Nonorthogonal HARQ for URLLC: Design and Analysis

The fifth generation (5G) of mobile standards is expected to provide ultrareliability 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., ultrareliability and low latency, is considered to be very challenging, especially in bandwidth-limited scenarios. Most communication strategies rely on the 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 the HARQ retransmission mechanism to achieve reliability with guaranteed packet-level latency and in-time delivery. The proposed nonorthogonal HARQ (N-HARQ) utilizes nonorthogonal 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.