  We study the diversity order vs rate of an additive white Gaussian noise
(AWGN) channel in the whole capacity region. We show that for discrete input as
well as for continuous input, Gallager's upper bounds on error probability have
exponential diversity in low and high rate region but only subexponential in
the mid-rate region. For the best available lower bounds and for the practical
codes one observes exponential diversity throughout the capacity region.
However we also show that performance of practical codes is close to Gallager's
upper bounds and the mid-rate subexponential diversity has a bearing on the
performance of the practical codes. Finally we show that the upper bounds with
Gaussian input provide good approximation throughout the capacity region even
for finite constellation.
