Featureless Chaotic Spread Spectrum Modulation of Arbitrary Data Constellations

Chaotic spread spectrum communication systems provide a number of advantages for secure communications due to the apparent randomness of the underlying spreading signal. Many of these chaotic signals exhibit colored spectra, providing discernible features that enable their detection independent of the transmitted data. A recent digital chaotic circuit [1] has been shown to exhibit a truly white spectrum in addition to apparent time-domain randomness. This maximal entropy characteristic supports Shannon's criteria for maximal channel capacity communication, low probability of interception/detection (LPI/LPD), and a compact bandlimited white spectrum. Such a signal is featureless, susceptible only by energy detection. The disadvantage of such spread communication systems, one shared equally by all spread spectrum systems, is that the channel capacity is constrained by the bandwidth increases required for spreading gain. A traditional approach to increasing bandwidth efficiency is generalizing the modulated digital signaling constellation to include multiple levels of amplitude and phase modulation, which enhances exploitable cyclostationary features unless compensated. This paper presents a framework for adapting the maximal entropy digital chaotic signal to featureless chaotic spread spectrum modulation of arbitrary discrete-time discrete-amplitude data constellations, permitting higher throughputs in chaotic spread spectrum communication systems without sacrificing any of the maximum entropy characteristics that provide LPI/LPD.