The situation that occurs when many stations transmit simultaneously using frequencies that are close together, i.e., with insufficient width of frequency guard bands or channel spacing. Note: Frequency spectrum congestion causes (a) difficulty in discrimination by tuning, (b) overlap of (i) a sideband and an adjacent carrier, or (ii) upper and lower sidebands, respectively, of adjacent carriers, and (c) interference that occurs when frequencies shift slightly or are phase shifted by ionospheric reflection.