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This paper deals with superposition modulation for a three-node system (two sources, one destination) in a cooperative ad hoc configuration. An information-theoretic achievable capacity region is given for both amplify-and-forward (AF) and decode-and-forward (DF) superposition schemes, and it is shown that it is dependent on the frame length. To simplify the analysis of the superposition modulation, an approximation of the achievable capacity region is proposed. This approximation uses only two well-defined time slots with appropriate capacity constraints and efficiently approaches the true capacity region for large frame lengths. Another issue that is discussed in this paper is the definition of the optimal superposition weight factor. In contrast with previously reported studies, which were based on simulation results, a theoretical framework that provides the optimal weight factor is also investigated. The proposed algorithm uses simplified outage probabilities of the system model, and the result only depends on the required spectral efficiency. Analytical results and simulation studies show the gains of the proposed schemes.