@misc{indiciae6fb722226dca,
  title = {Codes for QPSK modulation with invariance under 90 degrees rotation},
  author = {Ungerboeck, Gottfried and Pietrobon, Steven S.},
  year = {1988},
  url = {https://ntrs.nasa.gov/citations/19880016336},
  note = {Source identifier: 19880016336}
}