  We consider a multi-cell, frequency-selective fading, uplink channel (network
MIMO) where K user terminals (UTs) communicate simultaneously with B
cooperative base stations (BSs). Although the potential benefit of multi-cell
cooperation grows with B, the overhead related to the acquisition of channel
state information (CSI) will rapidly dominate the uplink resource. Thus, there
exists a non-trivial tradeoff between the performance gains of network MIMO and
the related overhead in channel estimation for a finite coherence time. Using a
close approximation of the net ergodic achievable rate based on recent results
from random matrix theory, we study this tradeoff by taking some realistic
aspects into account such as unreliable backhaul links and different path
losses between the UTs and BSs. We determine the optimal training length, the
optimal number of cooperative BSs and the optimal number of sub-carriers to be
used for an extended version of the circular Wyner model where each UT can
communicate with B BSs. Our results provide some insight into practical
limitations as well as realistic dimensions of network MIMO systems.
