Age-Optimal Two-Layer Coding over Fading Channels

We consider a system in which the source node delivers status updates to the monitor node over a fading channel with nonnegligible propagation delay. In this work, we propose a two-layer coding scheme which uses erasure-correction coding across packets and error-correction coding within each packet to achieve timely delivery of status updates. In addition, we employ the age of information (AoI) for the timeliness evaluation and design a rate-allocation method to match the fading channel and minimize the average AoI of the status update system with a given reliability constraint. By formulating and solving the age-optimal rate-allocation problem for the proposed two-layer coding scheme, explicit expressions of the optimal rate-allocation results are derived, based on which the optimal average AoI is obtained. Numerical analysis and results demonstrate the existence of tradeoff in allocating redundancy between erasure and error-correction codes. The results reveal that the selection of optimal coding rates relies on the channel conditions and the signal to noise ratio (SNR). We find that for any nontrivial fading statistics, the proposed two-layer coding scheme and age-optimal rate-allocation method have a significant benefit over the pure physical-layer coding.