Weighted soft decision of QPSK modulation in parallel combinatory spread spectrum system

In parallel combinatory spread spectrum (PCSS) system, the soft decision of QPSK modulation will make the superposed Pseudo-Noise (PN) sequences lose its correlation, which leads to the failure of the demodulation. According to the characteristics of QPSK modulation, the superposed sequence is mapped to two sequences, and then a new weighed summation algorithm is proposed to keep the correlation properties of superposed sequences. Applying the algorithm in QPSK modulation in PCSS system, soft decision can be obtained in the receiver. This weighed soft decision resolves the problem that conventional soft decision QPSK modulation cannot apply in PCSS system. The system model is built and simulated in the paper. The simulation results show that the reliability of proposed soft decision QPSK modulation is better than that of hard decision QPSK modulation and this method is useful in engineering application.