Achieving Two-Level Age by Free-Ride Coding in Preemptive Mission-Critical Networks

Abstract

Age of information (AoI), which characterizes the freshness of information, is of significance to real-time Internet of Things (IoT) applications. This paper focuses on the timely transmission of the payload as well as the extra data in mission-critical networks, e.g., vehicular networks. To transmit both kinds of data simultaneously, a free-ride code is applied, in which the encoded extra data is transmitted along with the low-density parity-check (LDPC) coded payload by superposition, consuming neither extra transmit power nor extra bandwidth. Under the preemptive status update with truncated automatic repeat request (ARQ), we derive the closed-form expression of average peak AoI (PAoI) by taking into account the status update generation time, and evaluate the average PAoI of both payload and extra data. Numerical simulations show that free-ride code achieves two-level PAoI in status update systems. Comparisons with the traditional coding scheme show that free-ride code not only attains the smaller average PAoI of extra data but also does not impair the performance of the payload data, which can be used to piggyback a small amount of important extra bits over the existing payload data.