In a MIMO channel, the ability to exploit space-division multiple-access techniques for spectrally efficient wireless communications is determined by the rank of the complex channel matrix H. (The rank of a matrix is defined by the number of independent columns in the matrix.)

For a given (Nt , Nr) antenna configuration, it is desirable that the rank of H equal the minimum one of Nt transmit and Nr receive antennas, for it is only then that we are able to exploit the full potential of the MIMO antenna configuration. Under special conditions, however, the rank of the channel matrix H is reduced to unity, in which case the scattering (fading) energy flow across the MIMO link is effectively confined to a very narrow pipe, and with it, the channel capacity is severely degraded.

Under the special conditions just described, a physical phenomenon known as the keyhole channel or pinhole channel is known to arise. Using a propagation layout of the MIMO link, describe how this phenomenon can be explained.