@article{guo2008relay,
  title={On relay node placement and assignment for two-tiered wireless networks},
  author={Guo, Wenxuan and Huang, Xinming and Lou, Wenjing and Liang, Cao},
  journal={Mobile Networks and Applications},
  volume={13},
  number={1},
  pages={186--197},
  year={2008},
  publisher={Springer}
}