HARQ Based Joint Uplink-Downlink Optimal Scheduling Strategy for Single-Loop WNCS

We present a hybrid-ARQ (HARQ) based uplink-downlink scheduling scheme for a single-loop wireless networked control system (WNCS) which includes two transmission scenarios: uplink transmission and downlink transmission. Uplink transmission can either transmits a new status update information or the previous information when a transmission failure occurs. By using the HARQ mechanism, retransmission can ensure a higher probability of successful reception. Uplink transmission can help the controller generate a more reliable state estimate. On the other hand, downlink transmission transmits commands to ensure timely control of the plant state. To achieve balance, we propose a new scheduling strategy to minimize the long-term average plant state mean square error (MSE) and model the optimization problem as a Markov decision process (MDP). Then we give the sufficient condition for the existence of the joint optimal strategy and obtain the joint optimal strategy by using the relative value iterative algorithm. Numerical simulation shows that compared with the existing scheduling strategies, the joint optimal strategy can reduces the long-term average plant state MSE effectively.