  The fifth-generation (5G) of mobile standards is expected to provide
ultra-reliability and low-latency communications (URLLC) for various
applications and services, such as online gaming, wireless industrial control,
augmented reality, and self driving cars. Meeting the contradictory
requirements of URLLC, i.e., ultra-reliability and low-latency, is considered
to be very challenging, especially in bandwidth-limited scenarios. Most
communication strategies rely on hybrid automatic repeat request (HARQ) to
improve reliability at the expense of increased packet latency due to the
retransmission of failing packets. To guarantee high-reliability and very low
latency simultaneously, we enhance HARQ retransmission mechanism to achieve
reliability with guaranteed packet level latency and in-time delivery. The
proposed non-orthogonal HARQ (N-HARQ) utilizes non-orthogonal sharing of time
slots for conducting retransmission. The reliability and delay analysis of the
proposed N-HARQ in the finite block length (FBL) regime shows very high
performance gain in packet delivery delay over conventional HARQ in both
additive white Gaussian noise (AWGN) and Rayleigh fading channels. We also
propose an optimization framework to further enhance the performance of N-HARQ
for single and multiple retransmission cases.
