Spread spectrum signals with low probability of chip rate detection

Spread-spectrum signals that use spreading codes chosen to give a low probability of chip rate detection are examined. Pulse shaping and pseudorandom amplitude weighting are used to defeat the interceptor who uses a nonlinear circuit to generate a tone at the chip rate of the signal. The case of low input signal-to-noise ratio (SNR) is considered. Under this assumption, the detector output spectrum is found, and the output SNR is then used as a measure of detectability. The effect of signal pulse shape, i.e., band-limiting, on performance is studied. It is shown that a combination of band-limiting and correctly chosen amplitude weightings can, ideally, defeat any such circuit. The performance of a fourth-power envelope detector is analyzed with a combination of signal pulse shape and amplitude weighting. The amplitude distributions are binary, Gaussian, and quantized Gaussian.<>