Rate Approximation: A New Paradigm for Multiuser MIMO Downlink Communications

In this paper we present a new paradigm for multiuser MIMO downlink communications called Rate Approximation with significant impact on the upcoming LTE standard. Rate Approximation combines flexibility and robustness needed for reliable communications with the terminal in the downlink and taking advantage of the channel state information. The scheme responds to two major developments in the recent literature: One observation is that it is not so important to approximate the channel but rather the rate itself. The second observation is that a fixed 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 brought to the base station subject to a controlled uniform error. The new metric is amenable to further numerical optimization. Analysis and simulations show the superior performance of the scheme and, furthermore, a strong impact on upcoming cooperative schemes is expected.