  We conduct to our knowledge a first measurement study of commercial 5G
performance on smartphones by closely examining 5G networks of three carriers
(two mmWave carriers, one mid-band carrier) in three U.S. cities. We conduct
extensive field tests on 5G performance in diverse urban environments. We
systematically analyze the handoff mechanisms in 5G and their impact on network
performance. We explore the feasibility of using location and possibly other
environmental information to predict the network performance. We also study the
app performance (web browsing and HTTP download) over 5G. Our study consumes
more than 15 TB of cellular data. Conducted when 5G just made its debut, it
provides a "baseline" for studying how 5G performance evolves, and identifies
key research directions on improving 5G users' experience in a cross-layer
manner. We have released the data collected from our study (referred to as
5Gophers) at https://fivegophers.umn.edu/www20.
