Practical limitations in limiting the rate-line detectability of spread spectrum LPI signals

Realizable filters (such as Butterworth, Chebyshev, and Bessel filters) were used to generate bandlimited symbol sequences so that practical limitations on symbol rate-line suppression could be investigated. The signal-to-noise ratio at the output of a square-law detector, with the input matched to the symbol shape, was used as the measure of performance. Analytical results were obtained for various filter orders. The analytical results are compared with computer simulation results obtained using filters with finite impulse response. It is shown that at low stopband attenuations (⟨20 dB), the Chebyshev symbol shape offered significant improvement in suppressing the rate line over the Butterworth or Bessel shapes, with the Bessel symbol shape being the worst of the three. It is also shown that for large stopband attenuation, the performance differences between the filters became small