This paper introduces a new mathematical framework, which is used to derivejoint uplink/downlink achievable rate regions for multi-user spatialmultiplexing between one base station and multiple terminals. The frameworkconsists of two models: the first one is a simple transmission model for uplinkand downlink, which is capable to give a lower bound on the capacity for thecase that the transmission is subject to imperfect CSI. A detailed model forconcrete channel estimation and feedback schemes provides parameter input tothe former model and covers the most important aspects such as pilot designoptimization, linear channel estimation, feedback delay, and feedbackquantization. We apply this framework to determine optimal pilot densities andCSI feedback quantity, given that a weighted sum of uplink and downlinkthroughput is to be maximized for a certain user velocity. We show that for lowspeed, and if downlink throughput is of particular importance, a significantportion 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 ofuplink capacity. We further show that for high velocities, it becomesbeneficial to use no CSI feedback at all, but apply random beamforming in thedownlink and operate in time-division duplex.
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