  This paper provides an analytical performance characterization of both uplink
(UL) and downlink (DL) user-centric network multiple-input multiple-output
(MIMO) systems, where a cooperating BS cluster is formed for each user
individually and the clusters for different users may overlap. In this model,
cooperating BSs (each equipped with multiple antennas) jointly perform
zero-forcing beamforming to the set of single-antenna users associated with
them. As compared to a baseline network MIMO systems with disjoint BS clusters,
the effect of user-centric clustering is that it improves signal strength in
both UL and DL, while reducing cluster-edge interference in DL. This paper
quantifies these effects by assuming that BSs and users form Poisson point
processes and by further approximating both the signal and interference powers
using Gamma distributions of appropriate parameters. We show that BS
cooperation provides significant gain as compared to single-cell processing for
both UL and DL, but the advantage of user-centric clustering over the baseline
disjoint clustering system is significant for the DL cluster-edge users only.
Although the analytic results are derived with the assumption of perfect
channel state information and infinite backhaul between the cooperating BSs,
they nevertheless provide architectural insight into the design of future
cooperative cellular networks.
