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This paper presents an optimal rate allocation scheme in joint source-channel coding (JSCC) for image transmission over CDMA networks. Our scheme first uses progressive joint source-channel codes [V. Stankovic et al., Sep. 2002] to generate operational transmission rate-distortion (TR-D) functions of different multiple-access channels with different BERs. The Lagrange multiplier is then employed to set the optimal rate allocation among different channels, subject to a total transmission rate constraint. Progressive JSCC offers the scalability feature that is very attractive for handling bandwidth variations in wireless communications. By extending the rate-distortion function in source coding to the TR-D function in JSCC, our work borrows the "equal slope" argument [Y. Shoham and A. Gersho, Sep. 1988] to effectively solve the rate allocation problem in JSCC for multi-channel image transmission. Experiments show that our scheme runs in real time and provides competitive performance at all intermediate transmission rates.