  5G millimeter wave (mmWave) cellular networks have been reported to deliver
1-2 Gbps downlink throughput, via speed-tests. However, these speed-tests
capture only a few seconds of throughput and are not representative of
sustained throughput over several minutes. We report the first measurements of
sustained throughput in three cities, Miami, Chicago, and San Francisco, where
we observe throughput throttling due to rising skin temperature of the phone
when it is connected to a deployed 5G mmWave base-station (BS). Radio Resource
Control (RRC) messaging between the phone and the BS indicates the reduction in
the number of aggregated mmWave channels from 4 to 1 followed by a switch to
4G. We corroborate these measurements with infra-red images as the phone heats
up. Thus, mmWave throughput will be limited not by network characteristics but
by device thermal management.
