[Paper Review] Secrecy Outage Analysis of Two-Hop Decode-and-Forward Mixed RF/UWOC Systems

Yi Lou, Ruofan Sun+3 more|arXiv (Cornell University)|Sep 1, 2020

Underwater Vehicles and Communication Systems12 references4 citations

TL;DR

This paper analyzes the secrecy outage probability (SOP) in a two-hop decode-and-forward mixed RF/UWOC system using exact and asymptotic closed-form expressions derived via bivariate H-functions. It reveals that SOP saturates at high SNR, enabling optimal power allocation to balance energy efficiency and security under varying UWOC and RF channel conditions modeled by EGG and α-μ distributions, respectively.

ABSTRACT

We analyze the secrecy performance of a two-hop mixed radio frequency (RF)/underwater wireless optical communication (UWOC) system using a decode-and-forward (DF) relay. All RF and UWOC links are modeled by the $α-μ$ and exponential-generalized Gamma distributions, respectively. We first derive the expressions of the secrecy outage probability (SOP) in exact closed-form, which are subsequently used to derive asymptotic expressions at high SNR that only includes simple functions for further insight. Moreover, based on the asymptotic expression, we can determine the optimal transmit power for a wide variety of RF and UWOC channel conditions. All analyses are validated using Monte Carlo simulation.

Motivation & Objective

• To analyze the physical layer secrecy performance of a two-hop mixed RF/UWOC system with a decode-and-forward relay.
• To model the RF links using the flexible α-μ distribution and the UWOC link using the more accurate exponential-generalized Gamma (EGG) distribution.
• To derive exact and asymptotic expressions for secrecy outage probability (SOP) to enable performance insight and optimal power control.
• To validate the analytical results through Monte Carlo simulations under diverse channel conditions.

Proposed method

• Model the source-to-relay (SR) and eavesdropper-to-relay (SE) RF links using the α-μ distribution, capturing non-linearity and multipath clustering via parameters α and μ.
• Model the relay-to-destination (RD) UWOC link using the EGG distribution to account for temperature gradients and bubble effects in freshwater and saltwater.
• Derive the exact SOP in closed-form using bivariate H-functions, enabling precise performance evaluation across diverse fading conditions.
• Develop asymptotic SOP expressions at high SNR using simple functions to reveal system behavior and saturation trends.
• Use the asymptotic expressions to determine optimal transmit power that minimizes SOP while maintaining energy efficiency.
• Validate all analytical results through extensive Monte Carlo simulations under varying channel parameters.

Experimental results

Research questions

• RQ1 How does the secrecy outage probability (SOP) behave in a two-hop mixed RF/UWOC system with a decode-and-forward relay under realistic fading models?
• RQ2 What is the impact of UWOC channel parameters (e.g., bubble levels, temperature gradients) on SOP performance?
• RQ3 How do RF channel parameters (α and μ) influence the secrecy performance and saturation behavior of SOP at high SNR?
• RQ4 Can asymptotic SOP expressions be derived that simplify system design and reveal optimal power allocation strategies?
• RQ5 What is the optimal transmit power for minimizing SOP while ensuring energy efficiency, based on channel quality?

Key findings

• The SOP expression saturates at high SNR, indicating a fundamental limit on secrecy performance regardless of further power increase.
• The saturation value of SOP is determined by the quality of the eavesdropping RF channel and the UWOC channel, with better UWOC links reducing SOP.
• When the UWOC channel quality improves (e.g., from [2.4, 0.05] to [4.7, 0.05]), SOP decreases significantly, improving system secrecy.
• For a UWOC parameter of [4.7, 0.05], the optimal transmission power is 10 dB, achieving an SOP of 0.45, beyond which further power increase yields no benefit.
• The asymptotic SOP expressions closely match simulation results from 0 dB SNR onward, enabling accurate and efficient system design.
• The eavesdropping channel quality inversely affects the SOP saturation level: better eavesdropping links lead to higher SOP saturation values.