Multi-User SWIPT Cooperative Networks: Is the Max–Min Criterion Still Diversity-Optimal?

This paper considers a general energy harvesting cooperative network with M source-destination (SD) pairs and one relay, where the relay schedules only m user pairs for transmissions. For the special case of m = 1, the addressed scheduling problem is equivalent to relay selection for the scenario with one SD pair and M relays. In conventional cooperative networks, the max-min selection criterion has been recognized as a diversity-optimal strategy for relay selection and user scheduling. The main contribution of this paper is to show that the use of the max-min criterion will result in loss of diversity gains in energy harvesting cooperative networks. Particularly, when only a single user is scheduled, analytical results are developed to demonstrate that the diversity gain achieved by the max-min criterion is only M+1/2 , much less than the maximal diversity gain M. This finding is important since it illustrates that the use of energy harvesting brings fundamental changes to the design of cooperative networks. Motivated by the loss of the max-min criterion, several user scheduling approaches tailored to energy harvesting networks are developed and their performance is analyzed. Simulation results are provided to demonstrate the accuracy of the developed analytical results and facilitate the performance comparison.