Discrete Time Chaos Dithering for LPI Spread Spectrum Signals
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Solution Overview
Problem
Communications systems face challenges in achieving low probability of interception (LPI) and low probability of detection (LPD) due to limitations in reducing cyclostationarity and spectral energy density, particularly in chaotic signal-based systems where synchronization drifts quickly, leading to low throughput and energy density issues.
Innovation Solution
The introduction of a discrete time chaos dither mechanism, known to both the transmitter and receiver, forces non-uniform sampling of chaotic spread waveforms, reducing energy density by digitally generating a chaotic sequence and varying the sampling interval based on a pseudo-random sequence, thereby enhancing LPI/LPD characteristics.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If chaotic sequences are used to spread the signal, then LPI/LPD characteristics are improved, but synchronization drift occurs quickly leading to low throughput
Solution Approach 1:
A dither sequence is introduced as an intermediary between the chaotic spreading sequence and the modulated waveform. This dither sequence acts as a mediator that further randomizes the signal characteristics, suppressing cyclostationarity and energy density while maintaining synchronization stability, thereby resolving the contradiction between improved LPI/LPD and maintained throughput
2Object-affected harmful factors
If the spreading ratio is increased to reduce energy density, then LPI/LPD characteristics are improved, but practical chaos generation rates are limited
Solution Approach 1:
The spreading process is segmented into two independent stages: first, chaotic sequence spreading at a practical rate; second, dither sequence spreading that provides additional spectral distribution. This segmentation allows the system to achieve a higher effective spreading ratio and lower energy density without requiring a single chaotic generator to operate at impractically high rates
3Measurement precision
If state information is exchanged more often to maintain synchronization, then synchronization accuracy is improved, but data rate decreases
Solution Approach 1:
The dither sequence is generated using the same chaotic algorithm and initial conditions at both transmitter and receiver, enabling self-synchronization without frequent state information exchange. The system serves itself by using the deterministic chaotic nature to automatically maintain alignment, thereby preserving high data rates while achieving accurate synchronization
Data Source
AI summary
The invention concerns a chaotic communications system, method and apparatus having a transmitter configured to spread an input data signal over a wide intermediate frequency band, by digitally generating a first chaotic sequence of values to form a spreading code. The spreading code is then used to form a digital IF chaotic spread spectrum signal having a uniform sampling interval. The duration of the sampling interval is then selectively varied in accordance with a first pseudo-random sequence, thereby introducing a known dither in the digital IF chaotic spread spectrum signal. After introducing the known dither, the digital IF chaotic spread spectrum signal is converted to an analog RF spread spectrum signal at a conversion rate that exceeds the sampling interval of the chaotic spread spectrum signal. A corresponding receiver recovers the input data from the spread transmitted signal. This spreading may utilize a chaotic sequence employing discrete time chaos dithering.