  Non-linearities in radio-frequency (RF) transceiver hardware, particularly in
power amplifiers, cause distortion in-band and out-of-band. Contrary to claims
made in recent literature, in a multiple-antenna system this distortion is
correlated across the antennas in the array. A significant implication of this
fact is that out-of-band emissions caused by non-linearities are beamformed, in
some cases into the same direction as the useful signal.
