  This paper studies the performance of block coding on an additive white
Gaussian noise channel under different power limitations at the transmitter.
Lower bounds are presented for the minimum error probability of codes
satisfying maximal and average power constraints. These bounds are tighter than
previous results in the finite blocklength regime, and yield a better
understanding on the structure of good codes under an average power limitation.
Evaluation of these bounds for short and moderate blocklengths is also
discussed.
