2) Multi-RAT Association: Networks will continue to become
increasingly heterogeneous as we move toward 5G. A key
feature therein will be increased integration between different
RATs, with a typical 5G-enabled device having radios capable
of supporting not only a potentially new 5G standard (e.g., at
mmWave frequencies), but also 3G, numerous releases of 4G
LTE including possibly LTE-Unlicensed [33], several types of
WiFi, and perhaps direct device-to-device (D2D) communication,
all across a great many spectral bands. Hence, determining
which standard(s) and spectrum to utilize and which BS(s) or
users to associate with will be a truly complex task for the
network [34] (see Fig. 1).