under imperfect and outdated CSI.
• To evaluate the impact of outdated RF CSI and imperfect FSO CSI on physical layer security.
• To design and compare four transmit antenna selection (TAS) schemes for enhancing secrecy performance.
• To derive closed-form expressions for PDF and CDF of the equivalent SNR over Nakagami-m and Málaga fading channels.
• To establish bounds on effective secrecy throughput (EST) and investigate asymptotic behavior as FSO SNR increases.

Proposed method

• Derives the probability density function (PDF) and cumulative distribution function (CDF) of the equivalent SNR at the legitimate receiver over Nakagami-m and Málaga fading channels with imperfect CSI.
• Proposes four TAS schemes to exploit spatial diversity and improve secrecy performance in the presence of multiple-antenna eavesdroppers.
• Uses statistical channel models to account for outdated CSI on RF links and imperfect CSI on FSO links, incorporating correlation and estimation errors.
• Applies Meijer’s G-function and special function identities (e.g., modified Bessel functions) to derive closed-form expressions for SNR statistics.
• Derives bounds on effective secrecy throughput (EST) and investigates asymptotic behavior using series expansions of Meijer’s G-function.
• Validates analytical results via Monte Carlo simulations, confirming accuracy under various system configurations.

Experimental results

Research questions

• RQ1 How does outdated CSI on the RF link affect the secrecy outage performance in mixed RF-FSO systems?
• RQ2 What is the impact of imperfect CSI on the FSO link on the overall secrecy performance?
• RQ3 How do different transmit antenna selection (TAS) schemes compare in enhancing secrecy throughput under imperfect CSI?
• RQ4 What are the closed-form expressions for the PDF and CDF of the equivalent SNR at the legitimate receiver over Nakagami-m and Málaga fading channels?
• RQ5 How does the effective secrecy throughput behave asymptotically as the FSO link’s electrical SNR approaches infinity?

Key findings

• Outdated CSI on the RF link has a strong negative impact on secrecy outage performance, significantly degrading system security.
• Imperfect CSI on the FSO link further degrades secrecy performance, especially when the eavesdropper is closely located to the transmitter or receiver.
• Increasing the number of transmit antennas at the source does not lead to significant improvements in secrecy performance due to the dominant effect of outdated CSI.
• The proposed TAS schemes effectively improve secrecy performance, with analytical bounds on effective secrecy throughput (EST) derived in closed form.
• As the electrical SNR of the FSO link increases, the asymptotic behavior of EST is characterized by the unfolding of Meijer’s G-function, providing insight into high-SNR performance.
• Monte Carlo simulations confirm the accuracy of the derived analytical expressions for PDF, CDF, and EST bounds under various channel conditions.