Nearly doubling the throughput of multiuser MIMO systems using codebook tailored limited feedback protocol

We present a new robust feedback and transmit strategy for multiuser MIMO downlink communication systems, termed Rate Approximation (RA), and analyze its performance. The new scheme combines flexibility and robustness needed for reliable communications with the user terminal under a limited feedback constraint. It responds to two important observations: One is that it is not so significant to approximate the channel but rather the (potential) rate itself so as to mimic the optimal scheduling decision at the base station. The second observation is that a fixed transmit codebook at the transmitter is often better when simultaneously the channel state information is more accurate. Both observations are incorporated in the new scheme where the transmit and feedback codebook are strictly separated and user rates are delivered to the base station subject to a controlled uniform error regardless what the scheduling decision is. The scheme is analyzed and proved to have better performance below a certain interference plus noise margin and better scaling properties than the classical Jindal formula when considered in the very same setting. LTE system simulations sustain the analytic results showing performance gains of up to 50% compared to zeroforcing when using multiple antennas at both the base station and the terminal, and up to 70% when using single antennas at the terminal. Finally, a new feedback protocol is developed which inherently considers the transmit codebook and which is able to deal with the complexity issue at the terminal.