Conventional half-duplex amplify-and-forward（AF） method always adopted cooperative communications model regardless the fading state of the channel

which made insufficient use of the degrees of freedom of the system in a way.So

the problem about "when to cooperate" and the optimum allocation of transmitted power were discussed.An exact ergodic capacity formulation and corresponding upper bound were first derived for a two-user AF cooperative communications system.And then an approximately optimum power allocation scheme（AOPA） was proposed by making use of the upper bound as cost function.Whether the AOPA scheme adopting the cooperative model or not based on the comparison about the channel gains of source-to-destination with that of relay-to-destination.Finally

the system per-formances were analyzed with AOPA scheme when the relay moved in terms of the random direction model.Theoretical analysis and simulation results show that the AOPA outperforms the EPA scheme both in ergodic capacity and symbol-error-rate（SER） performance.