Bounding techniques for the intrinsic uncertainty of channels

A channel can generally be defined by a probability distribution on a set of possible actions. These actions determine the output for any possible input, and are independently drawn. The intrinsic uncertainty of a channel is defined as the conditional entropy of the action given the input and output sequences. For many channels, such as the deletion channel, the insertion channel, and various permutation channels, e.g., the trapdoor channel, quantifying the intrinsic uncertainty is the main challenge in determining the capacity. In this paper, we derive an alternative expression for the intrinsic uncertainty via the Laplace variational principle, and utilize it to obtain a general lower bound for the capacity. As an example, we apply our bound to the binary deletion channel and show that for the special case of an i.i.d. input distribution and a range of deletion probabilities, it outperforms the best known lower bound for the mutual information.