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A new accurate mathematical analysis is presented for the efficient computation of the spectral efficiency of CDMA downlink wireless communication systems in the presence of multipath Rayleigh fading and log-normal shadowing. A new explicit expression is derived for the spectral efficiency, which is based on an accurate interference model that accounts for both intra-cell and inter-cell interferences. This is used to investigate the performance of downlink CDMA in different multipath delay profiles. The numerical results has shown that the loss of orthogonality deteriorates the spectral efficiency significantly at high signal-to-noise ratios (SNR) levels. On the other hand, the diversity gain will compensate the loss of orthogonality at low SNR levels.