  Emerging wireless control applications demand for extremely high closed-loop
reliability under strict latency constraints, which the conventional Automatic
Repeat reQuest (ARQ) solutions with static schedules fail to provide. To
overcome this issue and enable data-link layer error control for ultra reliable
low-latency communication (URLLC) services, we propose a novel protocol: the
Closed-Loop ARQ (CLARQ), which forces to accomplish an information exchange
round within a fixed loop-back latency, and dynamically re-allocates the
remaining resource between uplink and downlink slots upon the result of last
uplink transmission. The proposed method guarantees to meet the latency
requirement, while delivering high communication reliability and power
efficiency. It can be efficiently offline optimized by means of dynamic
programming techniques, and is capable of real-time deployment with a low-cost
implementation based on look-up tables. Numerical evaluations have verified
that CLARQ outperforms baselines with significantly improved closed-loop
reliability and reduced energy consumption. Especially, over a Rayleigh channel
with 0dB mean SNR, it is able to provide a closed-loop error rate below 1e-7
within 10ms loop-back latency, which makes our proposal competitive for
practical URLLC applications in future 5G-and-beyond networks.
