Current software-defined radio systems enable transmission at nearly arbitrary frequencies, presenting the possibility of harmful interference to existing communication services when broadcasting over-the-air. The Platform for Open Wireless Data-driven Experimental Research (POWDER) provides software radios whose output can be amplified and transmitted over-the-air. POWDER must include a spectrum monitoring system that can identify users who are transmitting outside allowed frequency bands to ensure wireless spectrum license holders do not experience harmful interference.

Power amplifiers in the transmit signal path can create emissions at center frequency harmonics and other spurious emissions. A spectrum monitoring system, coupled with signal paths after all amplifiers in the transmit chain, can detect these emissions. However, incident radio frequency energy combines with the output signal, which is no longer buffered by the amplifier. Incident and transmitted signals must be separated and isolated. The monitor can then analyze the isolated transmitted signal for out-of-band energy. This paper presents a system that can achieve isolation and identify users that broadcast out-of-band.