  This paper introduces a new mathematical framework, which is used to derive
joint uplink/downlink achievable rate regions for multi-user spatial
multiplexing between one base station and multiple terminals. The framework
consists of two models: the first one is a simple transmission model for uplink
and downlink, which is capable to give a lower bound on the capacity for the
case that the transmission is subject to imperfect CSI. A detailed model for
concrete channel estimation and feedback schemes provides parameter input to
the former model and covers the most important aspects such as pilot design
optimization, linear channel estimation, feedback delay, and feedback
quantization. We apply this framework to determine optimal pilot densities and
CSI feedback quantity, given that a weighted sum of uplink and downlink
throughput is to be maximized for a certain user velocity. We show that for low
speed, and if downlink throughput is of particular importance, a significant
portion of the uplink should be invested into CSI feedback. At higher velocity,
however, downlink performance becomes mainly affected by CSI feedback delay,
and hence CSI feedback brings little gain considering the inherent sacrifice of
uplink capacity. We further show that for high velocities, it becomes
beneficial to use no CSI feedback at all, but apply random beamforming in the
downlink and operate in time-division duplex.
