  We consider a MIMO fading broadcast channel where channel state information
is acquired at user terminals via downlink training and channel feedback is
used to provide transmitter channel state information (CSIT) to the base
station. The feedback channel (the corresponding uplink) is modeled as an AWGN
channel, orthogonal across users. The total bandwidth consumed is the sum of
the bandwidth/resources used for downlink training, channel feedback, and data
transmission. Assuming that the channel follows a block fading model and that
zeroforcing beamforming is used, we optimize the net achievable rate for
unquantized (analog) and quantized (digital) channel feedback. The optimal
number of downlink training pilots is seen to be essentially the same for both
feedback techniques, but digital feedback is shown to provide a larger net rate
than analog feedback.
