Abstract: A method for transmitting covertly employs three features in a novel combination to create a transmission waveform that has no detectable artifacts. First, the method employs spread spectrum, such as a direct sequence spread spectrum signal, to transmit the power level below the noise floor. Second, the method modulates the phase of each chip in the spread spectrum signal using a chaotic sequence. Third, the method filters the transmission signal using a pulse shaped filter to depress blind detection features in the amplitude modulation and higher order power spectral densities. The novel combination of these features results in a practically invisible and undetectable transmission waveform. Many other features are disclosed herein to optimize this combination.

Abstract: A method for transmitting covertly employs three features in a novel combination to create a transmission waveform that has no detectable artifacts. First, the method employs spread spectrum, such as a direct sequence spread spectrum signal, to transmit the power level below the noise floor. Second, the method modulates the phase of each chip in the spread spectrum signal using a chaotic sequence. Third, the method filters the transmission signal using a pulse shaped filter to depress blind detection features in the amplitude modulation and higher order power spectral densities. The novel combination of these features results in a practically invisible and undetectable transmission waveform. Many other features are disclosed herein to optimize this combination.

Abstract: A communications waveform schema integrates security at the physical layer and has many desirable features of traditional LPI/LPD waveforms. To provide additional covert aspects in communications a single-carrier base waveform is divided up in frequency and/or time and redistributed pseudo-randomly among subcarriers and channel assignment. Multiple matched polyphase filterbank channelizers are used so that each individual channel has a bandwidth well below a Nyquist rate for information carried by the base waveform, thereby making full data extraction from any individual channels a theoretic impossibility. The individual channels do not carry enough information from the base waveform to be useful and only in the aggregate can the entire base waveform be reconstructed. Individual channels are up-converted onto pseudo-randomly chosen carrier frequencies within the bandwidth of the high rate hardware IF in such a way as to obfuscate the order in which they occur in the base waveform.

Abstract: A highly efficient distributed synchronization (DS) uses waveforms that can be easily detected by the receiver while possessing desirable LPI/LPD characteristics. Differentially encoding the synchronization pattern reduces receiver computation because signal detection search space is collapsed to a single dimension. Dissimilar bursts are used for synchronization to remove correlated energy between different sets, while maintaining the same differentially encoded pattern. The DS functions only as a whole and not as individual carriers or channels. No single carrier conveys any useful information. All cross-channel coherence required for proper alignment and subsequent detection of the distributed synchronization set of bursts is possible due to large transmission hardware IF bandwidth.