—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.