[Paper Review] Covert Communication in Wireless Relay Networks

Jinsong Hu, Shihao Yan+3 more|arXiv (Cornell University)|Apr 17, 2017

Wireless Communication Security Techniques14 references3 citations

TL;DR

This paper investigates covert communication in one-way relay networks where a relay node secretly transmits its own data to the destination while forwarding the source's message, with the source acting as a warden to detect such unauthorized transmissions. The study derives the necessary condition for undetectable transmission and shows that boosting the relay's transmit power increases its covert rate, revealing a tradeoff between relaying performance and covert communication capability under detection constraints.

ABSTRACT

Covert communication aims to shield the very existence of wireless transmissions in order to guarantee a strong security in wireless networks. In this work, for the first time we examine the possibility and achievable performance of covert communication in one-way relay networks. Specifically, the relay opportunistically transmits its own information to the destination covertly on top of forwarding the source's message, while the source tries to detect this covert transmission to discover the illegitimate usage of the recourse (e.g., power, spectrum) allocated only for the purpose of forwarding source's information. The necessary condition that the relay can transmit covertly without being detected is identified and the source's detection limit is derived in terms of the false alarm and miss detection rates. Our analysis indicates that boosting the forwarding ability of the relay (e.g., increasing its maximum transmit power) also increases its capacity to perform the covert communication in terms of achieving a higher effective covert rate subject to some specific requirement on the source's detection performance.

Motivation & Objective

• To analyze the feasibility of covert communication in one-way relay networks where the relay secretly transmits its own data while forwarding the source's message.
• To identify the necessary condition under which the relay's covert transmission remains undetectable by the source, who acts as a warden.
• To derive the detection limit at the source in terms of false alarm and miss detection rates.
• To quantify the achievable effective covert rate under a constraint on the source’s detection performance.
• To reveal the tradeoff between the relay’s ability to assist the source and its capability to perform covert communication.

Proposed method

• The system model assumes half-duplex relaying over Rayleigh fading channels with independent, identically distributed (i.i.d.) complex Gaussian channel coefficients.
• The relay performs amplify-and-forward relaying and opportunistically transmits its own information to the destination during the second phase.
• The source employs a likelihood ratio test to detect the relay’s covert transmission, using a threshold-based detection scheme.
• The detection performance is characterized by the total error rate, combining false alarm and miss detection probabilities.
• The analysis derives the condition under which covert transmission is possible (i.e., detection is not guaranteed), and computes the effective covert rate as a function of transmit power and channel conditions.
• The optimal transmit power for the relay is determined under a constraint that the detection error rate must exceed a threshold (1−ε), ensuring a non-zero probability of undetected transmission.

Experimental results

Research questions

• RQ1 Under what condition can the relay transmit covertly without being detected by the source in a one-way relay network?
• RQ2 What is the detection limit of the source in terms of false alarm and miss detection rates when the relay attempts covert communication?
• RQ3 How does the relay’s maximum transmit power affect its ability to perform covert communication while still aiding the source’s transmission?
• RQ4 What is the maximum achievable effective covert rate under a given detection performance constraint at the source?
• RQ5 How does the channel gain between the source and relay influence the feasibility and rate of covert communication?

Key findings

• Covert communication is feasible in one-way relay networks when the relay’s transmit power is below a threshold that ensures the detection error rate remains above a non-trivial value, preventing certain detection by the source.
• The minimum total error rate ξ∗ decreases monotonically with increasing covert transmit power PΔ, indicating that higher power makes covert transmission easier to detect.
• The maximum allowable covert transmit power PΔε is significantly less than the upper bound PΔu, with practical values requiring ξ∗ ≥ 0.5 to avoid detection.
• The effective covert rate R̄c increases with |h_sr|², as stronger source-relay links increase the probability that the relay can transmit covertly without violating detection constraints.
• There exists a tradeoff between the relay’s ability to support the source’s transmission (via higher power) and its capability to perform covert communication, as higher power increases detection likelihood.
• The optimal PΔ under the constraint ξ∗ ≥ 1−ε can be efficiently searched due to the reduced feasible region, especially when PΔε is much smaller than PΔu.