The average outage probabilities of the amplify-and-forward (AF) relaying and the decode-and-forward (DF) relaying with/without diversity in Rayleigh fading were analyzed respectively, and the analytical expressions were presented. To minimize the average outage probability, with total transmit power constraint, the statistically optimal power allocation problem of the AF relaying and DF relaying were considered, and also the analytical expressions are provided. Then the statistical optimal relay selection was suggested to further minimize the average outage probability. Though the analytical expressions of power allocation for AF and DF with diversity were obtained under the condition of high transmit SNR, the simulation results show that they are also valid in the low transmit SNR region. Compared with fixed power allocation, the statistical optimal power allocation effectively reduces the average outage probability. Meanwhile, the optimal relay selection further reduces the average outage probability.%分析Rayleigh信道下分集/无分集的AF( Amplify and Forward)和DF( Decode and Forward)模式单中继传输的平均中断概率,给出解析表达式.在总功率约束下推导了4种模式下的统计最优功率分配的解析表达式来最小化系统的平均中断概率,并提出通过最佳中继选择进一步降低系统的平均中断概率.仿真结果表明,虽然AF模式和分集DF模式下的功率分配是在高发送SNR条件下推导的,但其在低发送SNR下依然有效；相对于固定功率分配,依据节点统计信息的最优功率分配可以有效降低系统的平均中断概率,多中继网络中的最佳节点选择进一步降低了系统的平均中断概率.
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