In a cognitive radio network, the non-conforming behavior of rogue transmitters is a major threat to opportunistic spectrum access. One approach for facilitating spectrum enforcement and security is to require every transmitter to embed a uniquely-identifiable authentication signal in its waveform at the PHY-layer. In existing PHY-layer authentication schemes, known as blind signal superposition, the authentication/identification signal is added to the message signal as noise, which leads to a tradeoff between the message signal’s signal-to-noise (SNR) and the authentication signal’s SNR under the assumption of constant average transmitted power. This implies that one cannot improve the former without scarifying the latter, and vice versa. In this paper, we propose a novel PHY-layer authentication scheme called hierarchically modulated duobinary signaling for authentication (HM-DSA). HM-DSA introduces some controlled amount of inter-symbol interference (ISI) into the message signal. The redundancy induced by the addition of the controlled ISI is utilized to embed the authentication signal. Our scheme, HM-DSA, relaxes the constraint on the aforementioned tradeoff and improves the error performance of the message signal as compared to the prior art.