2019 Mobile 5G Phones! blazes on ads across the trade media from Qualcomm. "You’ll see 5G in 2019 for sure," Qualcomm CEO Steve Mollenkopf promises. Verizon has begun a huge, rapid deployment of mmWave, with hundreds of sites installed.
Mollenkopf himself thought it would be 2020, but new technology is allowing him to chop a year off the schedule. His main foundry, TSMC, has begun "risk production" of 7 nm chips, the next generation. It's reasonable to expect volume production in 2019. TSMC is taking delivery in 2018 on $150M EUV machines to improve critical layers. Steve is going for 2019 because he can.
Verizon will be ready to serve mobile 5G "as soon as phones are available." China Mobile and AT&T are also looking to build early. They will be expensive and probably power hogs, but a true gigabit should be possible. ("Gig LTE" will usually be 75-300 megabits to the customer, per Qualcomm.)