  Future virtualized radio access network (vRAN) infrastructure providers (and
today's experimental wireless testbed providers) may be simultaneously
uncertain what signals are being transmitted by their base stations and legally
responsible for their violations. These providers must monitor the spectrum of
transmissions and external signals without access to the radio itself. In this
paper, we propose FDMonitor, a full-duplex monitoring system attached between a
transmitter and its antenna to achieve this goal. Measuring the signal at this
point on the RF path is necessary but insufficient since the antenna is a
bidirectional device. FDMonitor thus uses a bidirectional coupler, a
two-channel receiver, and a new source separation algorithm to simultaneously
estimate the transmitted signal and the signal incident on the antenna. Rather
than requiring an offline calibration, we also adaptively estimate the linear
model for the system on the fly. FDMonitor has been running on a real-world
open wireless testbed, monitoring 19 SDR platforms controlled (with bare metal
access) by outside experimenters over a seven month period, sending alerts
whenever a violation is observed. Our experimental results show that FDMonitor
accurately separates signals across a range of signal parameters. Over more
than 7 months of observation, it achieves a positive predictive value of 97%,
with a total of 20 false alerts.
