Carrier detection of unbalanced QPSK direct sequence signals

This paper reports on detecting the presence of direct sequence spread spectrum signals through detection of the 2f and 4f carrier components of the signals produced by nonlinear techniques. The modulation studied is a generic unbalanced quadrature phase-shift-keyed (UQPSK) modulation, i.e. a QPSK signal with unequal power in the two channels. The complete range of channel power ratio is covered, with equal emphasis on the generic unbalanced case and the two limiting cases of BPSK and (balanced) QPSK. Analytic expressions are presented for the detected signal-to-noise ratio of carrier harmonics 2f and 4f as a function of SNR, channel power ratio, and normalized input bandwidth. The interest is in detecting covert direct sequence spread spectrum signals, i.e. those with SNR <0 dB. Measurements confirm every aspect of the theory. The least detectable signal type is balanced QPSK, which is detectable (at 4f) at a threshold SNR ranging from -2 to -13 dB as the detection process gain (chip rate/detection bandwidth) is varied from 40 to 80 dB