4.7 Article

Optimizing Information Freshness in Two-Hop Status Update Systems Under a Resource Constraint

Journal

IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS
Volume 39, Issue 5, Pages 1380-1392

Publisher

IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC
DOI: 10.1109/JSAC.2021.3065060

Keywords

Relays; Markov processes; Spread spectrum communication; Optimal scheduling; Performance analysis; Delays; Throughput; Information freshness; age of information; status update; constrained Markov decision process; Markov chain

Funding

  1. Chinese University of Hong Kong (CUHK) [4055126]
  2. Australian Research Council (ARC) [DP190101988, DP210103410]
  3. Australian Research Council Laureate Fellowship [FL160100032]

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The paper investigates the age minimization problem for a two-hop relay system, proposing an optimal policy and a low-complexity alternative with two thresholds. Theoretical analysis and numerical results show that the low-complexity policy achieves near-optimal performance compared to the optimal policy.
In this paper, we investigate the age minimization problem for a two-hop relay system, under a resource constraint on the average number of forwarding operations at the relay. We first design an optimal policy by modelling the considered scheduling problem as a constrained Markov decision process (CMDP) problem. Based on the observed multi-threshold structure of the optimal policy, we then devise a low-complexity double threshold relaying (DTR) policy with only two thresholds, one for relay's AoI and the other one for the age gain between destination and relay. We derive approximate closed-form expressions of the average AoI at the destination, and the average number of forwarding operations at the relay for the DTR policy, by modelling the tangled evolution of age at relay and destination as a Markov chain (MC). Numerical results validate all the theoretical analysis, and show that the low-complexity DTR policy can achieve near optimal performance compared with the optimal CMDP-based policy. Moreover, the relay should always consider the threshold for its local age to maintain a low age at the destination. When the resource constraint is relatively tight, it further needs to consider the threshold on the age gain to ensure that only those packets that can decrease destination's age dramatically will be forwarded.

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