Simple Finite-Length Achievability and Converse Bounds for the Deletion Channel and the Insertion Channel

We develop upper bounds on code size for independent and identically distributed deletion (insertion) channel for given code length and target frame error probability. The bounds are obtained as a variation of a general converse bound, which, though available for any channel, is inefficient and not easily computable without a good reference distribution over the output alphabet. We obtain a reference output distribution for a general finite-input finite-output channel and provide a simple formula for the converse bound on the capacity employing this distribution. We then evaluate the bound for the deletion channel with a finite block length and show that the resulting upper bound on the code side is tighter than that for a binary erasure channel, which is the only alternative converse bound for this finite-length setting. Also, we provide the similar results for the insertion channel.