Real-Time Status Updates Over Erasure Channels
Real-time decision making relies on the availability of accurate data and, as such, delivering status updates in a timely fashion is of paramount importance. In this thesis, we first look at the interplay between average timeliness and design decisions made at the physical layer, for unreliable communication channels. Specifically, this study focuses on the natural tension between the protection afforded by additional redundancy and the decoding delay associated with longer codewords in the presence of feedback. The average timeliness is adopted as an evaluation criterion, and a framework to efficiently compute the performance of various transmission schemes for the binary erasure channel is developed. We show that the average timeliness decreases as we increase the feedback rate in a hybrid ARQ scheme. In particular, we provide design guidelines for codeword length selection for an hybrid ARQ scheme to improve the average information timeliness. We show numerical examples to further illustrate the applicability of our findings. Second, we study periodic transmission schemes for sources with highly correlated information. In contrast to the previous model, the source periodically sends the actual information interleaved with differential messages. This encoding scheme has been studied previously in the literature, and it is known that differential encoding can improve timeliness only under the assumption of perfect feedback with a cost.We show that differential encoding can have better average information timeliness even if no receiver feedback is available, by carefully selecting the codeword length of the differential messages.