Backlog-adaptive compression: Age of information

The end-to-end delay of streaming source coding is characterized by an age of information (AoI) metric that measures the number of symbol periods the decoder output lags behind the encoder input. The source encoder receives input source symbols one per unit time and sequentially outputs binary codewords to a constant rate channel that transmits bits to the decoder. We examine a system in which knowledge of the busy/idle state at the channel interface enables the encoder to switch among codebooks with different source blocklengths based on the backlog of symbols at the encoder. We start by introducing two source sequence parsing policies and show that in each of them the blocklength process can be modeled by a Markov chain. We show by experiments that blocklength adjustment based on the channel interface state provides lower average age than codes with fixed blocklength. Aiming to avoid unnecessary frequent blocklength changes by the encoder backlog, we propose maximum blocklength control scheme at the encoder to further reduce the average age.