Abstract: Based on the idea of cooperative relay, we proposed a system model that the secondary user assisted the primary user to transmit, and used the Lagrange dualoptimized power allocation algorithm based on decoding and forwarding for power allocation, which solved the problem of excessive interference caused by the secondary user accessing the primary user frequency band. The simulation results show that the proposed algorithm can greatly reduce the interference to the 
primary users under the premise of ensuring the normal communication of the secondary users, effectively increase the resistance of the primary user to the external environment and improve the system capacity of the primary user.

Key words: cognitive radio, cooperative relay, power allocation, Lagrangian operator