Ambiguous Energy Suppression in Encryption Derived Pseudo-Random BPSK Radar Signals
Type of DegreeMaster's Thesis
Electrical and Computer Engineering
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In this thesis, a pseudo-random binary phase shift keying (BPSK) radar signal is developed using Advanced Encryption Standard (AES-192). The primary objective of this encryption based BPSK (E-BPSK) signal is to provide a fast user controlled means of generating a secure a-periodic radar signal which presents several desirable characteristics. These include virtually no ambiguous range interval, high noise tolerance and spoofing security through unpredictability. Statistical analysis of a simulated data model of the E-BPSK signal is performed to verify the randomness of the encryption based sequence. A comparison signal of identical transmit properties with a different modulation code is used for a detailed comparative analysis. The standard matched filter response of E-BPSK has a sharp main lobe peak at a real target location. The random nature of the signal due to the encryption also causes a significant amount of ambiguous side-lobe energy in both the range and Doppler axis of the matched filter. A method of suppressing this side-lobe ambiguous energy is required to enable multi-target identification. This thesis covers mitigating the ambiguous energy in the proposed E-BPSK signal with a modified CLEAN type algorithm specifically designed for an a-periodic signal. CLEAN involves identifying targets and reprocessing the signal after extracting the identified target energy. CLEAN algorithms can become hardware intensive and are sensitive to the target model accuracy. Fitness functions for optimizing the CLEAN target model parameters are included to increase model accuracy. In simulation the modified CLEAN algorithm successfully reduces the ambiguous side-lobe energy in the E-BPSK matched filter response by up to -30 dB. CLEAN processing applied to a discrete random signal is shown to effectively mitigate the ambiguous lobes in the matched filter while preserving the range and Doppler ambiguity mitigation characteristics.