Constellation Superposition and Artificial Noise Against Interior and Exterior Eavesdropping in Bidirectional Untrusted Relaying Systems

Untrusted bidirectional relay systems confront both internal and external eavesdropping problems, but existing studies generally focus on only one of them. This letter proposes a new scheme combining constellation superposition and artificial noise to resist both. First, we exploit constellation superposition to cause aliasing of the constellations at the untrusted relay to prevent its decoding. Then, we embed carefully designed artificial noise in the transmitted signal, making it difficult for an external eavesdropper to remove. Theoretical analysis and simulations verify that the proposed scheme maintains a high bit error rate for the untrusted relay while ensuring a lower bound of bit error rate for the external eavesdropper. In contrast, the bit error rate of the legitimate users decreases as the signal-to-noise ratio (SNR) at the transmitter increases.