  In this paper, we propose network massive multiple- input multiple-output
(MIMO) systems, where three radio units (RUs) connected via one digital unit
(DU) support multiple user equipments (UEs) at a cell-boundary through the same
radio resource, i.e., the same frequency/time band. For precoding designs,
zero-forcing (ZF) and matched filter (MF) with vector or matrix normalization
are considered. We also derive the formulae of the lower and upper bounds of
the achievable sum rate for each precoding. Based on our analytical results, we
observe that vector normalization is better for ZF while matrix normalization
is better for MF. Given antenna configurations, we also derive the optimal
switching point as a function of the number of active users in a network.
Numerical simulations confirm our analytical
