Subcode-Based Early HARQ for 5G

Tactile Internet (TI) constitutes one of the major use cases for the development of the fifth generation (5G) mobile specification. TI services put high demand on the latency and reliability requirement, which is covered in the ultra-reliable low-latency communication (URLLC) discussion in 5G standardization. Hybrid Automatic Repeat Request (HARQ) is used in LTE to achieve high robustness in an efficient way with the cost of introducing additional latency. In this paper, we propose a new early HARQ scheme based on LDPC subcodes (SC E- HARQ), which enables to provide faster feedback and thus an earlier retransmission. The SC E-HARQ technique makes use of substructures in LDPC codes to start feedback calculation already on partially received codewords. This paper investigates the performance of SC E-HARQ in comparison with a second E-HARQ scheme based on log-likelihood ratio (LLR) estimation. The results show that SC E-HARQ achieves a comparable reliability to regular HARQ. In SNR regions relevant for URLLC, it clearly outperforms also the LLR-based E-HARQ in means of reliability as well as latency. Sub-millisecond latency with a total block error rate (BLER) of less than 10^(-4) is attained in TDL-C by allowing 1% false negative retransmissions.