Achieving large spectral efficiency with TDD and not-so-many base-station antennas

Time-Division duplexing (TDD) allows to estimate the downlink channels for an arbitrarily large number of base station antennas by using a finite number of orthogonal pilot signals in the uplink, by exploiting channel reciprocity. Based on this observation, a recently proposed “Massive MIMO” scheme was shown to achieve unprecedented system spectral efficiency using a very simple beamforming scheme and a very large number of base station antennas per active user per cell. In this work, we consider a more sophisticated system design that partition the users' population in geographically determined “bins” and for each bin selects the best scheme in a family of network- MIMO schemes defined by the type of linear beamforming, cooperation among base stations, frequency reuse, pilot reuse and user loading factors. The spectral efficiency obtained by the proposed architecture is similar to that achieved by “Massive MIMO”, with a 10-fold reduction in the number of antennas at the base stations (roughly, from 500 to 50 antennas).