In Band Signal Cancellation Circuit
When the interferer appearing in the signal (signal plus interferer), can be obtained from a different source minus the intentional signal (interferer alone), then by phase shifting the interferer by 180 degrees and adjusting the level to be equal to the interferer present in the signal, the level of interferer can be significantly reduced, often to the extent of eliminating EMI = (signal + interferer + (interferer 180 degrees phase shifted) = signal).
In the above example the intentional signal is received on the Rx antenna which also receives the interferer. The interfering signal is from the transmitting antenna. The signal from the transmitting antenna is attenuated and passed to the phase shifter. This signal is added into the signal from the Rx antenna and thus the interferer is significantly reduced.