This paper considers the embedding of a covert communication signal amongst radar backscatter by means of a tag/transponder that lies within the illuminated area of the radar. Past approaches have operated on an inter-pulse basis whereby a communication symbol/identifier is relayed to an intended receiver by imparting a Doppler-like phase-shift to each of a successive series of incident radar pulses. In contrast, the approach proposed in this paper operates on an intra-pulse basis whereby the incident radar waveform at the tag/transponder is “re-modulated” into one of a set of different waveforms each representing a different communication symbol. The particular design issues for these remodulated waveforms are discussed and three general design methods are proposed. The effectiveness of the different methods is assessed in terms of the probability of communication error as a function of the respective powers of the embedded communication signal, the masking radar backscatter, and noise. The relative “covertness” of the resulting waveforms is also discussed.
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