  The problem of achieving ultra-reliable and low-latency communication (URLLC)
in multi-terminal networks has gained traction in the recent past owing to new
wireless applications in vehicular networks. In the context of multi-hop
networks, which is a classic example for multi-party communication, recent
studies have shown that automatic-repeat-request (ARQ) based decode-and-forward
(DF) strategies are suitable for URLLC since the idea of distributing a given
number of ARQs across the nodes provides fine control on the features of
reliability and latency. Inspired by these developments, in this work, we
propose a cooperative ARQ sharing strategy for URLLC in multi-hop networks. At
the heart of the proposed scheme lies the idea that every node is given the
knowledge of the number of ARQs allotted to its preceding node in addition to
the ARQs allotted to itself. As a result, each node only needs to count the
number of unsuccessful attempts of its preceding node, and then borrow the
unused ARQs, thereby improving the reliability feature with no compromise in
the latency constraint. Using packet-drop-probability (PDP) as the reliability
metric for the proposed cooperative strategy, we formulate an optimization
problem of minimizing the PDP subject to a sum constraint on the total number
of ARQs allotted across all the nodes. Supported by theoretical analysis on the
behavior of PDP, we present low-complexity algorithms to compute near-optimal
ARQ distributions for our strategy, and show that our strategy outperforms the
existing non-cooperative strategies.
