Invariant detection for QPSK DS-SS signals

This paper investigates the invariant detection problem of quadrature phase-shift keying (QPSK) direct sequence spread spectrum (DS-SS) signals in additive white Gaussian noise (AWGN). Using the complex Gaussian mixture signal model and invariance principle, we consider the constant-false-alarm-rate (CFAR) invariant detectors and other sub-optimal invariant tests, including the uniformly-most-powerful-invariant (UMPI) test and the generalized likelihood ratio test (GLRT). We derive an asymptotic locally most-powerful invariant (ALMPI) test and obtain the incoherent weighted multi-cycle detectors, which act as the performance upper bound of the spreading sequence period detectors based on second-order cyclostationary statistics. Simulation results demonstrate that, with finite observation samples, the ALMPI test exhibits better detection performance than the cycle feature detectors.