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In this paper, the effect of imperfect channel estimation on the transmit diversity based on space-time block coding for the downlink of a direct-sequence code-division multiple-access system is studied. Two transmit antenna and one receiving antennas are employed. However, the results of this paper can be extended to the system with more receiving antennas. Each channel is modeled as frequency-selective Rayleigh fading and the pair of channels corresponding to two transmit antennas are mutually independent. Both spatial diversity gain and multipath diversity gain are obtained in the system. The system performance is evaluated in terms of bit-error rate under the perfect and imperfect channel estimation. A pilot-assisted channel-estimation scheme with one common spreading code sequence is exploited. It is shown that the inaccurate channel estimates suffering from multiple access and multipath interference significantly degrade the system performance and can be effectively improved by use of a simple low-pass filter. The investigation of the power ratio of pilot to data channels illustrates that the base station should dynamically adjust the transmit power of the pilot channel according to the varying system configurations in order to achieve the best performance.