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For a fixed total bandwidth expansion factor, and for a fixed channel code rate, we consider the problem of optimal bandwidth allocation between the source coder and the spread-spectrum unit for a multicarrier direct-sequence CDMA system operating over a frequency-selective fading channel with narrowband interference. Assuming a Gaussian source with the optimum scalar quantizer and a binary convolutional code with soft-decision decoding, we obtain both a lower and an upper bound on the end-to-end average source distortion. The optimal bandwidth allocation is then numerically computed by minimizing upper and lower bounds on the average distortion. We show that the upper bound based cost function is a convex function of the source code rate, and the optimal allocation depends on the system and the channel conditions, such as the total number of active users, the number of carriers, and the average jammer-to-signal power ratio.