  Mobility and blockage are two critical challenges in wireless transmission
over millimeter-wave (mmWave) and Terahertz (THz) bands. In this paper, we
investigate network massive multiple-input multiple-output (MIMO) transmission
for mmWave/THz downlink in the presence of mobility and blockage. Considering
the mmWave/THz propagation characteristics, we first propose to apply per-beam
synchronization for network massive MIMO to mitigate the channel Doppler and
delay dispersion effects. Accordingly, we establish a transmission model. We
then investigate network massive MIMO downlink transmission strategies with
only the statistical channel state information (CSI) available at the base
stations (BSs), formulating the strategy design problem as an optimization
problem to maximize the network sum-rate. We show that the beam domain is
favorable to perform transmission, and demonstrate that BSs can work
individually when sending signals to user terminals. Based on these insights,
the network massive MIMO precoding design is reduced to a network sum-rate
maximization problem with respect to beam domain power allocation. By
exploiting the sequential optimization method and random matrix theory, an
iterative algorithm with guaranteed convergence performance is further proposed
for beam domain power allocation. Numerical results reveal that the proposed
network massive MIMO transmission approach with the statistical CSI can
effectively alleviate the blockage effects and provide mobility enhancement
over mmWave and THz bands.
