Ted Rappaport of NYU, the world's foremost mmWave researcher, disagrees with my guess that mmWave 5G will be modest until 2021-2023. My opinion is backed up by opinions from several carriers and an estimate from Ovum that fewer than 1% of lines will be millimeter wave in 2021. Ted's opinion is shared by Verizon CEO Lowell McAdam, who will deploy in Boston and probably San Francisco as soon as Verizon can get the equipment. That should be late 2017 or early 2018. Nokia and Ericsson have hundreds of engineers working on 5G mmWave.
I will be delighted to be proven wrong and see more rapid progress. Since my comment, Rappaport and team have published a seminal paper, Millimeter Wave Wireless Communications: New Results for Rural Connectivity (Abstract below.) They were able to detect a 73 GHz signal 11 kilometers away from their transmitter, a carefully aligned antenna 110 meters above average terrain. (Pictured.) They used 1 watt of transmitter power, levels similar to today's mobile phones.