Robust LMMSE beamformer design by naive UL/DL duality and validation for non-cooperative massive MIMO

Massive multiple input multiple output (MaMIMO) is key to enabling the 1000 fold capacity improvement promised by 5G, thanks to the linear increase in system capacity with the number of base station (BS) antennas. In this work, we consider a MaMIMO interfering broadcast channel (IBC) with no coordination among the BSs under Time Division Duplexing (TDD). Each BS only has partial channel state information (CSI)

of its own UE. Under this setting, we propose a duality based approach to combine the received covariance information at the BS and the estimated uplink (UL) channel from the UE to derive the downlink (DL) beamformers. We also take into account the fact that the channel reciprocity is impacted by the effect of the transmit and receive chain and correct it using the estimated reciprocity calibration factors. Essentially, using reciprocity, we

transform the partial CSI at the receiver in the UL to a partial CSI at the transmitter in the downlink (DL). The technique is evaluated on the Eurecom OpenAirInterface Massive MIMO hardware testbed [1].