说明：  —In this paper, an energy-efficient optimal relay selection strategy which is jointly optimized with the energy- efficient optimal power allocation solution for AF cooperative transmission is proposed. The relay selection criterion is the distance to the optimal relay locations where the minimum transmission power of the source, the relay or their total can be achieved. To determine those most energy-efficient relay locations, a universal algorithm with low computational complexity and easy implementation is also presented in this paper. The simulations are conducted to validate our theoretical analysis. The results show that with the relay selected by the proposed strategy, the cooperative transmission can achieve considerably high energy-efficiency.