Low overhead effective CSI signaling for beam coordination in TDD multi-cell MIMO systems

We propose a decentralized algorithm and corresponding low overhead signaling concepts of effective channel state information (CSI) for weighted sum rate (WSR) maximization via linear downlink transmit-receive beamforming in multi-cell multi-user MIMO systems operating in the time division duplex (TDD) mode. The proposed iterative processing consists of optimization steps that are run locally by base stations (BS), and facilitated by a combination of over-the-air uplink pilot signaling and scalar backhaul information exchange. In the uplink pilot signaling phase, each multi-antenna user terminal (UT) transmits a single precoded pilot beam. In case the UT was actively receiving, the uplink beam reveals its effective channel to all the coordinating BSs. On the other hand, if the UT was not receiving, the beam indicates its strongest effective vector channel to the serving BS. The processing strategy follows the changes of the time-varying channel and performs spatial user scheduling dynamically. According to the results, the strategy converges on the average in static channels, and in time-varying channels it effectively tracks the changes.