@article{massivemimoisarealitywhatisnextfive,
  title = {Massive MIMO is a Reality -- What is Next? Five Promising Research Directions for Antenna Arrays},
  year = {2019},
  eprint = {1902.07678},
  archivePrefix = {arXiv},
  url = {http://arxiv.org/abs/1902.07678v2},
}