Uplink downlink balancing using variable feedback rates

The broadcast MIMO channel have gained much attention in recent years, and in particular in the context of cooperative multi cell transmission in cellular networks. Broadcast MIMO techniques can signifcantly increase the capacity in the downlink of cellular networks, at the price of channel state information (CSI) feedback from the mobiles. Thus, it creates a mechanism that can tradeoff uplink rate to downlink rate. In this work we quantify this tradeoff between the feedback rate (over the uplink) and the downlink rate. We show that uplink rate can be exchanged to downlink rate with a constant exchange ratio. This exchange ratio is an increasing function of the channel coherence time, and a decreasing function of the number of measured BSs. We also show that devoting a constant ratio of the uplink to CSI feedback can increase the downlink multiplexing gain continuously from 0 to 1. This capability to control the balance between the uplink and downlink rates is very important in cellular networks, where the operators need to respond to continuously changing user demands.