Confidence Levels in the Design of Direct Sequence LPE Spread Spectrum Signals

The probability density function of the power levels of transmitted DS signals is shown to depend on the type of spreading modulation and, in the case of BPSK, the amount of frequency offset from the carrier. Using the appropriate density function, confidence levels in the form of probabilities are derived and illustrated for LPD and LPE communications. It is shown that with confidence or probability levels near unity, DS modulation having about 2000 chips per bit (without noncoherent post detection integration) can be used to obtain a reduction in SNR of about 23 dB, a reduction in intercept range of not much more than about 10-12 dB, and can severely limit the ability of an intercept receiver to directly demodulate and recover the baseband message.