METHOD FOR DETECTING DANGEROUS DIGITAL RADIO SIGNALS

Abstract

Detection and recognition of digital radio signals remains a critical challenge, particularly in electromagnetically contested environments. This study examines low-pass filtering approaches characterized by either linear or quadratic dependence of the output on the input signal amplitude. These filters operate by coherently summing deterministic signal components while incoherently accumulating random noise contributions resulting in constructive reinforcement of the signal energy and sublinear growth of noise power, which substantially enhances the signal-to-noise ratio (SNR). A rectangular pulse, serving as a representative model of modern digital communication waveforms, was applied to both linear and quadratic filter architectures. A comprehensive statistical characterization including mathematical expectation, variance, root mean square deviation, correlation coefficient, and SNR was performed in both time and frequency domains for input and output signals. To enable objective evaluation of filtering performance, a novel efficacy metric referred to as the payoff coefficient was introduced to quantify improvements in detection reliability. Further simulations analyzed the envelope voltage at the output of an ideal bandpass filter when excited by rectangular pulses of varying durations, emulating signals typical of low-probability-of-intercept (LPI) communication systems. Transient responses and spectral leakage were examined under diverse signal-noise correlation conditions. The findings confirm that covert signals can be reliably extracted using two-dimensional likelihood density estimation, which effectively discriminates interfering components from the composite received waveform. At the system level, incorporating narrowband low-frequency filtering into the signal processing pipeline improved the noise immunity of airborne digital radio signal detection and recognition by 23%. This enhancement significantly strengthens operational resilience in hostile electromagnetic environments, with direct relevance to secure communications, electronic warfare, and signals intelligence applications.