  We consider the problem of designing low latency and low complexity schemes
for channel state feedback over the MIMO-MAC (multiple-input multiple-output
multiple access channel). We develop a framework for analyzing this problem in
terms of minimizing the MSE distortion, and come up with separated
source-channel schemes and joint source-channel schemes that perform better
than analog feedback. We also develop a strikingly simple code design based on
scalar quantization and uncoded QAM modulation that achieves the theoretical
asymptotic performance limit of the separated approach with very low complexity
and latency, in the case of single-antenna users.
