Blind interception of phase coded signals

Abstract

This paper proposes a blind phase coded signal interception method for a potential target to recognize whether it is being looked at. The interception method is based on identifying the carrier frequency (fc) of the received signal with unknown phase coding from the ratio of the second derivative of the signal with itself. It is shown that this ratio is equal to -(2πfc)2. The second derivative is estimated by successive differences of the highly sampled received signal. It is shown that average of the ratio of the estimated second derivative to the highly sampled received signal gives an accurate estimate of the carrier frequency. To reduce the additive noise effect, different segments of the received signal are convolved with themselves and their outputs cross-correlated before computing the second derivative. Convolution of the noisy cosine/sine chips in the received signal is identical to cross correlation of the cosine chips/cross-correlation of the sine chips multiplied by - 1, added to convolution of the noise segments. This is so because cosine/sine chips are symmetrical/anti-symmetrical and the lack of such symmetry in the noise results in higher signal-to-noise ratio. This technique along with a bandpass filter bank, results in successful carrier identification based signal interception in environments with SNR ≅ 12 dB or more. Such SNR levels could be considered as reasonable in the common application of signal interception for a potential target. The received signal in such a scenario is stronger since, it is proportional to only 1, versus 1 and the effect of target cross-section on reflected signals.