  Communications at frequencies above 10 GHz (the mmWave band) are expected to
play a major role for the next generation of cellular networks (5G), because of
the potential multi-gigabit, ultra-low latency performance of this technology.
mmWave frequencies however suffer from very high isotropic pathloss, which may
result in cells with a much smaller coverage area than current LTE macrocells.
High directionality techniques will be used to improve signal quality and
extend coverage area, along with a high density deployment of mmWave base
stations (BS). However, when propagation conditions are hard and it is
difficult to provide high quality coverage with mmWave BS, it is necessary to
rely on previous generation LTE base stations, which make use of lower
frequencies (900 MHz - 3.5 GHz), which are less sensitive to blockage and
experience lower pathloss. In order to provide ultra-reliable services to
mobile users there is a need for network architectures that tightly and
seamlessly integrate the LTE and mmWave Radio Access Technologies. In this
paper we will present two possible alternatives for this integration and show
how simulation tools can be used to assess and compare their performance.
