Waveform design for radar-embedded communications

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 re-modulated 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.