C-Band Key for Early 5G; So is 26 GHz to 29 GHz Millimeter Wave

The C band, around 3.3 GHz to 3.7 GHz, has emerged as a key platform for 5G globally. 

source: Everything RF

Globally, the most-used frequency bands to support 5G is in the C band, about 3.5 GHz to 3.7 GHz, followed by the 26 GHz to 29 GHZ millimeter bands. 

source: Global Mobile Suppliers Association