It looks like Verizon is the first U.S. carrier to officially publish and promote their 5G specification. Here's a quote from Adam Koeppe, Verizon’s vp of network technology planning:
The completion of the 5G radio specification is a key milestone toward the development of a complete 5G specification. The level of collaboration that we are seeing exceeds what we saw during 4G. This agile way of developing the specification and working with the ecosystem will enable us to get to market rapidly.
In the mean-time, other carriers aren't too far behind. Both AT&T and T-Mobile plan to expand or continue 5G testing with Nokia this year. Big Red was the first U.S. carrier to get on board the LTE bandwagon several years ago. It looks like they want to stay ahead of the curve again.
Source: PC Magazine