[Paper Review] Wireless Surveillance of Two-Hop Communications

Ganggang Ma, Jie Xu+2 more|arXiv (Cornell University)|Apr 25, 2017

Wireless Communication Security Techniques10 references19 citations

TL;DR

This paper proposes a half-duplex wireless surveillance framework for two-hop suspicious communications, where a legitimate monitor adaptively selects among passive eavesdropping, noise jamming on the first hop, or hybrid jamming on the second hop to maximize eavesdropping rate. By jointly optimizing mode selection and jamming power, the scheme significantly outperforms individual modes, especially in low-SINR environments.

ABSTRACT

Wireless surveillance is becoming increasingly important to protect the public security by legitimately eavesdropping suspicious wireless communications. This paper studies the wireless surveillance of a two-hop suspicious communication link by a half-duplex legitimate monitor. By exploring the suspicious link's two-hop nature, the monitor can adaptively choose among the following three eavesdropping modes to improve the eavesdropping performance: (I) \emph{passive eavesdropping} to intercept both hops to decode the message collectively, (II) \emph{proactive eavesdropping} via {\emph{noise jamming}} over the first hop, and (III) \emph{proactive eavesdropping} via {\emph{hybrid jamming}} over the second hop. In both proactive eavesdropping modes, the (noise/hybrid) jamming over one hop is for the purpose of reducing the end-to-end communication rate of the suspicious link and accordingly making the interception more easily over the other hop. Under this setup, we maximize the eavesdropping rate at the monitor by jointly optimizing the eavesdropping mode selection as well as the transmit power for noise and hybrid jamming. Numerical results show that the eavesdropping mode selection significantly improves the eavesdropping rate as compared to each individual eavesdropping mode.

Motivation & Objective

• To address the challenge of effective eavesdropping on two-hop suspicious communications when the monitor is far from the source.
• To overcome the limitations of passive eavesdropping in low-channel-gain scenarios by leveraging proactive jamming strategies.
• To design a half-duplex surveillance system that avoids the hardware complexity of full-duplex self-interference cancellation and message forwarding.
• To maximize the eavesdropping rate through joint optimization of eavesdropping mode selection and transmit power allocation.
• To provide a practical surveillance solution that exploits the multi-hop nature of suspicious links for improved interception performance.

Proposed method

• The monitor operates in half-duplex mode, enabling adaptive selection among three eavesdropping modes: passive eavesdropping on both hops, noise jamming on the first hop, and hybrid jamming on the second hop.
• For passive eavesdropping (Mode I), the monitor combines signals from both hops using maximal ratio combining (MRC) to improve signal detection.
• For proactive eavesdropping via noise jamming (Mode II), the monitor transmits artificial noise on the first hop to degrade Bob’s SINR and reduce the end-to-end rate.
• For proactive eavesdropping via hybrid jamming (Mode III), the monitor forwards the overheard signal from Alice combined with artificial noise to cause destructive interference at Bob.
• The system jointly optimizes eavesdropping mode selection and jamming power allocation using a unified optimization framework to maximize the eavesdropping rate.
• The optimization problem is solved via convex relaxation and first-order derivative analysis, with closed-form expressions derived for optimal power allocation and beamforming coefficients.

Experimental results

Research questions

• RQ1 How does the performance of passive eavesdropping compare to proactive eavesdropping in two-hop wireless surveillance?
• RQ2 What are the conditions under which noise jamming on the first hop or hybrid jamming on the second hop outperforms passive eavesdropping?
• RQ3 How does the monitor’s location relative to Alice and Bob influence the optimal eavesdropping mode selection?
• RQ4 What is the impact of path loss and fading on the eavesdropping rate and mode selection in a two-hop surveillance setup?
• RQ5 Can joint optimization of mode selection and jamming power significantly improve eavesdropping performance compared to fixed-mode strategies?

Key findings

• The proposed joint optimization of eavesdropping mode selection and jamming power allocation achieves a significantly higher eavesdropping rate than any individual mode.
• When the monitor is close to Alice or the relay, passive eavesdropping (Mode I) is optimal if the combined signal gain exceeds the end-to-end suspicious link rate.
• In low-SINR scenarios where passive eavesdropping fails, the monitor selects Mode II (noise jamming on first hop) if closer to Alice, and Mode III (hybrid jamming on second hop) if closer to Bob.
• Numerical results in Rayleigh fading channels show that the adaptive mode selection scheme achieves a higher average eavesdropping rate than fixed-mode approaches, with performance gains confirmed across 10,000 channel realizations.
• The optimal beamforming coefficient and jamming power allocation are derived in closed form, enabling efficient implementation without iterative methods.
• The half-duplex design eliminates the need for complex self-interference cancellation and real-time message forwarding, making it more practical than prior full-duplex approaches.