While reading the book Search Problems by Rudolf Ahlswede and Ingo Wegener (sadly, both deceased), I have come across a neat result by Ahlswede and Péter Gács, which can be thought of as a unified information-theoretic converse for noiseless and noisy channel codes. In particular, it contains as special cases the well-known Kraft’s inequality for prefix codes and the strong converse for discrete memoryless channels (DMC’s). In fact, Ahlswede and Gács establish achievability as well using a Feinstein-type maximal code construction, but I will focus on the converse.
Now, as shown recently by Yury Polyanskiy in his doctoral dissertation (see also the paper he wrote with Vince Poor and Sergio Verdú), most existing converse results for channel codes are consequences of a deceptively simple “meta-converse” involving the performance of binary hypothesis tests in the Neyman–Pearson framework. In this post, I will show that this is also the case for the Ahlswede–Gács converse.