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Traditional fixed-rate error correction codes are not suitable for use in time-varying channels, for example in cellular radio channels. To overcome this problem, an adaptive scheme with differentially coherent detection for DS/CDMA systems is proposed. In this scheme, the transmitter and receiver parameters, as well as their rates, are adjusted according to the channel conditions, such that the degradation of the quality of transmission can be negligible. Analytical results for the bit-error-rate performance of the system are derived and compared with simulation results. It is found that there are significant improvements in performance, especially in fading channels with shadowing. Finally, a brief comparison is made between this system and its non-coherent counterpart, while considering the effect of the delay of the feedback channel.