Disclosed herein are methods and systems for embedding a supplementary data channel in OFDM-based communication systems. One embodiment takes the form of a process that includes obtaining a primary data signal that includes a given symbol, where the given symbol includes primary payload data prepended with a given cyclic prefix. The process also includes obtaining supplementary payload data. The process also includes identifying an available portion of the given symbol. The process also includes generating a modified primary data signal at least in part by replacing the available portion of the given symbol with a subset of the supplementary payload data. The process also includes outputting the generated modified primary data signal for transmission via an air interface.