A coherent dual-channel QPSK modulator/demodulator for a CDMA system, and modulating/demodulating methods therefor, are disclosed, in which the crosstalk interferences between an I-channel and a Q-channel are eliminated by an orthogonal spreading method, which assigns different Walsh codes to the I-channel and Q-channel. Pseudo-random modulation and pseudo-random demodulation are carried out by using one PN code so as to minimize the channel parameter estimation errors, thereby improving receiving performance.
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