  High-performance real-time wireless connectivity is at the heart of the
Industrial Internet-of-Things (IIoT). Realizing wireless closed-loop control is
crucial for various mission-critical IIoT systems. Existing wireless
technologies fall short of meeting the stringent performance requirements of
closed-loop control. This paper presents a novel wireless solution, called
\textsf{GALLOP}, for realizing closed-loop control over multi-hop or single-hop
networks. \textsf{GALLOP} adopts a pragmatic design approach for addressing the
challenges of wireless closed-loop control. Key design aspects of
\textsf{GALLOP} include control-aware bi-directional scheduling for cyclic
exchange, robust retransmission techniques based on cooperative multi-user
diversity and low-overhead signaling for scalable operation. \textsf{GALLOP}
has been specifically designed for control loops that are closed over the whole
network with dynamics on the order of few milliseconds. Performance evaluation
based on extensive system-level simulations and hardware implementation on a
Bluetooth 5 testbed demonstrates that \textsf{GALLOP} provides high-performance
connectivity with very low and deterministic latency, very high reliability and
high scalability, to meet the stringent requirements of wireless closed-loop
control for versatile IIoT applications.
