On the selection of semi-orthogonal users for zero-forcing beamforming

  • Authors:
  • Alessandro Tomasoni;Giuseppe Caire;Marco Ferrari;Sandro Bellini

  • Affiliations:
  • Politecnico di Milano, Milan, Italy;University of Southern California, Los Angeles, California;CNR-IEIIT, Politecnico di Milano, Milan, Italy;Politecnico di Milano, Milan, Italy

  • Venue:
  • ISIT'09 Proceedings of the 2009 IEEE international conference on Symposium on Information Theory - Volume 2
  • Year:
  • 2009

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We reconsider the role of user selection in multiuser MIMO broadcast channels (downlink), in the relevant regime where the number of users K is linear in the number of transmitter (base station) antennas M. User selection is known to achieve mutually quasi-orthogonal user channels and, at the same time, a multiuser diversity effect in terms of receiver SNR. These goals are achieved in the regime of fixed number of transmit antennas, and very large number of users. In contrast, we show that when K = O(M) these effects cannot be achieved, and the role of user selection is marginal. In terms of system design, our results suggest that only a small number K ≈ M of users should feedback their channel state information at each point in time. This greatly alleviates the burden of the channel state information feedback, while achieving essentially optimal performance.