Techniques for Multi-User MIMO with Two-Way Training

We consider forward-link multiuser MIMO transmission, whereby K users are served by a base station with a large number of (potentially distributed) transmit antennas. We address the problem of channel state information (CSI) mismatch between the transmitter and the receivers with a two-way training scheme. In a quasi-static setting, the proposed scheme requires only K pilot symbols for uplink training and as little as one symbol for downlink training. In particular, we show that the performance of the system where the receiver has perfect channel knowledge can be approached with a small value of downlink training symbols. We consider two variants of linear MMSE precoders that take into account the quality of the channel state information at the transmitter and demonstrate that robust high sum-rate systems can be designed that rely on single-antenna receivers, provided that a large enough number of antennas is used for transmission. Finally, we sketch an example of implementations for the equal rate case.