Spread spectrum signals with low probability of chip rate detection

Spread spectrum signals are considered which use spreading codes chosen to give a low probability of chip rate detection. 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. It is shown that a combination of these techniques can, ideally, defeat any such circuit. The use of quantized multilevel symbols is shown to give performance approaching that of Gaussian symbols. A four-level symbol set is shown to be able to defeat quadratic and fourth-power circuits