Modern wireless systems are designed with excess synchronization bandwidth to ensure reliable operation under worst-case conditions. This paper presents a protocol-agnostic secondary signaling layer that exploits this unused synchronization margin by intentionally introducing small, controlled frequency offsets into an existing communication signal to embed a parallel stream of information. These offsets are naturally absorbed by the primary receiver's carrier frequency offset (CFO) correction mechanisms, allowing the primary data stream to remain intact while enabling simultaneous secondary communication. The proposed signaling method employs a frequency-shift-keyed overlay engineered to remain within the stable operating region of conventional carrier-synchronization loops, ensuring backward compatibility and transparency for legacy receivers. We analyze the system for single-carrier protocols, characterize the trade-offs between secondary throughput and the impact on the primary link, and validate performance through both simulation and over-the-air experiments using software-defined radios. Results demonstrate reliable secondary communication with negligible degradation to primary performance, making our system well-suited for ad hoc spectrum sharing and decentralized coordination at the tactical edge.

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