  We present and analyze a new robust feedback and transmit strategy for
multiuser MIMO downlink communication systems, termed Rate Approximation (RA).
RA combines the flexibility and robustness needed for reliable communications
with the user terminal under a limited feedback constraint. It responds to two
important observations. First, it is not so significant to approximate the
channel but rather the rate, such that the optimal scheduling decision can be
mimicked at the base station. Second, a fixed transmit codebook at the
transmitter is often better when therefore the channel state information is
more accurate. In the RA scheme the transmit and feedback codebook are
separated and user rates are delivered to the base station subject to a
controlled uniform error. The scheme is analyzed and proved to have better
performance below a certain interference plus noise margin and better behavior
than the classical Jindal formula. LTE system simulations sustain the analytic
results showing performance gains of up to 50% or 70% compared to zeroforcing
when using multiple antennas at the base station and multiple antennas or a
single antenna at the terminals, respectively. 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.
