@article{chen2021structured,
  title={Structured massive access for scalable cell-free massive {MIMO} systems},
  author={Chen, Shuaifei and Zhang, Jiayi and Bj{\"o}rnson, Emil and Zhang, Jing and Ai, Bo},
  journal={IEEE J. Sel. Areas Commun.},
  volume={39},
  number={4},
  pages={1086--1100},
  year={2021},
  month={Apr.},
  publisher={IEEE}
}