  With the advent of 5G, standardization and research are currently defining
the next generation of the radio access. Considering the high constraints
imposed by the future standards, disruptive technologies such as Massive MIMO
and mmWave are being proposed. At the heart of this process are wireless
channel models that now need to cover a massive increase in design parameters,
a large variety of frequency bands, and heterogeneous deployments. This
tutorial describes how channel models address this new level of complexity and
which tools the community prepares to efficiently but accurately capture the
upcoming changes in radio access design. We analyze the main drivers behind
these new modeling tools, the challenges they pose, and survey the current
approaches to overcome them.
