Two Measure is Two Know: Calibration-free Full Duplex Monitoring for Software Radio Platforms

Software-defined radio (SDR) platforms, despite being an enabler of spectrum and infrastructure sharing, must ensure that their transmitted signals comply with spectrum rules. Violations may occur because of malware, misconfiguration, bugs in software, or RF frontend nonlinearities. We present FDMonitor, a full-duplex monitoring system attached between a transmitter’s power amplifier and its antenna to monitor and control each SDR in the Platform for Open Wireless Data-driven Experimental Research (POWDER), an open city-scale SDR-based testbed. FDMonitor uses a bidirectional coupler, a two-port receiver, and a new source separation algorithm to simultaneously and adaptively estimate the transmitted signal and the signal incident on the antenna. FDMonitor has been running on POWDER since 2021, monitoring 19 SDR platforms accessible by outside experimenters. Its closed-loop feature sends alerts in real time whenever a violation is observed, and automatically turns off the SDR as necessary. Our experimental results show that FDMonitor accurately separates signals across a range of critical parameters. We further validate the system-wide performance of FDMonitor with 27 months of observation. Over this period, it achieves a positive predictive value of $95 \%$, with a total of 45 false alerts. Beyond its use on POWDER, FDMonitor, as a novel spectrum policy compliance solution, can be a key enabler of more dynamic sharing applications.