On fixed-length channel coding with feedback in the moderate deviations regime

Block coding over memoryless channels with causal and noiseless feedback is investigated in the moderate deviations regime. By means of a converse, we show that feedback does not improve the sub-exponential decay rate of the optimal error probability for a certain class of discrete memoryless channels, which includes symmetric ones, as well as the additive white Gaussian noise channel subject to an almost-sure power constraint.