Hybrid ARQ Feedback Prediction for 5G

In cellular communications, the transmission of information is sometimes plagued by problems (e.g., the corruption of data). In response, receivers request a retransmission of information. Hybrid automatic repeat request (HARQ) is a well-established mechanism that combines forward error-correcting coding with automatic repeat request (ARQ) error control.

HARQ has a tradition of use in Long-Term Evolution (LTE) technologies. Now, 5G applies the same mechanism to achieve high spectral efficiency. Whereas in the previous generation (with LTE) the HARQ protocol was limited by many restrictions, 5G allows a fully flexible operation to make maximum use of time and frequency diversity. However, a major drawback of HARQ is the introduction of added latency. This poses an issue for latency-sensitive communication, such as factory automation, self-driving cars, and control for VR streaming.

The Multimedia Communications group explores methods to minimize the added latency, while benefitting from HARQ features such as high reliability and high spectral efficiency. The approach is based on HARQ feedback prediction, which applies various machine learning methods to predict the decodability of a partially received signal. Here, an emphasis is placed on so-called low-density parity-check (LDPC) subcodes, which characterize the features in the partial signal of the original code. By incorporating the code structure into the prediction mechanism, we achieve significant improvement over simple average likelihood estimators such as average log-likelihood ratio (LLR) thresholding.