In numerous application areas, including biomedical engineering, radar, sonar and digital communi-

cations, the goal is to extract a useful signal corrupted by interferences and noises. Noise/interference

removal is facilitated when multiple sensors on dierent locations record the biomedical phe-

nomenon simultaneously.

For instance in recordings taken from the mother s skin during pregnancy, the electrical activity

from the fetal heartbeat can be masked by the stronger maternal cardiac activity. In electrocar-

diogram (ECG) recordings from atrial brillation suerers, the electrical activity from the atria

appears mixed with that from the ventricles. In electroencephalogram (EEG) recordings from

epileptic patients, epileptic discharges and the brain s background activity contribute simultane-

ously to the signals measured by scalp electrodes, and can be further corrupted by artifacts such

as eye blinks or body movements.