  The gold standard of a wireless network is that the throughput increases
linearly with the density of access points (APs). However, such a linear
throughput gain is suspended in the 5G mmWave network mainly due to the short
communication distances in mmWave bands and the dense deployments of mmWave
APs. As being operated in the interference-limited regime, the aggregate
interference resulted from the increasing mmWave APs will gradually become the
network performance bottleneck, which leads to the saturation of the
throughput. In this paper, we propose to overcome the densification plateau of
a mmWave network by employing spatial multiplexing at APs. To study the effect
of spatial multiplexing on mmWave networks, we first derive the coverage
probability as a function of spatial multiplexing gain. The fixed-rate coding
scheme is then used to provide the network throughput. We also introduce the
concept of densification gain to capture the improvement in network throughput
achieved through the AP densification. Our results indicate that without the
spatial multiplexing at APs, the throughput of mmWave networks will reach the
plateau when the density of APs becomes sufficiently large. By enabling the
spatial multiplexing at APs, however, the mmWave network can continuously
harvest the throughput gains as the number of APs grows. By deriving the upper
bound for the throughput of mmWave networks, we quantify the potential
throughput improvement as the spatial multiplexing gain increases. However, our
numerical results show that such a potential throughput gain cannot be explored
by the fixed-rate coding scheme. We then demonstrate the necessity for
deploying the multi-rate coding scheme in mmWave networks, especially when the
spatial multiplexing gain at mmWave APs is large.
