  The capacity of ideal MIMO channels has a high-SNR slope that equals the
minimum of the number of transmit and receive antennas. This letter analyzes if
this result holds when there are distortions from physical transceiver
impairments. We prove analytically that such physical MIMO channels have a
finite upper capacity limit, for any channel distribution and SNR. The high-SNR
slope thus collapses to zero. This appears discouraging, but we prove the
encouraging result that the relative capacity gain of employing MIMO is at
least as large as with ideal transceivers.
