  We propose a technique to derive upper bounds on Gallager's cost-constrained
random coding exponent function. Applying this technique to the non-coherent
peak-power or average-power limited discrete time memoryless Ricean fading
channel, we obtain the high signal-to-noise ratio (SNR) expansion of this
channel's cut-off rate. At high SNR the gap between channel capacity and the
cut-off rate approaches a finite limit. This limit is approximately 0.26 nats
per channel-use for zero specular component (Rayleigh) fading and approaches
0.39 nats per channel-use for very large specular components.
  We also compute the asymptotic cut-off rate of a Rayleigh fading channel when
the receiver has access to some partial side information concerning the fading.
It is demonstrated that the cut-off rate does not utilize the side information
as efficiently as capacity, and that the high SNR gap between the two increases
to infinity as the imperfect side information becomes more and more precise.
