  We present a method to compute, quickly and efficiently, the mutual
information achieved by an IID (independent identically distributed) complex
Gaussian input on a block Rayleigh-faded channel without side information at
the receiver. The method accommodates both scalar and MIMO (multiple-input
multiple-output) settings. Operationally, the mutual information thus computed
represents the highest spectral efficiency that can be attained using standard
Gaussian codebooks. Examples are provided that illustrate the loss in spectral
efficiency caused by fast fading and how that loss is amplified by the use of
multiple transmit antennas. These examples are further enriched by comparisons
with the channel capacity under perfect channel-state information at the
receiver, and with the spectral efficiency attained by pilot-based
transmission.
