  Broadcast MIMO techniques can significantly increase the throughput in the
downlink of cellular networks, at the price of channel state information (CSI)
feedback from the mobiles, sent over the uplink. Thus, it creates a mechanism
that can tradeoff some uplink capacity for increased downlink capacity. In this
work we quantify this tradeoff and study the exchange ratio between the
feedback rate (over the uplink) and the downlink rate. We study both finite and
infinite networks, and show that for high enough (but finite) SNR, the uplink
rate can be exchanged for increased downlink rate with a favorable exchange
ratio. This exchange ratio is an increasing function of the channel coherence
time, and a decreasing function of the number of measured base stations. We
also show that devoting a constant fraction of the uplink to CSI feedback can
increase the downlink multiplexing gain continuously from 0 to 1, in finite
networks. On the other hand, in infinite networks (with infinite connectivity)
our bounds can only show doubly logarithmic scaling of the rate with SNR. The
presented results prove that the adaptation of the feedback rate can control
the balance between the uplink and downlink rates. This capability is very
important in modern cellular networks, where the operators need to respond to
continuously changing user demands.
