Abstract: In order to increase the capacity of cooperative diversity system, a novel power allocation scheme based on water-filling algorithm is proposed under the background of multiple-relay cooperation. First, a novel node index mapping scheme is provided to enhance the orthogonality of signal transmitted by relay nodes. For the GABBA codes applying linear orthogonal decoding, the proposed scheme can decrease the symbol error rate. Then, by analyzing the relationship between SNR and the power allocated by relay nodes, a two-step solution with water-filling algorithm is proposed to solve the optimization problem. Simulation results indicate that, the scheme based on water-filling algorithm can obtain better performance on SER compared with equal allocation scheme. In addition, both the two schemes can obtain the same diversity order NtNr, which is same as MIMO scenario. Furthermore, this scheme can achieve higher capacity for different antenna-deployment. The capacity of cooperative diversity system is proportional to min{NtNr}log2M.