Age of Information for Short-Packet Relay Communications in Cognitive-Radio-Based Internet of Things With Outdated Channel State Information

Abstract

It is crucial to consider time-critical communications in cognitive-radio-based Internet of Things (CR-IoT), which increasingly rely on the exchange of short-packet information to efficiently monitor and control. Therefore, the age of information (AoI) is exploited as a timeliness metric in this paper to investigate the freshness of received information at the remote server with dual-hop short-packet communications in underlay CR-IoT. Since short-packet communications are sensitive to time delay, the influence of outdated factors in the channel state information (CSI) cannot be ignored, so we develop the system under the condition of outdated CSI with Nakagami-m fading channels. Considering the impact of outdated CSI and packets error, we investigate the performance of dual-hop short-packet communications under the classical automatic repeat-request (ARQ) protocol and truncated ARQ (TARQ) protocol, and obtain the approximated expressions of the average AoI and the average peak AoI (PAoI), respectively. Accordingly, the blocklength and the number of retransmissions can be optimized by Fibonacci and iterative algorithms to minimize the average PAoI under two protocols, respectively. Simulation results substantiate the influence of each parameter on the system performance differently under two different protocols. Moreover, the AoI of the dual-hop is also better than that of the single-hop due to the interference threshold of the primary user.