Interference-Assisted Secret Communication

Wireless communication is susceptible to adversarial eavesdropping due to the broadcast nature of the wireless medium. In this paper it is shown how eavesdropping can be alleviated by exploiting the superposition property of the wireless medium. A wiretap channel with a helping interferer (WT-HI), in which a transmitter sends a confidential message to its intended receiver in the presence of a passive eavesdropper, and with the help of an independent interferer, is considered. The interferer, which does not know the confidential message, helps in ensuring the secrecy of the message by sending independent signals. An achievable secrecy rate for the WT-HI is given. The results show that interference can be exploited to assist secrecy in wireless communications. An important example of the Gaussian case, in which the interferer has a better channel to the intended receiver than to the eavesdropper, is considered. In this situation, the interferer can send a (random) codeword at a rate that ensures that it can be decoded and subtracted from the received signal by the intended receiver but cannot be decoded by the eavesdropper. Hence, only the eavesdropper is interfered with and the secrecy level of the confidential message is increased.

💡 Research Summary

The paper introduces a novel approach to enhancing wireless secrecy by deliberately exploiting the superposition property of the wireless medium. The authors consider a “wiretap channel with a helping interferer” (WT‑HI) model, where a legitimate transmitter (X₁) wishes to send a confidential message to an intended receiver (Y₁) while a passive eavesdropper (Y₂) attempts to intercept it. A third node, the helping interferer (X₂), does not know the confidential message but transmits independent signals that can be decoded and cancelled by the legitimate receiver but remain undecodable to the eavesdropper. This cooperative interference creates artificial confusion at the eavesdropper, thereby increasing the equivocation rate and achieving secrecy.

The system is formally defined by input alphabets X₁, X₂, output alphabets Y₁, Y₂, and a transition probability p(y₁,y₂|x₁,x₂). The transmitter uses a stochastic encoder f₁ that maps each message W into a codeword X₁ⁿ, while the interferer uses an independent stochastic encoder f₂ to generate a random codeword X₂ⁿ. The legitimate receiver employs a deterministic decoder g(·) to recover W, and secrecy is measured by the equivocation rate (1/n) H(W|Y₂ⁿ). A secrecy rate Rₛ is achievable if, for any ε>0, there exists a code with block length n such that the error probability ≤ε and the equivocation ≥Rₛ−ε.

The core contribution is Theorem 1, which provides an achievable secrecy rate expressed in terms of multiple‑access channel (MAC) regions. Two MAC capacity regions are defined: R