A coherent dual-channel QPSK modulation for CDMA systems

For the reverse link in code division multiple access (CDMA) systems, noncoherent 64-ary orthogonal quadrature phase shift keying (QPSK) modulation has been studied intensively since it has been employed by the IS-95 CDMA system. The wideband-CDMA (W-CDMA) personal communications service (PCS) system, which is proposed to the Joint Technical Committee (JTC) by OKI Inc., employed a coherent QPSK modulation. In the W-CDMA system, imperfect estimation of the channel parameters introduces crosstalk between the I- and Q-channels, resulting in a performance degradation. A coherent dual-channel QPSK scheme which eliminates the crosstalk is proposed. Its performance is analyzed numerically for the reverse link of the CDMA system operating over Rayleigh fading channels and compared to W-CDMA QPSK modulation and noncoherent 64-ary orthogonal QPSK modulation. In addition, computer simulation is carried out by means of the COSSAP software tool.