We consider the problem of soft-covering with constant composition superposition codes and characterize the optimal soft-covering exponent. A double-exponential concentration bound for deviation of the exponent from its mean is also established. We demonstrate an application of the result to achieving the secrecy-capacity region of a broadcast channel with confidential messages under a per-codeword cost constraint. This generalizes the recent characterization of the wiretap channel secrecy-capacity under an average cost constraint, highlighting the potential utility of the superposition soft-covering result to the analysis of coding problems.
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