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Cooperative diversity systems have recently been proposed as a solution to provide spatial diversity for terminals where multiple antennas are not feasible to be implemented. As in multiple-input-multiple-output systems, space-time codes can be used to efficiently exploit the increase in capacity provided in cooperative diversity systems. In this paper, we propose a two-layer linear dispersion (LD) code for cooperative diversity systems and derive a simulation-based optimization algorithm to optimize the LD code and power allocation in terms of block error rate. The proposed code design paradigm can obtain optimal codes under arbitrary fading statistics. Performance comparisons are made to other cooperative diversity schemes. The effect that distances between source, relays, and destination terminals have on the energy allocation between the broadcast and cooperative intervals is also studied. Cooperative diversity, gradient estimation, linear dispersion (LD) codes, multiple-input-multiple-output (MIMO), stochastic approximation.