On the Effect of Spectral Location of Interferers on Linearity Requirements for Wideband Cognitive Radio Receivers

Since 2008, The Federal Communications Commission (FCC) allows the operation of Cognitive Radio (CR) in unused parts (i.e. white spots) of the DTV spectrum. Due to the nonlinearity of the radio receiver and the existence of strong DTV signals, different types of distortion products will be generated in the CR-receiver. This paper analyzes the spectral location of distortion products across the white spots depending on the location of the DTV signals in the RF spectrum, focusing on 3rd order distortion products. Based on this analysis, we show that a receiver is always limited by cross-modulation (XM3) and self-interference products. Thus true distortion free white spots do not exist if DTV signals are present after the RF-band filter. However, XM3 and self-interference distortion products are typically much weaker than 3rd order intermodulation (IM3) products. Thus it makes sense to monitor the level and spectral location of interferes and classify the "white spots" into two types, namely IM3-spots and IM3-free spots. This paper derives equations to quantify how much the 3rd order linearity requirements are relaxed when the CR operates at an IM3-free spot. The analysis not only takes into account narrowband interferers but also wideband interferers. The analysis is verified by measurements.