One of the critical challenges in dynamic spectrum sharing (DSS) is identifying non-conforming transmitters that violate spectrum access rules prescribed by a spectrum regulatory authority. One approach for facilitating identification of the transmitters in DSS is to require every transmitter to embed an uniquely-identifiable authentication signal in its waveform at the PHY-layer. In most of the existing PHY-layer authentication schemes, the authentication signal is added to the message signal as noise, which leads to a tradeoff between the message signal’s signal-to-noise ratio (SNR) and the authentication signal’s SNR under the assumption of constant average transmitted power. This implies that one cannot improve the former without sacrificing the latter, and vice versa. In this paper, we propose a novel PHY-layer authentication scheme called Hierarchical Modulation with Modified Duobinary Signaling for Authentication (HMM-DSA), which relaxes the constraint on the aforementioned tradeoff. HMM-DSA utilizes a modified duobinary filter to introduce some controlled amount of inter-symbol interference into the message signal, and embeds the authentication signal in the form of filter coefficients. Our results show that the proposed scheme, HMM-DSA, improves the error performance of the message signal as compared to the prior art.