On uncoded transmission and blocklength

This work considers the definition of the excess-distortion exponent, used to measure the asymptotic finite blocklength behavior of joint source-channel coding. We arrive at the conclusion that it is not a meaningful measure for the operational tradeoffs of a scheme. We propose a new definition, which makes a distinction between the processing block of the coding scheme (which implies delay and may be connected to complexity), the fidelity blocklength (reflecting the quality of the reconstruction as required by the application), and the resource blocklength (depending on hardware or shared medium considerations). As an aside, the exponent of uncoded schemes is analyzed. This results in finding the joint source-channel coding excess-distortion exponent in some cases where it was not known previously.