Spectrum security and enforcement is one of the major challenges that need to be addressed before spectrum-agile and opportunistic spectrum access technologies can be deployed. Rogue transmitters are a major threat to opportunistic spectrum access. One approach for deterring rogue transmissions is to enable receivers to authenticate or uniquely identify secondary transmitters. Although cryptographic mechanisms at the higher layers have been widely used to authenticate transmitters, the ability to authenticate transmitters at the physical (PHY) layer has a number of key advantages over higher-layer approaches. In existing schemes, the authentication signal is added to the message signal in such a way that the authentication signal appears as noise to the message signal and vice versa. Hence, existing schemes are constrained by a fundamental tradeoff between the message signal’s signal-to-noise ratio (SNR) and the authentication signal’s SNR. In this paper, we propose a novel PHY-layer authentication scheme called Precoded Duobinary Signaling for Authentication (P-DSA). P-DSA introduces some controlled amount of inter-symbol interference (ISI) into the data stream. The addition of the controlled ISI introduces redundancy in the message signal which can be utilized to embed the authentication signal. In this way, P-DSA relaxes the constraint on the aforementioned tradeoff. Our results show that P-DSA achieves superior detection performance compared to the prior art without sacrificing message throughput or increasing power.