The presence of a blocker signal or a large unwanted signal plays a big role on the performance of the receiver. In addition to causing IMD (intermodulation distortion) problems, the presence of an unwanted signal (commonly called a Blocker) can degrade the Noise Figure of the Receiver. So in addition to specifying the Noise Figure of a component, a Noise Figure with a Blocker present is also specified. In-Band Blockers, unwanted legitimate channels in the band, cannot be filtered at the input of the receiver.