Second Order Asymptotics for Communication under Strong Asynchronism

Abstract: The capacity under strong asynchronism was recently shown to be essentially unaffected by the imposed output sampling rate $\rho$ and decoding delay $d$---the elapsed time between when information is available at the transmitter and when it is decoded. This paper examines this result in the finite blocklength regime and shows that, by contrast with capacity, the second order term in the rate expansion is sensitive to both parameters. When the receiver must exactly locate the sent codeword, that is $d=n$ where $n$ denotes blocklength, the second order term in the rate expansion is of order $\Theta(1/\rho)$ for any $\rho=O(1/\sqrt{n})$---and $\rho =\omega(1/n)$ for otherwise reliable communication is impossible. However, if $\rho=\omega(1/\sqrt{n})$ then the second order term is the same as under full sampling and is given by a standard $O(\sqrt{n})$ term whose dispersion constant only depends on the level of asynchronism. This second order term also corresponds to the case of the slightly relaxed delay constraint $d\leq n(1+o(1))$ for any $\rho=\omega(1/n)$.