  This paper studies the cross-layer challenges and performance of Hybrid
Beamforming (HBF) and Multi-User Multiple-Input Multiple-Output (MU-MIMO) in 5G
millimeter wave (mmWave) cellular networks with full-stack TCP/IP traffic and
MAC scheduling. While previous research on HBF and MU-MIMO has focused on
link-level analysis of full-buffer transmissions, this work reveals the
interplay between HBF techniques and the higher layers of the protocol stack.
To this aim, prior work on full stack simulation of mmWave cellular network has
been extended by including the modeling of MU-MIMO and HBF. Our results reveal
novel relations between the networking layers and the HBF MU-MIMO performance
in the physical layer. Particularly, throughput can be increased in 5G networks
by means of Spatial Division Multiple Access (SDMA). However, in order to
achieve such benefits it is necessary to take into account certain trade-offs
and the implementation complexity of a full-stack HBF solution.
