Past exam of the communication Theory course of the University of Oxford / 2010 / 3 / a / Solution

For a finite joint distribution, the mutual information isZero-probability terms contribute zero. It measures the reduction in uncertainty about one variable upon observing the other.

A discrete memoryless channel has discrete input/output alphabets and a fixed transition matrix $W(y\mid x)$. Over successive uses it obeysThus the output at a use depends on that input and no other channel history. Its channel capacity isin bits per use for base-two logarithms. On finite alphabets the maximum exists by continuity on the probability simplex; for more general discrete alphabets one uses a supremum.